(CoL)

Section 3

**The CoL zoo of game operations**

3.1 Preview

3.2
Prefixation

3.3 Negation

3.6
Reduction

3.7
Blind operations

3.10
Toggling operations

3.11
Cirquents

3.1 Preview

As we
already
know,
logical operators in CoL stand for operations on games. There is an
open-ended
pool of operations of potential interest, and which of those to study
may
depend on particular needs and taste. Below is an incomplete list of
the
operators that have been officially introduced so far.

*Negation: **
*

*Conjunctions: **parallel*), *choice*), *sequential*), (*toggling*)

*Disjunctions: **parallel*), *choice*), *sequential*),
*toggling*)

*Recurrences: *** **(

*Corecurrences:**branching*), (*parallel*),
*sequential*),
*toggling*)

*Universal
quantifiers:** *** **(

*Existential
quantifiers:*** **(

*Implications***:** *parallel*), *choice*),
*sequential*),
*toggling*)

*Rimplications:**branching*),
*parallel*),
*sequential*),
*toggling*)

Among
these we
see
all operators of classical logic, and our choice of the classical
notation for
them is no accident. It was pointed out earlier
that
classical logic is nothing but the elementary, zero-interactivity
fragment of
computability logic. Indeed, after analyzing the relevant definitions,
each of
the classically-shaped operations, when restricted to elementary games, can
be easily seen to be
virtually the same as the corresponding operator of classical logic.
For
instance, if *A* and *B*
are elementary games, then so is *A**B*, and the
latter is
exactly the classical conjunction of *A*
and *B* understood as an (elementary)
game. In a general --- not-necessarily-elementary --- case, however, , , , become
more reminiscent of
(yet not the same
as) the corresponding multiplicative operators of linear
logic. Of course, here we are comparing apples with oranges
for, as noted
earlier, linear logic is a syntax while computability logic is a
semantics, and
it may be not clear in what precise sense one can talk about
similarities or
differences.

In the
same
apples
and oranges style, our operations , , , can
be perceived as relatives of the
additive connectives and
quantifiers of linear logic; , as
“multiplicative
quantifiers”; ,,, as
“exponentials”,
even though it is hard to guess which of the two groups

In this
section we are going to see formal definitions of all of the above-listed
operators. We agree that, throughout those definitions, Φ ranges over positions, Γ ranges over** **runs and e over (V,D)-valuations, where D
is the domain of the game G that is being defined and V is the set of
variables on which that game depends.** **All three metavariables should be considered universally quantified. Each definition has two clauses, one defining Lp_{e}^{G} (and thus also Lr_{e}^{G}) and the other Wn_{e}^{G}. The second clause, telling us who wins a given run Γ of G(e), always implicitly assumes that such a Γ is in **Lr**_{e}* ^{G}*.

Intuitive explanations of game operations will be provided as well. In such cases, unless otherwise implied by the context, the games to which those operations are applied are assumed to be constant.

This section
also provides many examples of particular
games. Let us agree that, unless otherwise suggested by the context, in
all
those cases we have the standard universe in mind. Often we let
non-numerals such
as people, Turing machines, etc. act in the roles of “constants”. These
should
be understood as abbreviations of the corresponding decimal numerals
that
encode these objects in some fixed reasonable encoding. It should also
be
remembered that algorithmicity is a minimum requirement on ’s strategies.
Some of
our examples implicitly assume stronger requirements, such as
efficiency or
ability to act with imperfect knowledge. For instance, the problem of
telling
whether there is or has been life on Mars is, of course, decidable, for
this is
a finite problem. Yet our knowledge does not allow us to actually solve
the
problem. Similarly, chess is a finite game and (after ruling out the
possibility of draws) one of the players does have a winning strategy
in it.
Yet we do not know specifically what (and which player’s) strategy is a
winning
one.

When
omitting
parentheses in compound expressions, we assume that all unary operators
(negation, repudiations, recurrences, corecurrences and quantifiers)
take precedence
over all
binary operators (conjunctions, disjunctions, implications,
rimplications),
among which implications and rimplications have the lowest precedence.
So, for
instance, *A* *B**C* should be
understood as *A* ((*B*)*C*) rather
than,
say, as (*A* *B*)*C* or as *A* ((*B**C*)).

**Theorem
3.1.1**
(proven in [Jap03, Jap08b,
Jap11a]) All operators listed
in
this subsection preserve the static property of games (i.e., when
applied to
static games, the resulting game is also static).

3.2 Prefixation

Unlike
the
operators
listed in the preceding subsection, the operation of prefixation is not
meant here to act as a logical operator in the formal language of CoL. Yet, it is
very
useful in characterizing and analyzing games, and we want to start our
tour of
the zoo with it.

**Definition
3.2.1**
Assume A = (Dm, Dn, Vr, A) is
a game and Ψ is** **a unilegal
position of *A *(otherwise the operation is undefined). The Ψ–** prefixation**
of

**Lp**_{e}=^{G}**{**Φ | 〈Ψ,Φ〉∈**Lr**_{e}^{A}**}**;**Wn**_{e}^{G}〈Γ^{ }**〉****=****Wn**〈_{e}^{A}**Ψ,Γ****〉.**

Intuitively, 〈**Ψ〉***A* is the game
playing which means playing *A*
starting (continuing) from position Ψ. That
is, 〈Ψ**〉***A** is the game
to which **A* ** evolves** (is

3.3 Negation

For
a run Γ, by Γ we mean the
“negative image” of Γ (green and red
interchanged). For instance, 〈α,β,γ**〉**** = **〈α,β,γ**〉. **

**Definition
3.3.1**** **Assume *A = *(*Dm, Dn, Vr, A*)
is a game**.
***A*** **(read "not A") ** **is defined as
the
game (*Dm, Dn, Vr, G*) such that:

**Lp**_{e}={Φ | Φ∈^{G}**Lp**_{e}^{A}**};****Wn**_{e}^{G}〈Γ^{ }**〉****=****Wn**〈Γ_{e}^{A}**〉****=**.

In
other words, *A* is *A*
with the roles of the two players
interchanged: Machine’s (legal) moves and wins become Environment’s
moves and
wins, and vice versa. So, when visualized as a tree, *A *is the exact
negative image of *A*, as illustrated
below:

Figure 3.3.2**:**
Negation

Let
*Chess* be
the game of chess (with draws ruled out) from the point of view of the
white
player. Then *Chess* is *Chess*
“upside down”, i.e., the game of
chess from the point of view of the black player:

Observe
that the classical double negation
principle *A*
= *A*
holds: interchanging the players’ roles twice restores the
original
roles of the players. It is also easy to see that we always have 〈**Ψ〉***A* = 〈** Ψ〉 ***A*. So, for
instance,
if α is Machine’s
legal move in the empty position of *A*
that brings *A* down to *B*, then the
same α **is
Environment’s legal move in the empty position of ***A*, and it
brings *A* down to *B*. Test the
game *A* of
Figure 3.3.2
to see that this is indeed so.

3.4 Choice operations

** Choice
conjunction **(read “

*A ** B* is a game
where, in the initial (empty) position,
only Environment has legal moves. Such a move should be either ‘0’ or
‘1’. If
Environment moves 0, the game continues as *A*,
meaning that 〈**0**〉(*A
** B*) = *A*;
if it
moves 1, then the game continues as *B*,
meaning that 〈**1**〉(*A
** B*) = *B*;
and
if it fails to make either move (“choice”), then it loses. *A
** B *is similar,* *with
the difference that here it is Machine who has initial moves and who
loses if
no such move is made.

**Definition
3.4.1 **Assume** ***A*_{0 }= (Dm, Dn, Vr_{}_{0}, A_{0}) and ** ***A*_{1 }=(Dm, Dn, Vr_{}_{1}, A_{1}) are games.

a)
*A*_{0 }_{ }*A*_{1}** **is defined as
the game
(*Dm, Dn,* Vr_{}_{0}^{}∪Vr_{}_{1}^{}^{}* ^{}*, G)
such that:

**Lp**_{}_{e}={〈 〉} ∪ {〈^{G},Φ〉 |*i**i*∈{0,1}, Φ∈**Lp**_{e}^{A}^{i}};**Wn**_{e}〈 〉 =^{G}**Wn**〈_{e}^{G},Γ〉 =*i***Wn**_{e}^{A}^{i}〈Γ〉.

b)
*A*_{0 }_{ }*A*_{1}** **is defined as
the game (*Dm, Dn, *Vr_{}_{0}^{}∪Vr_{}_{1}^{}^{}* ^{}*, G)
such that

**Lp**={〈 〉} ∪ {〈^{G},Φ〉 |*i**i*∈{0,1}, Φ∈**Lp**^{A}^{i}};**Wn**〈 〉 = ;^{G}**Wn**〈^{G},Γ〉 =*i***Wn**^{A}^{i}〈Γ〉.

The
game *A*
of Figure 3.3.2 can now
be easily seen to be ( ) ( ), and its
negation
be ( ) ( ). The De
Morgan
laws familiar from classical logic persist: we always have (*A* *B*) = *A * *B* and (*A ** B*) = *A ** **B*. Together
with the
earlier observed double negation principle, this means that *A
* *B *=
(*A* *B*) and *A ** B
*= (*A * *B*). Similarly
for the
quantifier counterparts and
of
and
.

Given
a game *A*(*x*),
the ** choice universal
quantification**
(read “

Specifically,
*xA*(*x*)
is a game where, in the initial
position, only Environment has legal moves, and such a move should be
one of
the constants. If Environment moves *c*,
then the game continues as *A*(*c*), and
if Environment fails to make an
initial move/choice, then it loses.
*xA*(*x*)
is similar,* *with the difference
that here it is Machine who has initial moves
and who loses if no such move is made.
So, we always have 〈*c*〉*xA*(*x*)=*A*(*c*)
and 〈*c*〉*xA*(*x*)=*A*(*c*).
Below is a formal definition of choice quantifiers.

