# Left Module over Commutative Ring induces Bimodule

## Theorem

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

Let $\struct{G, +_G, \circ}$ be a left module over $\struct {R, +_R, \times_R}$.

Let $\circ’ : G \times R \to G$ be the binary operation defined by:

$\forall \lambda \in R: \forall x \in G: x \circ’ \lambda = \lambda \circ x$

Then $\struct{G, +_G, \circ, \circ’}$ is a bimodule over $\struct {R, +_R, \times_R}$.

## Proof

From Left Module over Commutative Ring induces Right Module, $\struct{G, +_G, \circ’}$ is a right module.

Let $\lambda, \mu \in R$ and $x \in G$.

Then:

 $\displaystyle \lambda \circ \paren{x \circ’ \mu}$ $=$ $\displaystyle \lambda \circ \paren{\mu \circ x}$ Definition of $\circ’$ $\displaystyle$ $=$ $\displaystyle \paren {\lambda \circ \mu} \circ x$ Left module axiom $(M \,3)$ : Associativity of Scalar Multiplication $\displaystyle$ $=$ $\displaystyle \paren {\mu \circ \lambda} \circ x$ Ring product $\circ$ is commutative $\displaystyle$ $=$ $\displaystyle \mu \circ \paren{\lambda \circ x}$ Left module axiom $(M \,3)$ : Associativity of Scalar Multiplication $\displaystyle$ $=$ $\displaystyle \paren{\lambda \circ x} \circ’ \mu$ Definition of $\circ’$

Hence $\struct{G, +_G, \circ, \circ’}$ is a bimodule over $\struct {R, +_R, \times_R}$ by definition.

$\blacksquare$