Function composition facts for kids

Kids Encyclopedia Facts

In mathematics, function composition is a way of making a new function from two other functions through a chain-like process.

More specifically, given a function f from X to Y and a function g from Y to Z, then the function "g composed with f", written as g ∘ f, is a function from X to Z (notice how it is usually written in the opposite way to how people would expect it to be).

The value of f given the input x is written as f(x). The value of g ∘ f given the input x is written as (g ∘ f)(x), and is defined as g(f(x)).

As an example. let f be a function which doubles a number (multiplies it by 2), and let g be a function which subtracts 1 from a number. These two functions can be written as:

f(x) = 2x

g(x) = x - 1

Here, g composed with f would be the function which doubles a number, and then subtracts 1 from it. That is:

(g \circ f)(x) = 2x - 1

On the other hand, f composed with g would be the function which subtracts 1 from a number, and then doubles it:

(f \circ g)(x) = 2(x-1)

Composition of functions can also be generalized to binary relations, where it is sometimes represented using the same $\circ$ symbol (as in $R \circ S$ ).