**Definition
3.4.2 **Assume
*A*(*x*) = (Dm, Dn, Vr, A)
is a game.

a)
*xA*(*x*)
** **is
defined as the game (Dm, Dn, Vr-{*x*}, *G*) such that:

**Lp**_{e}={〈 〉} ∪ {〈^{G},Φ〉:*c**c*∈*Constants*, Φ∈**Lp**_{e}^{A}^{(c)}};**Wn**_{e}〈 〉 =^{G}**Wn**〈_{e}^{G},Γ〉 =*c***Wn**_{e}^{A}^{(c)}〈Γ〉.

b)
*xA*(*x*)
** **is
defined as the game (Dm, Dn, Vr-{*x*},* G*) such that:

**Lp**_{e}={〈 〉} ∪ {〈^{G},Φ〉:*c**c*∈*Constants*, Φ∈**Lp**_{e}^{A}^{(c)}};**Wn**_{e}〈 〉^{G}**=****Wn**〈_{e}^{G},Γ〉 =*c***Wn**_{e}^{A}^{(c)}〈Γ〉.

Now
we are already able to express traditional
computational problems using formulas. Traditional problems come in two
forms:
the problem of computing a function *f*(*x*),
or the problem of deciding
a predicate p(x). The former can be written as *x**y*(*y*=*f*(*x*)), and the
latter as *x*(*p*(*x*)
*p*(*x*)). So, for
instance,
the constant “successor” game of Figure
2.3.4 will
be written as *x**yA*(*y*=*x*+1),
and the unary “generalized successor” game of Figure
2.3.5 as *x**yA*(*y*=*x*+*z*).
The following game, which is about deciding the “evenness”
predicate,
could be written as *x*(*y*(*x=2y*)
*y*(*x=2y*)) ( will be
officially
defined later, but,
as promised, its meaning is
going to be exactly classical when applied to an elementary game like

Classical
logic has been repeatedly criticized for its
operators not being constructive. Consider, for example,** ***x**y*(*y*=*f*(*x*)). It is true
in the
classical sense as long as* f* is a
total function. Yet its truth has little (if any) practical import, as “*y*” merely
signifies *existence*
of *y*, without implying that such a *y*
can actually be *found*.**
**And,
indeed, if *f* is an incomputable
function, there is no method for finding *y*.**
**On the other hand, the choice operations of CoL *are*
constructive.**
**Computability (“truth”) of *x**y*(*y*=*f*(*x*)) means more
than
just existence of *y*;
it means the possibility to actually *find* (*compute*,
*construct*) a corresponding *y*
for every *x*.

*x**y*(*Halts*(*x,y*)
*Halts*(*x,y*)) *x**y*(*Halts*(*x,y*)
*Halts*(*x,y*)).

*Chess* *Chess*,
on the other hand, is an example of a
computable-in-principle yet “practically incomputable” problem, with no
real
computer anywhere close to being able to handle it.

There
is no need to give a
direct definition for the remaining choice operation of ** choice
implication** (“

**Definition
3.4.3** *A**B *=_{def
} _{ }*A** B*.

Each of the other sorts of disjunctions (parallel, sequential and toggling) generates the corresponding implication the same way.

3.5 Parallel operations

The
** parallel conjunction **(read “

This
is very different from *Chess ** Chess*. In the
latter needs
to choose between the
two
components and then win the chosen one-board game, which makes *Chess ** Chess* essentially
as hard to win as either *Chess *or* Chess.*
A game of the form *A**B* is generally
easier (at
least, not harder) to win than *A**B*,*
*the latter is easier to win than *A**B*, and the
latter in turn is
easier to win than *A**B*.

Technically,
a move** **α** **in the left [resp. right] -conjunct or -disjunct is
made by prefixing α with ‘0.’ [resp. ‘1.’].
For instance, in (the initial position of) (*A**B*)(*C**D*), the move
‘1.0’ is legal
for , meaning
choosing the left -conjunct in
the second -disjunct of
the game. If such a move is made, the game continues as (*A**B*)*C*. The player , too, has
initial legal moves in (*A**B*)(*C**D*), which are
‘0.0’ and
‘0.1’.

In
the formal definitions of
this section and throughout the rest of this webpage, we use the
important
notational convention according to which:

**Notation
3.5.1**
For a
run Γ and string α, Γ^{
}^{α} means
the result of removing
from Γ all moves
except those of
the form αβ, and
then deleting the
prefix ‘α’ in the
remaining moves.

So,
for instance, 〈**1.2,1.0,0.33〉**^{1}^{.}^{ }= 〈2,0〉** **and** ** 〈**1.2,1.0,0.33〉**^{0}** ^{. }**= 〈

**Definition
3.5.2 **Assume** ***A*_{0 }= (Dm, Dn, Vr_{}_{0, }*A*_{0}) and ** ***A*_{1 }= (Dm, Dn, Vr_{}_{1, }*A*_{1}) are games.

a)
*A*_{0}*A*_{1 }**is defined as
the game
(***Dm, Dn,* Vr_{}_{0}^{}∪Vr_{}_{1}^{}^{}* ^{}*, G)
such that

- Φ∈
**Lp**_{e}iff every move of Φ has the prefix ‘0.’ or ‘1.’ and, for both^{G}*i*∈{0,1}, Φ^{i}∈^{.}**Lp**_{e}^{A};^{i} **Wn**_{e}〈Γ^{G}**〉****=***i***∈{0,1},****Wn**_{e}^{A}〈^{i}**Γ**^{i}〉^{.}**=.**

b)
*A*_{0}*A*_{1}** ****is defined as
the game
(***Dm, Dn,* Vr_{}_{0}^{}∪Vr_{}_{1}^{}^{}* ^{}*, G)
such that

- Φ∈
**Lp**_{}_{e}iff every move of Φ has the prefix ‘0.’ or ‘1.’ and, for both^{G}*i*∈{0,1}, Φ^{i}∈^{. }**Lp**_{e}^{A};^{i} **Wn**_{e}〈^{G}**Γ〉****=***i***∈{0,1},****Wn**_{e}^{A}〈^{i}**Γ**^{i}〉^{.}**=.**

When
*A* and *B*
are (constant) finite games, the depth
of *A**B* or *A**B* is the sum of
the
depths of *A* and *B*,
as seen below.

** **Figure 3.5.3

This
signifies
an exponential growth of the breadth, meaning
that, once we have reached the level of parallel operations, continuing
drawing
trees in the earlier style becomes no fun. Not to be disappointed
though:
making it possible to express large- or infinite-size game trees in a
compact
way is what our game operators are all about after all.

Whether
trees
are or are not helpful in visualizing
parallel combinations of unistructural games, prefixation is still very
much so
if we think of each unilegal position Φ of *A*
as the game 〈Φ〉*A*.
This way, every unilegal
run G of *A*
becomes a sequence of games as illustrated in the following example.

**Example
3.5.4 ** To the
(uni)legal
run Γ = 〈1.7, 0.7, 0.49,
1.49**〉**** of
game ***A*
= *x**y*(*y*≠*x*^{2}) *x**y*(*y*=*x*^{2})
induces the following sequence, showing how things evolve as Γ runs, i.e.,
how the moves of Γ affect/modify
the game that is being played:

*x**y*(*y*≠*x*^{2})*x**y*(*y*=*x*^{2}) i.e.*A**x**y*(*y*≠*x*^{2})*y*(*y*=7^{2}) i.e. 〈1.7〉*A**y*(*y*≠7^{2})*y*(*y*=7^{2}) i.e. 〈1.7,0.7〉*A*- 49≠7
^{2}*y*(*y*=7^{2}) i.e. 〈1.7,0.7,0.49〉*A* - 49≠7
^{2}49=7^{2}i.e. 〈1.7,0.7,0.49,1.49〉*A*

The
run hits
the true proposition 49≠7^{2}
49=7^{2},
and hence is won by the
machine.

As we
may
guess, the ** parallel
universal quantification **(“

**Definition
3.5.5 **Assume
*A*(*x*) = (Dm, Dn, Vr, A)
is a game.

a)
*xA*(*x*)** **** **is
defined as the game (Dm, Dn, Vr-{*x*}, *G*) such that:

- Φ∈
**Lp**_{e}iff every move of Φ has the prefix ‘^{G}*c*.’ for some*c*∈*Constants*and, for all such*c*, Φ^{c}∈^{. }**Lp**_{e}^{A}^{(c)}; **Wn**_{e}〈Γ〉^{G}**=***c*∈*Constants*,**Wn**〈Γ_{e}^{A}^{c}〉^{.}**=.**

b)
*xA*(*x*)** **** **is
defined as the game (Dm, Dn, Vr-{*x*},* G*) such that:

- Φ∈
**Lp**_{e}iff every move of Φ has the prefix ‘^{G}*c*.’ for some*c*∈*Constants*and, for all such*c*, Φ^{c}∈^{. }**Lp**_{e}^{A}^{(c)}; **Wn**_{e}〈Γ〉^{G}**=***c*∈*Constants*,**Wn**〈Γ_{e}^{A}^{c}〉^{.}**=.**

The next group of parallel operators are ** parallel
recurrence** (“

**Definition
3.5.6 **Assume
*A = *(*Dm, Dn, Vr, A*) is
a game.

a)
*A*** **is defined as
the game (*Dm, Dn, Vr, G*)
such that:

- Φ∈
**Lp**_{e}iff every move of Φ has the prefix ‘^{G}*c*.’ for some*c*∈*Constants*and, for all such*c*, Φ^{c}∈^{. }**Lp**_{e};^{A} **Wn**_{e}〈Γ〉^{G}**=***c*∈*Constants*,**Wn**〈Γ_{e}^{A}^{c}〉^{.}**=.**

b)
*A*** ****is defined as
the game (***Dm, Dn, Vr, G*)
such that:

- Φ∈
**Lp**_{e}iff every move of Φ has the prefix ‘^{G}*c*.’ for some*c*∈*Constants*and, for all such*c*, Φ^{c}∈^{. }**Lp**_{e};^{A} **Wn**_{e}〈Γ〉^{G}**=***c*∈^{}*Constants*,**Wn**〈Γ_{e}^{A}^{c}〉^{.}**=.**

