# Definition:Composition of Mappings/Also known as

## Definition

Let $f_1: S_1 \to S_2$ and $f_2: S_2 \to S_3$ be mappings such that the domain of $f_2$ is the same set as the codomain of $f_1$.

Let $f_2 \circ f_1$ denote the **composition** of $f_1$ with $f_2$.

In the context of analysis, this is often found referred to as a **function of a function**, which (according to some sources) makes set theorists wince, as it is technically defined as a **function on the codomain of a function**.

Some sources call $f_2 \circ f_1$ the **resultant of $f_1$ and $f_2$** or the **product of $f_1$ and $f_2$**.

Some authors write $f_2 \circ f_1$ as $f_2 f_1$.

Some use the notation $f_2 \cdot f_1$ or $f_2 . f_1$.

Some use the notation $f_2 \bigcirc f_1$.

Others, particularly in books having ties with computer science, write $f_1; f_2$ or $f_1 f_2$ (note the reversal of order), which is read as **(apply) $f_1$, then $f_2$**.

## Sources

- 1951: Nathan Jacobson:
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*Abstract Algebra*... (previous) ... (next): Exercise $1.3: \ 4$ - 1967: John D. Dixon:
*Problems in Group Theory*... (previous) ... (next): Introduction: Notation - 1968: Ian D. Macdonald:
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*Set Theory and Abstract Algebra*... (previous) ... (next): $\S 5$. Induced mappings; composition; injections; surjections; bijections:*Remark $4$* - 1977: K.G. Binmore:
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- 2011: Robert G. Bartle and Donald R. Sherbert:
*Introduction to Real Analysis*(4th ed.) ... (previous) ... (next): $\S 1.1$: Sets and Functions