Combinations
4.3
An r-combination of elements of a set is an unordered selection of r
elements from the set.
In other words, this is an r-element subset of the set.
2-combinations of {a,b,c}:
{a,b} {a,c} {b,c}
The number of all r-combinations of a set of n distinct elements is
denoted by
C(n,r)
or (also) by
n
r
Thus, C(3,2)=3.
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