Most of the functions we have dealt with in previous chapters have been defined explicitly: by a formula in terms of the variable. We can also define functions recursively: in terms of the same function of a smaller variable. In this way, a recursive function "builds" on itself.

A recursive definition has two parts:

1. Definition of the smallest argument (usually f (0) or f (1)).
2. Definition of f (n), given f (n - 1), f (n - 2), etc.

Here is an example of a recursively defined function:


We can calculate the values of this function:

This recursively defined function is equivalent to the explicitly defined function f (n) = 2n + 5. However, the recursive function is defined only for nonnegative integers.

Here is another example of a recursively defined function:


The values of this function are:

This recursively defined function is equivalent to the explicitly defined function f (n) = n². Again, the recursive function is defined only for nonnegative integers.

Here is one more example of a recursively defined function:

The values of this function are:

This is the recursive definition of the factorial function, F(n) = n!.

Not all recursively defined functions have an explicit definition.

One special recursively defined function, which has no simple explicit definition, yields the Fibonacci numbers:


The values of this function are:

Thus, the sequence of Fibonacci numbers is 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, …. These numbers have many interesting properties that will be studied in higher math. They recur often in mathematics and even in nature.