Recursively defined functions
Recursive definition - definition of an object (such as function, set,
etc.) in terms of itself.
To recursively (inductively) define a function with the set of
nonnegative integers as its domain,
1. Specify the value of the function at zero.
2. Give a rule for finding its value at an integer from its values at smaller
Then, by the second principle of mathematical induction, this function
will be defined for all nonnegative integer.