Recursive Functions

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A definition that defines an object in terms of itself is said to be recursive. In computer science, recursion refers to a function or subroutine that calls itself, and it is a fundamental paradigm in programming. A recursive program is used for solving problems that can be broken down into sub-problems of the same type, doing so until the problem is easy enough to solve directly.


Fibonacci Numbers

A common recursive function that you’ve probably encountered is the Fibonacci numbers: 0, 1, 1, 2, 3, 5, 8, 13, and so on. That is, you get the next Fibonacci number by adding together the previous two. Mathematically, this is written as


Try finding f(10). No doubt, you have the correct answer, because you intuitively stopped when you reach $f(1)$ and $f(0)$. To be formal about this, we need to define when the recursion stops, called the base cases. The base cases for the Fibonacci function is $f(0)=1$, and $f(1)=1$. The typical way to write this function is as follows: $$f(N)=\cases{1 & if $N=0$\cr 1 & if $N=1$\cr f(N-1)+f(N-2) & if $N > 1$}$$

Here is a Python implementation of the Fibonacci function:

def Fibonacci(x):
  if (x == 0) return 0
  if (x == 1) return 1
  return Fibonacci(x-1) + Fibonacci(x-2)

(As a challenge to the reader: How could you implement the Fibonacci function without using recursion?)

Factorial Function

Consider the factorial function, $n! = N * (N-1) * ... * 1$, with 0! defined as having a value of 1. We can define this recursively as follows:

$$f(x)=\cases {1 & if $x=0$\cr x*f(x-1) & if $x\gt 0$\cr }$$

WIth this definition, the factorial of a negative number is not defined.

Here is a Python implementation of the factorial function:

def Factorial(x):
  if (x<=0) return 1
  return x*Factorial(x-1)

In the implementation above, the base case is listed as x<=0 rather than x<0 to return a value 1 when called with a negative number. Had the implementation been coded with <, then calling it with a negative number would lead the function never returning. This is called infinite recursion.

Some Definitions

A few definitions: Indirection recursion is when a function calls another function which eventually calls the original function. For example, A calls B, and then, before function B exits, function A is called (either by B or by a function that B calls). Single recursion is recursion with a single reference to itself, such as the factorial example above. Multiple recursion, illustrated by the Fibonacci number function, is when a function has multiple self references.

Many beginning programmers have a tough time writing recursive programs. So if this is confusing to you, not to worry!

This ACSL category focuses on mathematical recursive functions rather than programming procedures; but you’ll see some of the latter.

Sample Problems

Online Resources


The following videos show the solution to problems that have appeared in previous ACSL contests.

Recursion Example 1 (CalculusNguyenify)

The video walks through the solution to a straight-forward single-variable recursive function, that is, $f(x)=\cases{....}$ The problem appeared in ACSL Senior Division Contest #1, 2014-2015.

Recursion Example 2 (CalculusNguyenify)

The video walks through the solution to a 2-variable recursive function, that is, $f(x,y)=\cases{....}$ . The problem appeared in ACSL Senior Division Contest #1, 2014-2015.

Recursive Functions ACSL Example Problem (Tangerine Code)

The video walks through the solution to a 2-variable recursive function, that is, $f(x,y)=\cases{....}$ .

Other Videos

The follow YouTube videos cover various aspects of this topic; they were created by authors who are not involved (or aware) of ACSL, to the best of our knowledge. Some of the videos contain ads; ACSL is not responsible for the ads and does not receive compensation in any form for those ads.