# Definition:Pointwise Operation/Real-Valued Functions

## Definition

Let $\R^S$ be the set of all mappings $f: S \to \R$, where $\R$ is the set of real numbers.

Let $\oplus$ be a binary operation on $\R$.

Define $\oplus: \R^S \times \R^S \to \R^S$, called **pointwise $\oplus$**, by:

- $\forall f, g \in \R^S: \forall s \in S: \map {\paren {f \oplus g} } s := \map f s \oplus \map g s$

In the above expression, the operator on the right hand side is the given $\oplus$ on the real numbers.

### Pointwise Addition

Let $f, g: S \to \R$ be real-valued functions.

Then the **pointwise sum of $f$ and $g$** is defined as:

- $f + g: S \to \R:$
- $\forall s \in S: \map {\paren {f + g} } s := \map f s + \map g s$

where the $+$ on the right hand side is real-number addition.

### Pointwise Multiplication

Let $f, g: S \to \R$ be real-valued functions.

Then the **pointwise product of $f$ and $g$** is defined as:

- $f \times g: S \to \R:$
- $\forall s \in S: \map {\paren {f \times g} } s := \map f s \times \map g s$

where $\times$ on the right hand side denotes real multiplication.

- $\lambda \cdot f: S \to \R, \map {\paren {\lambda \cdot f} } s := \lambda \cdot \map f s$

as is done on Pointwise Scalar Multiplication

### Multiary Operations

For ease of notation, write $\left[{S \to \R}\right]$ for $\R^S$.

Let $I$ be some index set.

Let $\oplus^I: \R^I \to \R$ be an $I$-ary operation on $\R$.

Then $\oplus^I: \left[{S \to \R}\right]^I \to \left[{S \to \R}\right]$, referred to as **pointwise $\oplus^I$**, is defined as:

- $\forall \left({f_i}\right)_{i \mathop \in I} \in \left[{S \to \R}\right]^I: \forall s \in S: \left({\oplus^I \left({f_i}\right)_{i \in I} }\right) \left({s}\right) := \oplus^I \left({f_i \left({s}\right) }\right)_{i \in I}$

## Also known as

When $\oplus$ or $\oplus^I$ has a distinguished name, it is usual to name the corresponding **pointwise operation** by prepending **pointwise** to that name.

## Also defined as

Sometimes an operation cannot be consistently defined on all of $\R^I$. Often one then still speaks about a **pointwise operation** by suitably restricting above definition, adapting it wherever necessary.

Examples of such suitably restricted pointwise operations are listed under Partial Examples below.

## Examples

- Definition:Pointwise Addition of Real-Valued Functions
- Definition:Pointwise Multiplication of Real-Valued Functions
- Definition:Pointwise Scalar Multiplication of Real-Valued Functions
- Definition:Pointwise Maximum of Real-Valued Functions
- Definition:Pointwise Minimum of Real-Valued Functions
- Definition:Absolute Value of Real-Valued Function

## Partial Examples

## Also see

- Definition:Pointwise Operation on Number-Valued Functions: a more general concept

## Sources

- 1970: B. Hartley and T.O. Hawkes:
*Rings, Modules and Linear Algebra*... (previous) ... (next): Chapter $1$: Rings - Definitions and Examples: $2$: Some examples of rings: Ring Example $8$ - 1977: K.G. Binmore:
*Mathematical Analysis: A Straightforward Approach*... (previous) ... (next): $\S 7.9$