# Recursive definition

In mathematics and computer science, a **recursive definition**, or **inductive definition**, is used to define the elements in a set in terms of other elements in the set (Aczel 1977:740ff). Some examples of recursively-definable objects include factorials, natural numbers, Fibonacci numbers, and the Cantor ternary set.^{[1]}

A recursive definition of a function defines values of the function for some inputs in terms of the values of the same function for other (usually smaller) inputs. For example, the factorial function *n*! is defined by the rules

This definition is valid for each natural number *n*, because the recursion eventually reaches the **base case** of 0. The definition may also be thought of as giving a procedure for computing the value of the function *n*!, starting from *n* = 0 and proceeding onwards with *n* = 1, *n* = 2, *n* = 3 etc.

The recursion theorem states that such a definition indeed defines a function that is unique. The proof uses mathematical induction.^{[2]}

An inductive definition of a set describes the elements in a set in terms of other elements in the set. For example, one definition of the set **N** of natural numbers is:

Properties of recursively defined functions and sets can often be proved by an induction principle that follows the recursive definition. For example, the definition of the natural numbers presented here directly implies the principle of mathematical induction for natural numbers: if a property holds of the natural number 0 (or 1), and the property holds of *n*+1 whenever it holds of *n*, then the property holds of all natural numbers (Aczel 1977:742).

Most recursive definitions have two foundations: a base case (basis) and an inductive clause.

The difference between a circular definition and a recursive definition is that a recursive definition must always have *base cases*, cases that satisfy the definition *without* being defined in terms of the definition itself, and that all other instances in the inductive clauses must be "smaller" in some sense (i.e., *closer* to those base cases that terminate the recursion) — a rule also known as "recur only with a simpler case".^{[3]}

In contrast, a circular definition may have no base case, and even may define the value of a function in terms of that value itself — rather than on other values of the function. Such a situation would lead to an infinite regress.

That recursive definitions are valid – meaning that a recursive definition identifies a unique function – is a theorem of set theory known as the recursion theorem, the proof of which is non-trivial.^{[4]} Where the domain of the function is the natural numbers, sufficient conditions for the definition to be valid are that the value of *f*(0) (i.e., base case) is given, and that for *n* > 0, an algorithm is given for determining *f*(*n*) in terms of *n*, *f*(0), *f*(1), …, *f*(*n* − 1) (i.e., inductive clause).

More generally, recursive definitions of functions can be made whenever the domain is a well-ordered set, using the principle of transfinite recursion. The formal criteria for what constitutes a valid recursive definition are more complex for the general case. An outline of the general proof and the criteria can be found in James Munkres' *Topology*. However, a specific case (domain is restricted to the positive integers instead of any well-ordered set) of the general recursive definition will be given below.^{[5]}

The set of prime numbers can be defined as the unique set of positive integers satisfying

The primality of the integer 1 is the base case; checking the primality of any larger integer *X* by this definition requires knowing the primality of every integer between 1 and *X*, which is well defined by this definition. That last point can be proved by induction on *X*, for which it is essential that the second clause says "if and only if"; if it had said just "if" the primality of for instance 4 would not be clear, and the further application of the second clause would be impossible.

It is chiefly in logic or computer programming that recursive definitions are found. For example, a well-formed formula (wff) can be defined as:

The value of such a recursive definition is that it can be used to determine whether any particular string of symbols is "well formed".