As
was the
case with choice operations, we can see
that the definition of each parallel operation seen so far in this
section can be
obtained
from the definition of its dual by just interchanging with
. Hence it is
easy
to verify that we always have (*A* *B*) = *A**B*,
(*A* *B*) = *A**B*,
*xA*(x) = *x**A*(x),
*xA*(*x*) = *x**A*(*x*), *A *= *A*, *A *= *A*,
This, in turn, means that each parallel operation is definable in the
standard
way in terms of its dual operation and negation. For instance, *A**B *can be defined
as* **(**A**B*), and *A *as *A*.
Three more parallel operations defined in
terms of negation and other parallel operations are ** parallel implication** (“

**Definition
3.5.7 **a) *A*_{
}_{ }*B*** **=_{def
}** ***A*_{ }_{ }*B*

*
*b)* A*_{ }_{ }*B*** **=_{def}** ***A*_{ }_{ }*B*_{ }

c) *A*** **=_{def}** ***A*_{ }_{}

Note
that,
just like negation and unlike choice
operations, parallel operations preserve the elementary property of
games. When
restricted to elementary games, the meanings of , and
coincide
with those of
classical conjunction,
disjunction and implication. Further, as long as all individuals of
the
universe have naming constants, the meanings of and
coincide
with those of
classical universal
quantifier and existential quantifier. The same conservation of
classical
meaning (but without any conditions on the universe) is going to be the
case with the blind quantifiers

While
all
classical tautologies automatically remain
valid when parallel operators are applied to elementary games, in the
general
case the class of valid (“always computable”) principles shrinks. For
example, *P ** P**P*, i.e. *P*(*P**P*), is not
valid. Back to our
chess example, one
can see that the earlier copycat
strategy successful for
*Chess**Chess* would be
inapplicable to *Chess*(*Chess**Chess*). The best
that can
do in this three-board
game is to synchronize *Chess* with one of
the two
conjuncts of *Chess**Chess*. It is
possible
that then *Chess* and the
unmatched *Chess* are both lost, in
which case the
whole game will be lost.

The
principle *P ** P**P* is valid in
classical logic because the latter sees no difference between *P* and *P**P. *On the other
hand,
in virtue of its semantics, CoL is *resource-conscious*,
and in it *P*
is by no means the same as *P**P* or *P**P*. Unlike
*P ** P**P*,*
* *P ** P**P* is a valid
principle. Here, in the antecedent, we have infinitely many “copies” of
*P*. Pick any two copies and, via
copycat,
synchronize them with the two conjuncts of the consequent. A win is
guaranteed.

3.6 Reduction

Intuitively,
*A**B* is the
problem of *reducing**B* to *A*:
solving *A**B* means solving
*B* while having *A*
as a computational resource. Specifically, may
observe how *A* is being solved (by the environment),
and utilize this information
in its own solving *B*. As already
pointed out, resources
are symmetric
to problems: what is a problem to solve for one player is a resource
that the
other player can use, and vice versa. Since *A*
is negated in *A**B* and negation
means
switching the roles, *A* appears as a
resource rather than problem for in
*A**B*. Our
copycat
strategy
for *Chess**Chess* was an
example of
reducing *Chess*
to *Chess*.
The same strategy was
underlying Example 3.5.4,
where *x**y*(*y*=*x*^{2})
was reduced to itself.

Let us
look at
a
more meaningful example: reducing the acceptance problem to the
halting
problem. The former, as a decision problem, will be written as *x**y* (*Accepts*(*x*,*y*)
*Accepts*(*x*,*y*)), where *Accepts*(*x*,*y*) is the predicate
“Turing machine *x* accepts input *y*”.
Similarly, the halting problem is
written as *x**y* (*Halts*(*x*,*y*)
* Halts*(*x*,*y*)). Neither problem has an algorithmic
solution, yet the following pimplication does:

*x**y* (*Halts*(*x*,*y*) * Halts*(*x*,*y*)) *x**y* (*Accepts*(*x*,*y*) * Accepts*(*x*,*y*))

Here is
Machine’s
winning strategy for the above game. Wait till Environment makes moves 1.*m*
and 1.*n*
for some *m* and *n*.
Making these moves essentially means asking the question “*Does machine m accept input n?*”. If such moves are never
made, you win.
Otherwise, the moves bring the game down to

*x**y* (*Halts*(*x*,*y*) * Halts*(*x*,*y*)) *Accepts*(*m*,*n*) * Accepts*(*m*,*n*)

Make
the moves
0.*m* and 0.*n*,
thus asking the counterquestion “*Does machine m
halt on input n?*”.
Your moves
bring the game down to

*Halts*(*m*,*n*) * Halts*(*m*,*n*) *Accepts*(*m*,*n*) * Accepts*(*m*,*n*)

Environment
will
have to answer this question, or else it loses (why?).** **If
it answers by
move 0.0 (“*No,
m
does not halt on n*”),
you make the move 1.0
(say “*m does
not accept n*”).
The game will be
brought down to *Halts*(*m*,*n*) *Accepts*(*m*,*n*).
You win, because if *m*
does not halt on *n*,
then it does not accept *n*,
either. Otherwise, if
Environment answers by move 0.1 (“*Yes,
m halts
on n*”),
start simulating *m*
on *n*
until *m*
halts. If you see
that *m*
accepted *n*,
make the move 1.1 (say “*m accepts
n*”);
otherwise make the
move 1.0 (say “*m
does not accept
n*”).
Of course, it is a possibility that the simulation goes on forever. But
then Environment
has lied when saying “*m
halts on n*”;
in other
words, the antecedent is false, and you still win.

Note
that what
Machine did when following the above
strategy was indeed reducing the acceptance problem to the halting
problem: it
solved the former using an external (Environment-provided) solution of
the
latter.

There
are many
natural concepts of reduction, and a
strong case can be made in favor of the thesis that
the sort of reduction captured by is
most basic among them.
For this reason, we
agree that, if we simply say “reduction”, it always means the sort of
reduction
captured by . A great
variety of
other reasonable concepts of reduction is expressible in terms of . Among
those is Turing
reduction. Remember
that a predicate *q*(*x*)
is said to be *Turing reducible* to a
predicate *p*(*x*)
if *q*(*x*)
can be decided by a
Turing machine equipped with an oracle for *p*(*x*).
For a positive integer *n*,
*n*-*bounded
Turing reducibility* is defined in the same way, with the only
difference
that here the oracle is allowed to be used only *n*
times. It turns out that parallel rimplication is
a conservative
generalization of Turing
reduction. Namely, when *p*(*x*) and *q*(*x*) are elementary games
(i.e. predicates), *q*(*x*)
is Turing reducible to *p*(*x*)
if and only if the problem *x*(*p*(*x*) * p*(*x*))
*x*(*q*(*x*)
* q*(*x*)) has an
algorithmic
solution. If here we change back
to , we get the
same
result for 1-bounded Turing reducibility. More generally, as one might
guess, *n*-bounded Turing reduction
will be
captured by

*x*_{1}(*p*(*x*_{1})
* p*(*x*_{1}))
… *x*_{n}(*p*(*x*_{n})
* p*(*x*_{n})) *x*(*q*(*x*) * q*(*x*)).

If,
instead,
we write

*x*_{1}…*x*_{n}((*p*(*x*_{1})
* p*(*x*_{1})) …
(*p*(*x*_{n})
* p*(*x*_{n}))) *x*(*q*(*x*) * q*(*x*)),

then
we get a
conservative generalization of *n*-*bounded
weak truth-table reduction*. The latter differs from *n*-bounded
Turing reduction in that here
all *n* oracle queries should be made
at once, before seeing responses to
any of those queries. What is called *mapping*
(*many-one*) *reducibility*
of *q*(*x*)
to *p*(*x*)
is nothing but
computability of *x**y*(*q*(*x*)↔*p*(*y*)), where A↔B abbreviates (*A**B*)(*B**A*). We
could go on and on with
this list.

And
yet many
other natural concepts of reduction
expressible in the language of CoL
may
have no established names in the literature. For example, from the
previous
discussion it can be seen that a certain reducibility-style relation
holds
between the predicates *Accepts*(*x*,*y*)
and *Halts*(*x*,*y*)
in an even stronger sense than the algorithmic
winnability of

*x**y* (*Halts*(*x*,*y*) * Halts*(*x*,*y*)) *x**y* (*Accepts*(*x*,*y*) * Accepts*(*x*,*y*)).

In
fact, not
only
the above problem has an algorithmic solution, but also the generally
harder-to-solve problem

*x**y* (*Halts*(*x*,*y*) *
Halts*(*x*,*y*) *Accepts*(*x*,*y*) * Accepts*(*x*,*y*)).

Among
the
merits of CoL is that it offers a formalism
and deductive machinery for systematically expressing and studying
computation-theoretic
relations such as reducibility, decidability, enumerability, etc., and
all
kinds of variations of such concepts.

Back to
reducibility, while the standard approaches only allow us to talk about
(a
whatever sort of) reducibility as a *relation*
between problems, in our
approach reduction becomes an *operation* on
problems, with reducibility
as a relation simply meaning computability of the corresponding
combination
(such as *A**B*) of games.
Similarly for other relations or
properties such as the property of *decidability*.
The latter becomes the
operation of *deciding* if we define the problem of
deciding a predicate
(or any computational problem) *p*(*x*) as the game *x*(*p*(*x*) * p*(*x*)). So, now we
can
meaningfully ask questions such as “*Is
the* *reduction
of
the problem of deciding q(x) to the
problem of deciding p(x) always reducible to the mapping
reduction of
q(x) to
p(x)?*”.
This
question would be equivalent to whether the
following formula is valid in CoL:

*x**y*(*q*(*x*)
↔ *p*(*y*)) (*x*(*p*(*x*)
* p*(*x*)) *x*(*q*(*x*)
* q*(*x*))).

The answer
turns out to be “Yes”, meaning that mapping
reduction is at least as strong as reduction. Here is a strategy which
wins this
game no matter what particular predicates*
p*(*x*)
and *q*(*x*)
are:

1.** **Wait till, for
some *m*,
Environment brings the game down to ** **

*x**y*(*q*(*x*) ↔ *p*(*y*)) (*x*(*p*(*x*)
* p*(*x*)) *q*(*m*)
* q*(*m*)).

2.
Bring
the game down to ** **

*y*(*q*(*m*) ↔ *p*(*y*)) (*x*(*p*(*x*)
* p*(*x*)) *q*(m)
* q*(m)).

