# Definition:Pointwise Multiplication

## Contents

## Definition

Let $\mathbb F$ be one of the standard number sets: $\Z, \Q, \R$ or $\C$.

Let $\mathbb F^S$ be the set of all mappings $f: S \to \mathbb F$.

The (binary) operation of **pointwise multiplication** is defined on $\mathbb F^S$ as:

- $\times: \mathbb F^S \times \mathbb F^S \to \mathbb F^S: \forall f, g \in \mathbb F^S:$
- $\forall s \in S: \map {\paren {f \times g} } s := \map f s \times \map g s$

where the $\times$ on the right hand side is conventional arithmetic multiplication.

## Also denoted as

Using the other common notational forms for multiplication, this definition can also be written:

- $\forall s \in S: \map {\paren {f \cdot g} } s := \map f s \cdot \map g s$

or:

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

## Specific Number Sets

Specific instantiations of this concept to particular number sets are as follows:

### Integer-Valued Functions

Let $f, g: S \to \Z$ be integer-valued functions.

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

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

where the $\times$ on the right hand side is integer multiplication.

### Rational-Valued Functions

Let $f, g: S \to \Q$ be rational-valued functions.

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

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

where the $\times$ on the right hand side is rational multiplication.

### Real-Valued Functions

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.

### Complex-Valued Functions

Let $f, g: S \to \Z$ be complex-valued functions.

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

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

where the $\times$ on the right hand side is complex multiplication.

## Specific Instances

### Pointwise Scalar Multiplication

When one of the functions is the constant mapping $f_\lambda: S \to \mathbb F: \map {f_\lambda} s = \lambda$, the following definition arises:

The (binary) operation of **pointwise scalar multiplication** is defined on $\mathbb F \times \mathbb F^S$ as:

- $\times: \mathbb F \times \mathbb F^S \to \mathbb F^S: \forall \lambda \in \mathbb F, f \in \mathbb F^S:$
- $\forall s \in S: \map {\paren {\lambda \times f} } s := \lambda \times \map f s$

where the $\times$ on the right hand side is conventional arithmetic multiplication.

## Pointwise Multiplication on Ring of Mappings

Let $\struct {R, +, \circ}$ be a ring.

Let $S$ be a set.

Let $\struct {R^S, +', \circ'}$ be the ring of mappings from $S$ to $R$.

The pointwise operation $\circ'$ induced by $\circ$ on the ring of mappings from $S$ to $R$ is called **pointwise multiplication** and is defined as:

- $\forall f, g \in R^S: f \circâ€™ g \in R^S :$
- $\forall s \in S : \map {\paren {f \circâ€™ g}} x = \map f x \circ \map g x$

## Pointwise Multiplication on Ring of Sequences

Let $\struct {R, +, \circ}$ be a ring.

Let $\struct {R^\N, +', \circ'}$ be the ring of sequences over $R$.

The pointwise operation $\circ'$ induced by $\circ$ on the ring of sequences is called **pointwise multiplication** and is defined as:

- $\forall \sequence {x_n}, \sequence {y_n} \in R^{\N}: \sequence {x_n} \circ' \sequence {y_n} = \sequence {x_n \circ y_n}$

## Also see

- Definition:Pointwise Operation on Number-Valued Functions: a more general concept of which these definitions can be seen to be instantiations.