Function composition

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:

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

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

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

Properties

Function composition can be proven to be associative, which means that:

However, function composition is in general not commutative, which means that:

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

 Related pages 

 Chain rule

References

Category:Mathematics