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Recursion is a way of specifying a process by means of itself. More precisely (and to dispel the appearance of circularity in the definition), "complicated" instances of the process are defined in terms of "simpler" instances, and the "simplest" instances are given explicitly.

Examples of mathematical objects often defined recursively are functions and sets.

The canonical example of a recursively defined function is the following definition of the factorial function:

0! = 1

n! = n * (n-1)!   for any natural number n>0

Therefore, given this definition, we work out 3! as follows:

3! = 3 * (3-1)!
   = 3 * 2!
   = 3 * 2 * (2-1)!
   = 3 * 2 * 1! 
   = 3 * 2 * 1 * (1 - 1)!
   = 3 * 2 * 1 * 1
   = 6

In set theory there is a theorem guaranteeing that such functions exist.

The recursion theorem. Given a set X, an element a of X and a function f:X->X, then there is a unique function F:N->X such that

F(0)=a, and

F(n+1)=f(F(n)).

The canonical example of a recursively defined set is the natural numbers:

0 is in N

if n is in N, then n+1 is in N

The natural numbers can be defined as the smallest set satisfying the definition.

Another interesting example is the set of all true propositions in an axiomatic system.

if a proposition is an axiom, it is true.

if a proposition can be obtained from true propositions by means of inference rules, it is true.

Here is another, perhaps simpler way to understand recursive processes:

1. Are we done yet? If so, return the results.
2. If not, simplify the problem and send it to 1.

Any function that can be evaluated by a computer can be expressed in terms of recursive functions, without use of iteration. Indeed, some languages designed for functional programming provide recursion as the only means of repetition directly available to the programmer. Such languages generally make tail recursion as efficient as iteration, letting programmers express other repetition structures (such as Scheme's map and for) in terms of recursion.

Recursion is deeply embedded in the theory of computation, with the theoretical equivalence of recursive functions and Turing machines at the foundation of ideas about the universality of the modern computer.

See also:

• Recursion
• Self-reference
• Primitive recursive function