3. Wait
till,
for
some *n*,
Environment brings the game down to ** **

(*q*(*m*) ↔ *p*(*n*)) (*x*(*p*(*x*)
* p*(*x*)) *q*(m)
* q*(*m*)).

4.
Bring the
game
down to ** **

(*q*(*m*) ↔ *p*(*n*)) (*p*(*n*)
* p*(*n*) *q*(*m*)
* q*(*m*)).

5.
Wait
till Environment brings the game down to one of
the following: ** **

5a.
(*q*(*m*) ↔ *p*(*n*)) (*p*(*n*) *q*(m)
* q*(*m*)). In this
case,
further bring it down to (*q*(*m*) ↔ *p*(*n*)) (*p*(*n*) *q*(*m*)), and you have
won.

5b.
(*q*(*m*) ↔ *p*(*n*)) (*p*(*n*) *q*(*m*)
* q*(*m*)). In this
case,
further bring it down to (*q*(*m*) ↔ *p*(*n*)) (*p*(*n*) *q*(*m*)), and you have
won,
again.

We can
also
ask: “*Is the mapping reduction of* *q*(*x*)
*to* *p*(*x*)
*always reducible to the reduction
of the problem of deciding q*(*x*)
*to the problem of deciding* *p*(*x*)?”.
This question would be equivalent to whether the following formula is
valid:

( *x*(*p*(*x*)
* p*(*x*)) *x*(*q*(*x*)
* q*(*x*))) *x**y*(*q*(*x*) ↔ *p*(*y*)).

The
answer
here turns
out to be “No”, meaning that mapping reduction is properly stronger
than
reduction. This negative answer can be easily obtained by showing that
the
above formula is not provable in the deductive system **CL4**
that we are
going to see later. **CL4**
is sound and
complete with respect to validity. Its completeness implies that any
formula
which is not provable in it (such as the above formula) is not valid.
And the
soundness of **CL4** implies that every provable
formula is valid. So, had
our ad hoc attempt to come up with a strategy for *x**y*(*q*(*x*) ↔ *p*(*y*)) (*x*(*p*(*x*)
* p*(*x*)) *x*(*q*(*x*)
* q*(*x*))) failed, its
validity could have been easily
established by finding a **CL4**-proof of it.

To summarize, CoL offers not only a convenient language for specifying computational problems and relations or operations on them, but also a systematic tool for answering questions in the above style and beyond.

3.7 Blind operations

Our
definition
of
the ** blind universal quantifier** (“

**Definition
3.7.1****
**Assume *A*(*x*) = (Dm, Dn, Vr, A) is a game
unistructural in *x*.

a)
*xA*(*x*)
** **is
defined as the game (Dm, Dn, Vr-{x}, G) such that:

**Lp**_{e}=^{G}**Lp**_{e}^{A}^{(}^{x}^{)};**Wn**_{e}〈Γ^{G}**〉****=***a*∈Dm^{},^{}**Wn**_{e}^{A}^{(a) }〈Γ**〉****=**

b)
*xA*(*x*)
** **is
defined as the game (Dm, Dn, Vr-{x}, G) such that:

**Lp**_{e}=^{G}**Lp**_{e}^{A}^{(}^{x}^{)};**Wn**_{e}〈Γ^{G}**〉****=***a*∈Dm^{},^{}**Wn**_{e}^{A}^{(a) }〈Γ**〉****=.**

Intuitively,
playing *xA*(*x*) **or ***xA*(*x*) **means
just playing**** ***A*(*x*)
“blindly”, without knowing the value of *x*.
In *xA*(*x*),
Machine wins iff the play it
generates is successful for every possible value of *x* from the domain,
while in *xA*(*x*)** **being
successful for just one value is
sufficient. When applied to elementary games, the blind quantifiers act
exactly
like the quantifiers of classical logic, even if not all individuals of
the
universe have naming constants.

From
the
definition
one can see a perfect symmetry between and

Unlike *xA*(*x*)
which is a game on infinitely many
boards, both *xA*(*x*)
and *xA*(*x*)
are one-board games. Yet, they are very different from
each other. To
see this difference, compare the problems *x*(*Even*(*x*)
* Odd*(*x*)) and *x*(*Even*(*x*)
* Odd*(*x*)). The former
is an
easily winnable game of depth 2: Environment selects a number, and
Machine
tells whether that number is even or odd. The latter, on the other
hand, is a
game which is impossible to win. This is a game of depth 1, where the
value of *x* is not specified by
either player, and
only Machine moves --- tells whether (the unknown) *x*
is even or odd. Whatever Machine says, it loses, because there is
always a value for *x* that makes the
answer wrong.

This
should
not
suggest that nontrivial -games can
never be
won. For instance, the problem

*x*(*Even*(*x*)*Odd*(*x*)*y*(*Even*(*x*+*y*)*Odd*(*x*+*y*)))*x*(*Even*(*x*)*Odd*(*x*)*Even*(*x*+5)*Odd*(*x*+5))*x*(*Even*(*x*)*Even*(*x*+5)*Odd*(*x*+5))*x*(*Even*(*x*)*Odd*(*x*+5))

Machine
won
because
the play hit the true *x*(*Even*(*x*) *Odd*(*x*+5)). Notice how *x* persisted
throughout the sequence. Generally, the (,

** Exercise 3.7.2** Make your best guess regarding whether the
following problems are always
computable.

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*xA*(*x*) *xA*(*x*)
answer

*x*(*A*(*x*)*B*(*x*)) *xA*(*x*)*xB*(*x*)
answer

*x*(*A*(*x*)*B*(*x*)) *xA*(*x*)* xB*(*x*)
answer

*xA*(*x*)*xB*(*x*) *x*(*A*(*x*)*B*(*x*)) answer

3.8 Branching operations

There
are only
four
branching operations that we consider:** **recurrence

We have already seen one (parallel) sort of recurrence
operations, and more are still to come. What is
common
for
the members of the family of game operations called *recurrence*** operations**
is that, when applied to a game

There
is more
than
one naturally emerging recurrence operation. The differences between
various
recurrence operations are related to how “repetition” or “reusage” is
exactly
understood. Imagine a computer that has a program successfully playing
chess.
The resource that such a computer provides is obviously something
stronger than
just *Chess*,
for it
permits to play *Chess*
as
many times as the user wishes, while *Chess*,
as such, only assumes one play. The simplest operating system would
allow to
start a session of *Chess*,
then --- after finishing or abandoning and destroying it --- start a
new play
again, and so on. The game that such a system plays --- i.e. the
resource that
it supports/provides --- is *Chess*, which
assumes an unbounded
number of plays of
*Chess* in a
sequential
fashion. A formal definition of the operation , called *sequential recurrence*, will
be
given later is Section 3.9.

A more
advanced
operating system, however, would not require to destroy the old
sessions before
starting new ones; rather, it would allow to run as many parallel
sessions as
the user needs. This is what is captured by *Chess*, meaning
nothing
but the infinite parallel conjunction *Chess
** Chess
** Chess
*** **...**
**As
we remember
from Section
3.5, is
called *parallel recurrence*.

Yet a
really
good
operating system would not only allow the user to start new sessions of
*Chess*
without destroying old ones;
it would also make it possible to branch/replicate any particular stage
of any
particular session, i.e., create any number of
“copies” of any already reached position of the multiple
parallel plays
of *Chess*,
thus giving the
user the possibility to try different continuations from the same
position.
What corresponds to this intuition is the ** branching
recurrence **(“

At the
intuitive
level, the difference between and
is
that in

The
less
flexible
type of reusage of *A*
assumed by *A*, on the other
hand, is closer to what infinitely many
autonomous physical resources would naturally offer, such as an
unlimited
number of independently acting robots each performing task *A*,
or an unlimited
number of computers with limited memories, each one only capable of and
responsible
for running a single thread of process *A*.
Here the effect of
forking an
advanced stage of *A*
cannot be achieved unless, by good luck, there are two
identical copies of the stage, meaning that the corresponding two
robots or
computers have so far acted in precisely the same ways.

The
formal
definitions of *A* and its dual ** branching
corecurrence** (“

**Notation
3.8.1**
Where Γ is a run and *w* is a ** bitstring** (finite
or
infinite sequence of 0s and 1s), Γ

**Definition
3.8.2 **Assume
*A = *(*Dm, Dn, Vr, A*) is
a game.

a)
*A*** **is defined as
the game (*Dm, Dn, Vr, G*) such that:

- Φ∈
**Lp**_{e}iff every move of Φ has the prefix ‘^{G}*u*.’ for some finite bitstring*u*, and, for every infinite bitstring*w*, Φ^{≤}^{w}**∈****Lp**_{e};^{A} **Wn**_{e}〈Γ〉^{G}**=***w*,**Wn**〈Γ_{e}^{A}^{≤}^{}〉^{w}

b)
*A*** **is defined as
the game (*Dm, Dn, Vr, G*) such that:

- Φ∈
**Lp**_{e}iff every move of Φ has the prefix ‘^{G}*u*.’ for some finite bitstring*u*, and, for every infinite bitstring*w*, Φ^{≤}^{w}**∈****Lp**_{e};^{A} **Wn**_{e}〈Γ〉^{G}**=***w*,**Wn**〈Γ_{e}^{A}^{≤}^{}〉^{w}

It
is obvious
that *A=**A*
and *A=**A*,
hence *A=**A *and
*A=**A*.

Branching
recurrence can be shown
to be
stronger than its parallel counterpart
, in the sense
that
the principle

** Branching
rimplication** (“

**Definition
3.8.3 **

a)** ***A*_{
}*B*_{ }** **=_{def
}** ***A*_{ }_{ }*B*_{}

b)** **** ***A*_{ }** **=_{def
}*A*_{ }_{ }

**Exercise** **3.8.4**
The *Kolmogorov
complexity* *k*(*x*)
of a number *x* is the size of the
smallest Turing machine which outputs *x*
on input 0. The Kolmogorov complexity problem
*x**y*(*y*=*k*(*x*)) has no
algorithmic
solution. Nor is it reducible to the halting problem in the strong
sense of , meaning that
the
problem *x* *y*(*Halts*(*x,y*)
* Halts*(*x,y*)) *x**y*(*y*=*k*(*x*)) has no
algorithmic
solution, either. The Kolmogorov complexity problem, however, is
reducible to
the halting problem in the weaker sense of , meaning that
Machine has a winning strategy for
*x*
*y*(*Halts*(*x,y*)
* Halts*(*x,y*))
*x**y*(*y*=*k*(*x*)).
Describe
such a
strategy, informally.

