Function composition

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In mathematics, function composition is a way to make a new function from two other functions.

If we let f be a function from X to Y and g be a function from Y to Z then we say that g composed with f is written as g ∘ f a function from X to Z (notice how it is usually written in the opposite way to how people would it expect it to be as we will explain below).

The value of f given the input x is written as f(x). The value of g ∘ f given the input x is written (g ∘ f)(x) and is defined as g(f(x)) (which means our way of writing g composed with f makes sense).

Here is another 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 would be written as:

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

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

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

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)

Properties[change | change source]

Function composition can be proven to be associative,[1] which means:

f \circ (g \circ h) = (f \circ g) \circ h

Function composition is in general not commutative however,[2] which means:

f \circ g \neq g \circ f

This can be seen in the first example where (g ∘ f)(2) = 2*2 - 1 = 3 and (f ∘ g)(2) = 2*(2-1) = 2.

References[change | change source]