# Definition:Left Module

## Definition

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

Let $\struct {G, +_G}$ be an abelian group.

A left module over $R$ is an $R$-algebraic structure $\struct {G, +_G, \circ}_R$ with one operation $\circ$, the (left) ring action, which satisfies the left module axioms:

 $(\text M 1)$ $:$ Scalar Multiplication (Left) Distributes over Module Addition $\ds \forall \lambda \in R: \forall x, y \in G:$ $\ds \lambda \circ \paren {x +_G y}$ $\ds =$ $\ds \paren {\lambda \circ x} +_G \paren {\lambda \circ y}$ $(\text M 2)$ $:$ Scalar Multiplication (Right) Distributes over Scalar Addition $\ds \forall \lambda, \mu \in R: \forall x \in G:$ $\ds \paren {\lambda +_R \mu} \circ x$ $\ds =$ $\ds \paren {\lambda \circ x} +_G \paren {\mu \circ x}$ $(\text M 3)$ $:$ Associativity of Scalar Multiplication $\ds \forall \lambda, \mu \in R: \forall x \in G:$ $\ds \paren {\lambda \times_R \mu} \circ x$ $\ds =$ $\ds \lambda \circ \paren {\mu \circ x}$

## Left vs Right Modules

In the case of a commutative ring, the difference between a left module and a right module is little more than a notational difference.

See:

But this is not the case for a ring that is not commutative.

From:

it is known that it is not sufficient to simply reverse the scalar multiplication to get a module of the other ‘side’.

From:

to obtain a module of the other ‘side’ it is, in general, also necessary to reverse the product of the ring.

From:

a left module induces a right module and vice versa if and only if actions are commutative.

Given the remarks above, a theorem about modules, where the type of module is unspecified, is in effect two theorems:

one about left modules where all modules are left modules
the other about right modules where all modules are right modules.

The theorem may not be true for a mix of left and right modules, unless the modules are over a commutative ring.

The proof of such a theorem is generally given for one type of module only.

The proof for the other type is proved similarly with the scalar applied on the other side.

For example, see Direct Product of Modules is Module.

Where a theorem does involve a mix of left and right modules it is necessary to explicitly identify which modules are the left modules and which are the right modules.

For example, see Left Module over Ring Induces Right Module over Opposite Ring.

## Also see

• Results about left modules can be found here.