Both and are conservative
generalizations of Turing
reduction. Specifically, for any predicates *p*(*x*) and *q*(*x*), the problem
*x*(*p*(*x*)
* p*(*x*))
*x*(*q*(*x*)
* q*(*x*))
is
computable iff *q*(*x*)
is Turing reducible to *p*(*x*) iff the
problem *x*(*p*(*x*)
* p*(*x*))
*x*(*q*(*x*)
* q*(*x*))
is
computable. This
means that, when restricted to traditional sorts of problems such as
decision
problems, the behaviors of and
are
indistinguishable. This
stops being the
case when these operators are applied to problems with higher degrees
of
interactivity though. For instance, the following problem is
computable, but
becomes incomputable with instead
of :

*y**x*(*Halts*(*x*,*y*)
*Halts*(*x*,*y*)) *y*(*x*(*Halts*(*x*,*y*) *Halts*(*x*,*y*))
*x*(*Halts*(*x*,*y*) *Halts*(*x*,*y*))).

Generally,
(*A**B*) (*A**B*) is valid but
(*A**B*) (*A**B*) is not.

While
both and
are
weaker than and
hence more general, is
still a more interesting
operation of
reduction than . What makes
it
special is the following belief. The latter, in turn, is based on the
belief
that (and
by no means ) is
the operation allowing
to reuse its
argument in the strongest algorithmic sense possible.

Let
A and B be computational problems
(games). We say that B is brimplicationally [resp. primplicationally,
pimplicationally, etc.] reducible to A iff *A**B* [resp. AB,
AB, etc.] has an algorithmic
solution (winning strategy).

**Thesis
3.8.5** Brimplicational
reducibility is an
adequate
mathematical counterpart of our intuition of reducibility in the
weakest (and
thus most general) algorithmic sense possible. Specifically:

(a) Whenever a problem* B* is
brimplicationally reducible to a problem *A*, *B*
is also algorithmically reducible to *A*
according to anyone’s reasonable intuition.

(b) Whenever a problem* B*
is algorithmically
reducible to a problem *A* according
to
anyone’s reasonable intuition, *B* is
also brimplicationally
reducible to *A*.

This
is pretty
much in the same sense as (by the
Church-Turing thesis), a function *f*
is computable by a Turing
machine iff *f***
** has an
algorithmic solution according to everyone’s
reasonable intuition.

3.9 Sequential operations

The
** sequential conjunction**
(“

The
original
formal definition of *A**B* and *A**B* found in [Jap08b] was a direct formalization
of the above description.
Definition 3.9.1
given below, while less direct,
still faithfully formalizes the above intuitions as long as only static
games
are considered, and we opt for it because it is technically simpler.
Specifically, Definition 3.9.1 allows either player to continue making
moves in
*A* even after a switch takes place;
such moves are meaningless but harmless. Similarly, it allows either
player to
make moves in *B* without waiting for
a
switch to take place, even though a smart player would only start
making such
moves if and when a switch happens.

**Definition
3.9.1**** ****Assume**** ***A*_{0 }= (Dm, Dn, Vr_{}_{0},* A*_{0}) and ** ***A*_{1 }= (Dm, Dn, Vr_{}_{1}, A_{1}) are games.

a)
*A*_{0}*A*_{1}** **is defined as
the
game (Dm, Dn, Vr_{}_{0}* ^{}*∪Vr

- Φ∈
**L****p**_{e}iff Φ=〈Ψ,Θ〉 or Φ=〈Ψ,1,Θ〉, where every move of 〈Ψ,Θ〉^{G}*i*∈{0,1}, 〈Ψ,Θ〉^{i}∈^{.}**L****p**_{e}^{A};^{i} - If
*switch*”) move 1, then**Wn**_{e}〈Γ〉 =^{G}**Wn**_{e}^{A}^{0}〈Γ^{0.}〉; otherwise**Wn**_{e}〈Γ〉 =^{G}**Wn**^{A}^{1}〈Γ^{1.}〉.

b)
*A*_{0}*A*_{1}** **** **is defined as
the
game (Dm, Dn, Vr_{}_{0}* ^{}*∪Vr

- Φ∈
**Lp**_{e}iff Φ=〈Ψ,Θ〉 or or Φ=〈Ψ,1,Θ〉, where every move of 〈Ψ,Θ〉^{G}*i*∈{0,1}, 〈Ψ,Θ〉^{i}∈^{.}**L****p**_{e}^{A};^{i} - If
*switch*”) move 1, then**Wn**_{e}〈Γ〉^{G}**=****Wn**_{e}^{A}^{0}〈Γ^{0.}〉; otherwise**Wn**_{e}〈Γ〉^{G}**=****Wn**^{A}^{1}〈Γ^{1.}〉.

Recall that, for a predicate *p*(*x*), *x*(*p*(*x*)
* p*(*x*)) is the problem
of
deciding *p*(*x*).
Then what is the similar-looking *x*(*p*(*x*)
*p*(*x*))? If you’ve
guessed that this is the problem of *semideciding*
*p*(*x*),
you are right. Machine has a winning strategy in this game if and only
if *p*(*x*)
is semidecidable, i.e., recursively enumerable. Namely, if *p*(*x*)
is recursively
enumerable, a winning strategy by Machine is to wait until Environment
brings
the game down to *p*(*n*)
*p*(*n*) for some
particular *n*. After
that, Machine starts
looking for a
certificate of *p*(*n*)’s
being true. If and when such a certificate is found (meaning
that *p*(*n*)
is indeed true), Machine makes a switch move turning
*p*(*n*)
*p*(*n*) into the true *p*(*n*);
and if no certificate exists
(meaning that *p*(*n*)
is false), then Machine
keeps looking for a non-existent certificate forever and thus never
makes any
moves, meaning that the game ends as *p*(*n*),
which, again, is true. And vice
versa: any effective winning strategy for *x*(*p*(*x*)
*p*(*x*)) can obviously
be
seen as a semidecision procedure for *p*(*x*),
which accepts an input *n*
iff the strategy ever makes a switch
move in the scenario where Environment’s initial choice of a value for *x* is *n*.

Existence
of effective winning strategies for games
has been shown^{[Jap03]} to be closed
under the rules ‘*from
A**B and A
conclude*
*B*’,
‘*from A* *and
B conclude A**B*’, ‘*from A
conclude **xA*’, ‘*from A
conclude **A*’. In view of
these
closures, the validity of the principles discussed below implies
certain known
facts from the theory of computation. Needless to say, those examples
once
again demonstrate how CoL can be used as a systematic tool for defining
new
interesting properties and relations between computational problems,
and not
only reproducing already known theorems but also discovering an
infinite
variety of new facts.

The
valid formula *x*(*p*(*x*)
*p*(*x*)) *x*(*p*(*x*)
*p*(*x*)) *x*(*p*(*x*)
*p*(*x*)) “expresses”
the
well known fact that, if both a predicate *p*(*x*)
and its complement *p*(*x*)
are recursively enumerable, then *p*(*x*)
is decidable. Actually, the validity of this formula means something
more: it
means that the problem of deciding *p*(*x*)
is reducible to the (-conjunction
of) the
problems of semideciding *p*(*x*) and *p*(*x*).
In fact, a reducibility in an even
stronger sense (in a sense that has no name) holds, expressed by the
formula *x*((*p*(*x*)
*p*(*x*)) ((*p*(*x*) (*x*)) *p*(*x*) *p*(*x*)).

The
formula *x**y*(*q*(*x*) ↔ *p*(*y*)) *x*(*p*(*x*) *p*(*x*))* * *x*(*q*(*x*) *q*(*x*)) is also
valid,
which implies the known fact that, if a predicate *q*(*x*) is mapping reducible
to a predicate *p*(*x*)
and *p*(*x*)
is recursively enumerable, then so is
*q*(*x*).
Again, the validity of this formula, in fact, means something even
more: it
means that the problem of semideciding *q*(*x*)
is reducible to the problems of
mapping reducing *q*(*x*)
to *p*(*x*)
and semideciding *p*(*x*).

Certain
other reducibilities hold only in the sense of
rimplications rather than implications. Here is an example. Two Turing
machines
are said to be *equivalent* iff they
accept exactly the same inputs. Let
*Neq*(*x*,*y*) be the
predicate ‘Turing machines *x* and *y*
are not equivalent’. This predicate is neither semidecidable nor
co-semidecidable. However, the problem of its semideciding primplicationally (and hence also brimplicationally) reduces to
the
halting problem. Specifically,
Machine has
an effective winning strategy for the following game:

*z**t*(*Halts*(*z*,*t*)
*Halts*(*z*,*t*)) *x**y*(*Neq*(*x*,*y*)
*Neq*(*x*,*y*)).

A
strategy here is to wait till Environment specifies
some values *m* and *n*
for *x* and *y*.
Then, create a variable i,
initialize it to 1 and do the following. Specify
*z* and *t*
as *m* and *i
*in one yet-unused copy of the
antecedent, and as *n* and *i* in another
yet-unused copy. That is,
ask Environment whether *m* halts on
input *i* and whether *n*
halts on the same input. Environment
will have to provide the correct pair of answers, or else it loses. (1)
If the
answers are “No,No”, increment *i* to
*i*+1 and repeat the step. (2) If the
answers are “Yes,Yes”, simulate both *m*
and *n* on input *i*
until they halt. If both machines accept or both reject,
increment *i* to *i*+1
and repeat the step. Otherwise, if one accepts and one rejects,
make a switch move in the consequent and celebrate victory. (3) If the
answers
are “Yes,No”, simulate *m* on *i* until
it halts. If *m*
rejects *i*, increment *i*
to *i*+1 and repeat the step.
Otherwise, if *m* accepts *i*, make a
switch move in the consequent
and you win. (4) Finally,
if the answers are “No,Yes”, simulate *n*
on *i* until it halts. If *n* rejects *i*,
increment *i* to *i*+1
and repeat the step. Otherwise, if *n*
accepts *i*, make a switch move in
the consequent and you win.

