# Definition:Algebra over Field

## Definition

Let $F$ be a field.

An **algebra over $F$** is an ordered pair $\left({A, *}\right)$ where:

- $A$ is a vector space over $F$
- $* : A^2 \to A$ is a bilinear mapping

That is, it is an algebra $\left({A, *}\right)$ over the ring $F$ where:

- $F$ is a field
- the $F$-module $A$ is a vector space.

## Multiplication

The bilinear mapping $*$ is often called **multiplication**.

The symbol $A$ is often used for such an algebra, more so as the level of abstraction increases.

## Also defined as

Some sources insist that an **algebra over a field** requires that the bilinear mapping $*$ must have an identity element $1_A$ such that:

- $\forall a \in A: a * 1_A = 1_A * a = a$

... that is, that $\left({A, *}\right)$ has to be a unitary algebra.

It is worth being certain of what is meant in any works read.

Especially in commutative algebra, an **algebra** over a field is often defined as a **unital associative commutative algebra**.

## Also known as

Some sources refer to an **algebra over a field** as a **linear algebra**.

Others call it a **hypercomplex number system**.

## Also see

## Sources

- 1992: George F. Simmons:
*Calculus Gems*... (previous) ... (next): Chapter $\text {B}.26$: Extensions of the Complex Number System. Algebras, Quaternions, and Lagrange's Four Squares Theorem