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Function Composition in Mathematics: Definition, Examples, and Applications

Function composition is an important concept in mathematics that allows us to use the output of one function as the input of another. This idea is widely used in algebra, calculus, modeling, and engineering. In this article, we explain the definition of function composition, how to compute it, how it differs from other operations, and provide practical examples using a clean HTML structure.

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~2 min read • Updated Feb 9, 2026

1. What Is Function Composition?


Function composition is the process in which the output of one function becomes the input of another. If we have two functions f and g, their composition is defined as:


(f ∘ g)(x) = f(g(x))

This means we first apply the function g to x, and then apply f to the result.


2. Difference Between Composition and Other Operations


  • Function addition: outputs are added
  • Function multiplication: outputs are multiplied
  • Function composition: the output of one function becomes the input of another

Therefore, composition is a sequential operation, and the order matters.


3. Order Matters in Function Composition


In composition, the order of functions is extremely important. Usually:


(f ∘ g)(x) ≠ (g ∘ f)(x)

This means composition is not commutative.


Example:

f(x) = x + 2
g(x) = 3x

Compute the compositions:


(f ∘ g)(x) = f(g(x)) = f(3x) = 3x + 2
(g ∘ f)(x) = g(f(x)) = g(x + 2) = 3(x + 2) = 3x + 6

Result: the two compositions are different.


4. Domain in Function Composition


For the composition to be defined, the output of g must lie within the domain of f. Therefore, the domain of the composite function is often more restricted than the domain of each function individually.


Domain Example:

f(x) = √x
g(x) = x - 1

To define (f ∘ g)(x), we must have:


g(x) = x - 1 ≥ 0

Thus, the domain of the composition is x ≥ 1.


5. Complete Example of Function Composition


Consider the following functions:

f(x) = x^2
g(x) = x + 3

Compute (f ∘ g)(x):

(f ∘ g)(x) = f(g(x)) = f(x + 3) = (x + 3)^2

Compute (g ∘ f)(x):

(g ∘ f)(x) = g(f(x)) = g(x^2) = x^2 + 3

Result: the compositions are different.


6. Applications of Function Composition


  • Modeling multi‑step systems
  • Building complex functions from simpler ones
  • Chain rule in calculus
  • Applications in computer science and signal processing

7. Conclusion


Function composition is a powerful tool for constructing new functions and analyzing multi‑stage relationships. In this process, the output of one function becomes the input of another, and the order of application is crucial. Understanding composition is essential for advanced topics such as the chain rule, inverse functions, and mathematical modeling.


Written & researched by Dr. Shahin Siami

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