The
** sequential universal
quantification **(“

**Definition
3.9.2 **Assume ** ***A*(*x*) = (Dm, Dn, Vr, A) is a game.

a)
*xA*(*x*)**
**is defined as the game (Dm, Dn, Vr-{x}, G) such that:

- Φ∈
**Lp**_{e}iff Φ=〈Ψ^{G}_{0}, 1,_{1}, 2, Ψ_{2}, …,*n*, Ψ〉 (_{n}*n*≥0), where every move of 〈Ψ_{0},_{1}, Ψ_{2},…, Ψ〉 has the prefix ‘_{n}*c*.’ for some constant*c*and, for every constant*c*, 〈Ψ_{0},_{1}, Ψ_{2},…, Ψ〉_{n}^{c}∈^{. }**Lp**_{e}^{A}^{(c)}; - Call the
moves 1,2,,…
*switch*moves. If**Wn**_{e}〈Γ〉 =^{G}**Wn**_{e}^{A}^{(0)}〈Γ^{0.}〉; if Γ contains infinitely many switch moves, then**Wn**_{e}〈Γ〉 = ;^{G}*n*is the last switch move of Γ,**Wn**_{e}〈Γ〉 =^{G}**Wn**_{e}^{A}^{(n)}〈Γ^{n}〉.^{.}

b)
*xA*(*x*)_{ }is defined as the game (Dm, Dn, Vr-{x}, G) such that:

- Φ∈
**L****p**_{e}iff Φ=〈Ψ^{G}_{0}, 1, Ψ_{1},2, Ψ_{2}, …,*n*, Ψ〉 (_{n}*n*≥0), where every move of 〈Ψ_{0},_{1}, Ψ_{2}, …, Ψ〉 has the prefix ‘_{n}*c*.’ for some constant*c*and, for every constant*c*, 〈Ψ_{0},_{1}, Ψ_{2}, …, Ψ〉_{n}^{c}∈^{. }**Lp**_{e}^{A}^{(c) }; - Call the
moves 1,2,3,…
*switch*moves. If**Wn**_{e}〈Γ〉 =^{G}**Wn**^{A}^{(0)}〈Γ^{0.}〉; if Γ contains infinitely many switch moves, then**Wn**_{e}〈Γ〉 =;^{G}*n*is the last switch move of Γ,**Wn**_{e}〈Γ〉 =^{G}**Wn**_{e}^{A}^{(n)}〈Γ^{n}〉.^{.}

**Definition
3.9.3 **Assume** ***A* = (Dm, Dn, Vr, A)_{
}is a game.

a)
*A*** **is defined as
the game (Dm, Dn, Vr, G) such that:

- Φ∈
**Lp**_{e}iff Φ=〈^{G}**Ψ**_{}_{0}, 1,**Ψ**_{}_{1}, 2,**Ψ**_{}_{2}, …,*n*,**Ψ**_{}〉 (_{n}*n*≥0), where every move of 〈**Ψ**_{}_{0},**Ψ**_{}_{1},**Ψ**_{}_{2},…,**Ψ**〉 has the prefix ‘_{n}*c*.’ for some constant*c*and, for every constant*c*, 〈**Ψ**_{0},**Ψ**_{1},**Ψ**_{2},…,**Ψ**〉_{n}^{c}∈^{. }**L**p_{e};^{A} - Call the
moves 1,2,…
*switch*moves. If**Wn**_{e}〈Γ〉 =^{G}**Wn**_{e}〈Γ^{A}^{0.}〉; if Γ contains infinitely many switch moves, then**Wn**_{e}〈Γ〉 = ;^{G}*n*is the last switch move of Γ,**Wn**_{e}〈Γ〉 =^{G}**Wn**_{e}〈Γ^{A}^{n}〉.^{.}

b)
*A*_{ }**is defined as
the game (Dm, Dn, Vr, G) such that, for any (Vr,Dm)-valuation ***e*, we
have:

- Φ∈
**L**p_{e}^{}iff Φ=〈^{G}**Ψ**_{}_{0},**Ψ**_{}_{1}, 2,**Ψ**_{}_{2}, …,*n*,**Ψ**_{}〉 (_{n}*n*≥0), where every move of 〈**Ψ**_{}_{0},**Ψ**_{}_{1},**Ψ**_{}_{2},…,**Ψ**〉 has the prefix ‘_{n}*c*.’ for some constant*c*and, for every constant c, 〈**Ψ**_{}_{0},**Ψ**_{}_{1},**Ψ**_{}_{2},…,**Ψ**〉_{n}^{c}∈^{. }**L**p_{e}^{};^{A} - Call the
moves 1,2,…
*switch*moves. If**Wn**_{e}^{}〈Γ〉 =^{G}**Wn**_{e}^{}〈Γ^{A}^{0.}〉; if Γ contains infinitely many switch moves, then**Wn**_{e}^{}〈Γ〉 =;^{G}*n*is the last switch move of Γ,**Wn**_{e}^{}〈Γ〉 =^{G}**Wn**_{e}^{}〈Γ^{A}^{n}〉.^{.}

For illustration, remember the Kolmogorov
complexity function *k*(*x*). The
value of *k*(*x*) is known to be always smaller (in fact, logarithmically so) than x itself. While *x**y*(*y*=*k*(*x*)) is not
computable,
Machine does have an algorithmic winning strategy for the problem *x**y*(*y*=*k*(*x*)). It goes like
this:
Wait till Environment specifies a value *m*
for *x*, thus asking “what is the
Kolmogorov complexity of *m*?” and
bringing the game down to *y*(*y*=*k*(*m*)). Answer that the complexity is *m*, i.e. specify *y* as *m*. After that, start simulating, in parallel, all machines *n*
of sizes smaller than m on input 0. Whenever
you find a machine *n* that returns *m*
on input 0 and is smaller than any of
the previously found such machines, make a switch move and, in the new
copy
of *y*(*y*=*k*(*m*)), specify *y*
as the size |*n*|
of *n*. This obviously
guarantees success: sooner or later the real Kolmogorov complexity *c* of *m*
will be reached and named; and, even though the strategy will never be
sure
that *k*(*m*)
is not something yet
smaller than *c*, it will never
really
find a reason to further reconsider its latest claim that *c*=*k*(*m*).

**Exercise
3.9.4**
Describe a
winning strategy for *x**y*(*k*(*x*)=(*x*-*y*)).

As
expected, ** sequential
implication **(“

**Definition
3.9.5 **a) *A*_{
}*B*** **=_{def}** ***A*_{ }_{ }*B*

*
*b)* A*_{ }*B*** **=_{def}** ***A*_{ }_{ }*B*

c)* ** A*_{ }** **=_{def}** ***A*_{ } **
**

3.10 Toggling operations

As
all other
sorts of conjunctions and disjunctions, ** toggling
conjunction** (“

An
alternative
way to characterize *A**B* is to say
that it
is played exactly like *A**B*, with the
only
difference that Machine is allowed to make a ‘choose *A*’
or ‘choose *B*’ move any number of times. If infinitely many choices are made,
Machine
loses. Otherwise, the winner in the play will be the player who wins in
the
component that was chosen last (“the eventual choice”). The case of
Machine
having made no choices at all is treated as if it had chosen *A*. Thus, as in sequential disjunction,
the leftmost component is the “default”, or “automatically made”,
initial
choice.

It
is
important to note that the adversary (or perhaps
even the machine itself) never knows whether a given choice of a
component of *A**B* is the last
choice
or not. Otherwise, if Machine was required to indicate that it has made
its
final choice, then the resulting operation, for static games, would
essentially
be the same as *A**B*. Indeed, as
we
remember, in static games it never hurts a player to postpone making
moves, so
Environment could just inactively wait till the last choice is
declared, and
start playing the chosen component only after that, as in the case of *A**B*; under these
circumstances, making some temporary choices before making the final
choice
would not make any sense for Machine, either.

What
would
happen if we did not require that Machine
can change its mind only finitely meany times? There would be no “final
choice”
in this case. So, the only natural winning condition in the case of
infinitely
many choices would be to say that Machine wins iff it simply wins in
one of the
components. But then the resulting operation would be essentially the
same as , as a smart
machine
would always opt for keeping switching between components forever. That
is,
allowing infinitely many choices would amount to not requiring any
choices at
all, as is the case with *A**B*.

One
may also
ask what would happen if we allowed
Machine to make an arbitrary initial choice between *A*
and *B* and then
reconsider its choice only (at most) once? Such an operation on games,
albeit meaningful, would not be basic. That is because it can be
expressed
through our
primitives as (*A**B*) (*B**A*).

The
very weak
sort of choice captured by is
the kind of choice that,
in real life, one
would ordinarily call *choice after trial
and error*. Indeed, a problem is generally considered to be
solved after
trial and error (a correct choice/solution/answer found) if, after
perhaps
coming up with several wrong solutions, a true solution is eventually
found.
That is, mistakes are tolerated and forgotten as long as they are
eventually
corrected. It is however necessary that new solutions stop coming at
some
point, so that there is a last solution whose correctness determines
the success
of the effort. Otherwise, if answers have kept changing all the time,
no answer
has really been given after all. Or, imagine Bob has been married and
divorced
several times. Every time he said “I do”, he probably honestly believed
that
this time, at last, his bride was “the one”, with whom he would live
happily
ever after. Bob
will be considered to
have found his Ms. Right after all if and only if one of his marriages
indeed
turns out to be happy and final.

As
we remember, for a
predicate *p*(*x*), *x*(*p*(*x*)
*p*(*x*)) is the problem
of deciding *p*(*x*),
and *x*(*p*(*x*)
*p*(*x*)) is the weaker
(easier to
solve) problem of semideciding *p*(*x*).
Not surprisingly, *x*(*p*(*x*)*p*(*x*)) is also a
decision-style problem, but still weaker than the problem of
semideciding *p*(*x*).
This problem has been studied in the literature under several names,
the most
common of which is *recursively
approximating**p*(*x*). It means telling whether *p*(*x*)
is true or not, but doing so in the same style as semideciding does in
negative
cases: by correctly saying “Yes” or “No” at some point (after perhaps
taking
back previous answers several times) and never reconsidering this
answer
afterwards. Observe
that semideciding *p*(*x*)
can be seen as always saying “No” at the beginning and then, if this
answer is
incorrect, changing it to “Yes” at some later time; so, when the answer
is
negative, this will be expressed by saying “No” and never taking back
this
answer, yet without ever indicating that the answer is final and will
not
change. Thus, the difference between semideciding and recursively
approximating
is that, unlike a semidecision procedure, a recursive approximation
procedure
can reconsider *both* negative and
positive answers, and do
so several
times rather than only once.

It is
known
that a
predicate *p*(*x*)
is recursively approximable (i.e., the problem of its recursive
approximation has an algorithmic solution) iff *p*(*x*) has the arithmetical
complexity Δ_{2}, i.e., if
both *p*(*x*)
and its complement *p*(*x*)
can be written in the form *z**y* *s*(*z*,*y*,*x*),
where *s*(*z*,*y*,*x*)
is a decidable
predicate. Let us see that, indeed, algorithmic solvability of *x*(*p*(*x*)
*p*(*x*))
is
equivalent to *p*(*x*)’s
being of complexity Δ_{2}.

First,
assume *p*(*x*)
is of complexity Δ_{2},
specifically, for some decidable predicates *q*(*z*,*y*,*x*)
and *r*(*z*,*y*,*x*)
we have *p*(*x*)
= *z**y* *q*(*z*,*y*,*x*)
and *p*(*x*)
= *z**y* *r*(*z*,*y*,*x*).
Then *x*(*p*(*x*) * p*(*x*))
is
solved by the
following strategy. Wait till Environment specifies a value *m*
for *x*,
thus bringing the game down to *p*(*m*)*p*(*m*).
Then create variables *i* and *j*,
initialize both to 1,
choose the *p*(*m*) component, and do the
following:

*Step 1:* Check whether
*q*(*i*,*j*,*m*)
is true. If yes, increment *j* to *j*+1
and repeat Step 1. If not, switch to
the *p*(*m*)
component, reset *j* to 1, and go to
Step 2.

*Step 2: *Check whether *r*(*i*,*j*,*m*)
is true. If yes, increment *j* to *j*+1
and repeat Step 2. If not, switch to
the *p*(*m*)
component, reset *j* to
1, increment *i* to *i*+1,
and go to Step 1.

With
a little
thought, one can see that the above
algorithm indeed solves *x*(*p*(*x*)
*p*(*x*)).

For
the
opposite direction, assume a given algorithm *Alg*
solves *x*(*p*(*x*)
*p*(*x*)).
Let *q*(*z*,*y*,*x*) be the predicate such that *q*(*i*,*j*,*m*)
is true iff, in the scenario where the environment specified *x* as *m*
at the beginning of the play, so that the game was brought down to *p*(*m*)*p*(*m*),
we have: (1) at the *i*th computation
step, *Alg* chose the *p*(*m*)
component; (2) at
the *j*th computation step, *Alg* did not
move. Quite similarly, let *r*(*z*,*y*,*x*)
be the predicate such that *r*(*i*,*j*,*m*) is true iff, in the scenario where
the environment specified *x* as *m* at
the beginning of the play, so that
the game was brought down to *p*(*m*)*p*(*m*),
we have: (1) either
i=1 or, at the *i*th computation
step, *Alg* chose the *p*(*m*)
component; (2) at the *j*th
computation step, *Alg* did not move.
Of course, both *q*(*z*,*y*,*x*)
and *r*(*z*,*y*,*x*)
are decidable predicates, and hence so are *y*>*z * *q*(*z*,*y*,*x*)
and *y*>*z * *r*(*z*,*y*,*x*).
Now, it is not hard to see that *p*(*x*)
= *z**y*(*y*>*z* *q*(*z*,*y*,*x*)) and *p*(*x*)
= *z**y*
(*y*>*z* *r*(*z*,*y*,*x*)). So, *p*(*x*)
is indeed of complexity Δ_{2}.

As
a real-life
example of a predicate which is
recursively approximable but neither semidecidable nor
co-semidecidable,
consider the predicate *k*(*x*)<*k*(*y*), saying that number *x*
is simpler than number *y* in the
sense of Kolmogorov
complexity. As noted earlier,
*k*(*z*)
(the Kolmogorov
complexity of *z*) is always smaller than z.
Here is an algorithm which recursively approximates the predicate *k*(*x*)<*k*(*y*),
i.e., solves the problem *x**y*(*k*(*x*)<k(y)
*k*(*x*)< *k*(*y*)). Wait till
Environment brings the game down to *k*(*m*)<k(n)
*k*(*m*)< *k*(*n*)
for
some *m* and *n*.
Then start simulating, in parallel, all Turing machines t of sizes smaller than t≤max(*m*,*n*)
on
input 0. Whenever
you see that a machine *t* returns *m*
and the size of *t* is
smaller
than that of any other previously found machines that return *m* or *n*
on input 0, choose *k*(*m*)<
*k*(*n*).
Quite similarly,
whenever you see that a machine *t*
returns *n* and the size of *t* is
smaller than that of any other
previously found machine that returns *n*
on input 0, as well as smaller or equal to the size of any other
previously
found machines that return *m* on
input 0, choose *k*(*m*)<k(*n*). Obviously, the correct choice between *k*(*m*)<*k*(*n*)
and *k*(*m*)<*k*(*n*)
will be made sooner or later and never reconsidered afterwards. This
will
happen when the procedure hits --- in the role of *t*
--- a smallest-size machine that returns either *m*
or *n*
on input 0.

Anyway,
here
is our formal definition of the toggling
conjunction and disjunction:

**Definition
3.10.1 ****Assume**** ***A*_{0 }= (Dm, Dn, Vr_{}_{0}, A_{0}) and ** ***A*_{1 }= (Dm, Dn, Vr_{}_{1}, A_{1}) are games.

a)
*A*_{0}*A*_{1}** **is defined as
the
game (Dm, Dn, Vr_{}_{0}∪Vr_{}_{1}, G) such that:

- Φ∈
**Lp**_{e}iff Φ=〈Ψ^{G}_{0},*i*_{1}, Ψ_{1},*i*_{2}, Ψ_{2},…,*i*_{n}, Ψ〉 (_{n}*n*≥0), where*i*_{1},*i*_{2},…,*i*∈{0,1}, every move of 〈Ψ_{n }_{0}, Ψ_{1}, Ψ_{2},…, Ψ〉 has the prefix ‘0.’ or ‘1.’ and, for both_{n}*i*∈{0,1}, 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…,Ψ〉_{n}^{i}∈^{. }**L****p**_{e}^{}^{A};^{i} - Call 0 and 1
*switch moves*. If**Γ does not contain any switch moves, then****Wn**_{e}^{}〈^{G}**Γ〉 =****Wn**_{e}^{}^{A}^{0}〈Γ^{0.}〉; if**Γ has infinitely many occurrences of switch moves, then****Wn**_{e}^{}〈^{G}**Γ〉 = ;***i*is the last switch move,**Wn**_{e}^{}〈Γ〉 =^{G}**Wn**^{A}〈^{i}**Γ**^{i}〉.^{.}

b)
*A*_{0}*A*_{1}** **** **is defined as
the
game (Dm, Dn, Vr_{}_{0}∪Vr_{}_{1}, G) such that:

- Φ∈
**L****p**_{e}iff Φ=〈Ψ^{G}_{0},*i*_{1}, Ψ_{1},*i*_{2}, Ψ_{2},…,*i*_{n}, Ψ〉 (_{n}*n*≥0), where*i*_{1},*i*_{2},…,*i*∈{0,1}, every move of 〈Ψ_{n }_{0}, Ψ_{1}, Ψ_{2},…, Ψ〉 has the prefix ‘0.’ or ‘1.’ and, for both_{n}*i*∈{0,1}, 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…,Ψ_{n}〉_{}^{i}∈^{. }**L****p**_{e}^{A};^{i} - Call 0 and 1
*switch moves*. If**Γ does not contain any switch moves, then****Wn**_{e}〈^{G}**Γ〉 =****Wn**_{e}^{A}^{0}〈**Γ**^{0.}〉; if**Wn**_{e}〈^{G}**Γ〉 =;***i*is the last switch move,**Wn**_{e}〈Γ〉 =^{G}**Wn**^{A}〈^{i}**Γ**^{i}〉.^{.}

As
expected,
the ** toggling
universal quantification **(“

**Definition
3.10.2 **Assume *A*(*x*) = (Dm, Dn, Vr, A) is a game.

a)
*xA*(*x*)**
**is defined as the game (Dm, Dn, Vr-{x}, G) such that:

- Φ∈
**L****p**_{e}iff Φ=〈Ψ^{G}_{0},*c*_{1}, Ψ_{1},*c*_{2}, Ψ_{2}, …,*n*, Ψ〉 (_{n}*n*≥0), where every move of 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}〉 has the prefix ‘_{}*c*.’ for some constant*c*and, for every constant*c*, 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}_{}_{}_{}_{}_{}〉_{}^{c}∈^{. }**L****p**_{e}^{A}^{(c)}; - Call the
moves 0,1,2,…
*switch*moves. If**Wn**_{e}〈Γ〉 =^{G}**Wn**_{e}^{A}^{(0)}〈Γ^{0.}〉; if Γ has infinitely many occurrences of switch moves, then**Wn**_{e}〈Γ〉 = ;^{G}*n*is the last switch move of Γ,**Wn**_{e}〈Γ^{G}**〉 =****Wn**_{e}^{A}^{(n)}〈Γ^{n}〉.^{.}

b)
*xA*(*x*)_{ }is defined as the game (Dm, Dn, Vr-{x}, G) such that:

- Φ∈
**L****p**_{e}^{}iff Φ=〈Ψ^{G}_{0},*c*_{1}, Ψ_{1 },*c*_{2}, Ψ_{2}, …,*n*, Ψ〉 (_{n}*n*≥0), where every move of 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}_{}_{}_{}_{}_{}〉 has the prefix ‘_{}*c*.’ for some constant*c*and, for every constant*c*, 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}_{}_{}_{}_{}_{}〉_{}^{c}∈^{. }**L****p**_{e}^{}^{A}^{(c) }; - Call the
moves 0,1,2,…
*switch*moves. If**Wn**_{e}^{}〈Γ〉 =^{G}**Wn**_{e}^{}^{A}^{(0)}〈Γ^{0.}〉; if Γ has infinitely many occurrences of switch moves, then**Wn**_{e}^{}〈Γ〉 =;^{G}*n*is the last switch move of Γ,**Wn**_{e}^{}〈Γ〉 =^{G}**Wn**_{e}^{}^{A}^{(n)}〈Γ^{n}〉.^{.}

For
an example
illustrating toggling quantifiers at
work, remember that Kolmogorov
complexity *k*(*x*)
is not a computable function, i.e., the problem *x**y*(*y*=*k*(*x*)) has no
algorithmic
solution. However, replacing *y* with *y* in it yields
an
algorithmically solvable problem. A solution for *x**y*(*y*=*k*(*x*)) goes like
this.
Wait till the environment chooses a number *m*
for *x*, thus bringing the game down
to
*y*(*y*=*k*(*m*)), which is
essentially nothing but 0=*k*(*m*)
1=*k*(*m*)
2=*k*(*m*) …
Initialize *i*
to a sufficiently large number, such as *i*=*π*(*m*)
where *π* is the bound function mentioned earlier. After that perform the following routine: Switch to the disjunct *i*=*k*(*m*) of 0*=k*(*m*)
1=*k*(*m*)
2=*k*(*m*) …, and then
start
simulating on input 0, in parallel, all Turing machines whose sizes are
smaller
than *i*; if and when you see that
one
of such machines returns *m*, update *i*
to the size of that machine, and
repeat the present routine.

We
close this
sections with formal definitions of *toggling*** recurrence**
(“

**Definition
3.10.3 **Assume** ***A* = (Dm, Dn, Vr, A) is a game.

a)
*A*** **is defined as
the game (Dm, Dn, Vr, G) * ^{}*such that:

- Φ∈
**L****p**_{e}iff Φ=〈Ψ^{G}_{0},*c*_{1}, Ψ_{1},*c*_{2}, Ψ_{2}, …,*c*, Ψ_{n}〉 (_{n}*n*≥0), where every move of 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}_{}_{}_{}〉 has the prefix ‘_{}*c*.’ for some constant*c*and, for every constant*c*, 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}_{}_{}_{}〉_{}^{c}∈^{. }**L****p**_{e};^{A} - Call the
moves 0,1,2,…
*switch*moves. If**Wn**_{e}〈Γ〉^{G}**=****Wn**_{e}〈Γ^{A}^{0.}〉; if Γ has infinitely many occurrences of switch moves, then**Wn**_{e}〈Γ^{G}**〉 = ;***n*is the last switch move of Γ,**Wn**_{e}〈Γ^{G}**〉****=****Wn**_{e}〈Γ^{A}^{n}〉.^{.}

b)
*A*_{ }**is defined as
the game (Dm, Dn, Vr, G) *** ^{}*such that:

- Φ∈
**L****p**_{e}iff Φ=〈Ψ^{G}_{0},*c*_{1}, Ψ_{1},*c*_{2}, Ψ_{2}, …,*n*, Ψ〉 (_{n}*n*≥0), where every move of 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}_{}_{}_{}_{}〉 has the prefix ‘_{}*c*.’ for some constant*c*and, for every constant*c*, 〈Ψ_{0}, Ψ_{1}, Ψ_{2},…, Ψ_{n}_{}_{}_{}〉_{}^{c}∈^{. }**L****p**_{e};^{A} - Call the
moves 0,1,2,…
*switch*moves. If**Wn**_{e}〈Γ^{G}**〉****=****Wn**_{e}〈G^{A}^{0.}〉; if Γ has infinitely many occurrences of switch moves, then**Wn**_{e}〈Γ^{G}**〉****=;***n*is the last switch move of Γ,**Wn**_{e}〈Γ^{G}**〉 =****Wn**_{e}〈Γ^{A}^{n}〉.^{.}

**Definition
3.10.4 **a) *A*_{
}*B*** **=_{def}** ***A**B*

*
*b)* A**B*** **=_{def}** ***A*_{ }_{ }*B*_{ }

*
*c)* **A*** **=_{def}** ***A* **
**

** **

3.11
Cirquents

The syntactic
constructs called ** cirquents**,
briefly mentioned earlier,
take the expressive power of CoL to a qualitatively higher level,
allowing us
to form, in a systematic way, an infinite variety of game operations.
Each
cirquent is --- or can be seen as --- an independent operation on
games,
generally not expressible via composing operations taken from some
fixed finite
pool of primitives, such as the operations seen in the preceding
subsections of the present section.

For simplicity
considerations, we are not going to
introduce cirquents and their semantics in full generality and formal
detail
here. This is done in [Jap06c,
Jap08a,
Jap11b, Jap13a].
Instead, to get
some intuitive insights, let us only focus on cirquents that look like
Boolean
circuits with - and -gates. Every
such
cirquent *C* can be seen as an *n*-ary
operation on games, where *n* is the number of inputs of
*C*. For instance, the cirquent

represents the
3-ary game operation ♥ informally
defined as follows. Playing ♥(*A*,*B*,*C*), as is the
case with all parallel operations, means
playing simultaneously in all
components of it. In order to win, Machine needs to win in
at least two
out of the three components. Any attempt to express this operation in
terms of , or
other already defined
operations is going
to fail. For instance, the natural candidate (*A**B*)(*A**C*)(*B**C*) is
dramatically
inadequate. This is a game on six rather than three boards, with *A* played on boards #1 and #3, *B* on
boards #2 and #5, and *C*
on boards #4 and #6. A special case of
it is (*A**A*)(*A**A*)(*A**A*), which fails
to
indicate for instance that the 1^{st} and the 3^{rd}
occurrences of *A* stand for the same
copy of *A* while the 2^{nd}
occurrence for a different copy where a different run can be generated.

As we
just had
a
chance to see, even at the most basic (,) level,
cirquents
are properly more expressive than formulas. It is this added
expressiveness and
flexibility that, for some fragments of CoL, makes a difference between
axiomatizability and unaxiomatizability: even if one is only trying to
set up a
deductive system for proving valid formulas, intermediate steps in
proofs of
such formulas still inherently require using cirquents that cannot be
written
as formulas.^{[Jap06c,
Jap13a, Jap13b] }An
example is the system **CL15**
presented
in Section 6.4.

In the
present
piece
of writing we are exclusively focused on the formula-based version of
CoL,
seeing cirquents (in Section 6.4)
only as
technical servants to formulas. This explains why we do not attempt to
define
the semantics of cirquents formally.
It
should however be noted that cirquents are naturally called for not
only within
the specific formal framework of CoL, but also in the framework of all
resource-sensitive approaches in logic. Such approaches intrinsically
require
the ability to account for the possibility of *resource
sharing*, as sharing is a ubiquitous phenomenon in the
world of resources. Yet formula- or sequent-based approaches do not and
cannot
possess such an ability. The following naive example may provide some
insights.
It is about the earlier
discussed ♥(*A*,*A*,*A*)
combination, seen as a combination of resources in the abstract/naive
sense
rather than the specific game-semantical sense of CoL.

Victor has
three 25c-coins, and he knows that two of
them are true while one is perhaps false (but he has no way to tell
which one
is false). Could he get a candy? The answer depends on
the
number of slots that the machine has. Consider two cases: machine *M2* with two slots, and machine *M3* with
three slots. Victor would have
no problem with *M3*: he can insert
his
three coins into the three slots, and the machine, having received ≥50c, will
dispense a candy. With *M2*, however,
Victor is powerless. He can try inserting arbitrary
two of his three coins into the two slots of the machine, but there is
no
guarantee that one of those two coins is not false, in which case
Victor will
end up with no candy and only 25 cents remaining in his pocket.

Both
*M2* and *M3*
can be understood as resources ---
resources turning coins into a candy. And note that these two resources
are not
the same: *M3* is obviously stronger
(“better”), as it allows Victor to get a candy whereas *M2*
does not, while, at the same time, anyone rich enough to be able
to make *M2* dispense a candy would
be
able to do the same with *M3* as
well. Yet, formulas
fail to capture this
important difference. With , , here
understood as abstract
multiplicative-style operations on resources, *M2*
and *M3* can be written
as *R2**Candy * and
*R3**Candy*,
respectively: they
consume a certain resource *R2* or *R3*
and produce *Candy*.
What makes *M3*
stronger than *M2* is that the
subresource *R3* that it consumes is
weaker (easier to supply) than the subresource *R2*
consumed by *M2*.
Specifically, with one false and two true coins, Victor is able to
satisfy *R3* but not *R2*.

The
resource *R2*
can be represented as the cirquent

which,
due to
being tree-like, can also be adequately
written as the formula 25c 25c.
As for the resource *R3*, either one of the following
two
cirquents is an adequate
representation of it, with one of them probably showing the relevant
part of the
actual physical circuitry used in *M3*:

**Figure
3.11.1**:
Cirquents
for the
“two out of three” combination of resources

Unlike
*R2*,
however, *R3* cannot be represented
through a formula. 25c25c does not
fit the
bill, for it represents *R2* which,
as
we already agreed, is not the same as *R3*.
In another attempt to find a formula, we might try to rewrite one of
the above
two cirquents --- let it be the one on the right --- into an
“equivalent”
formula in the standard way, by duplicating and separating shared
nodes, as we
did earlier when
tackling ♥(*A*,*A*,*A*).
This results in (25c25c)(25c25c)(25c25c), which,
however, is not any more adequate than 25c25c. It
expresses
not *R3* but the resource consumed by
a
machine with six coin slots grouped into three pairs, where (at least)
one slot
in each of the three pairs needs to receive a true coin. Such a machine
thus
dispenses a candy for ≥75 rather than
≥50 cents,
which makes Victor's resources insufficient.

The
trouble
here is related to the inability of
formulas to explicitly account for resource sharing or the absence
thereof. The
cirquent on the right of the above figure stands for a
(multiplicative-style)
conjunction of three resources, each conjunct, in turn, being a
disjunction of two
subresources of type 25c. However, altogether there are three rather
than six
25c-type subresources, each one being shared between two different
conjuncts of
the main resource.