# Right Module induces Left Module over same Ring iff Actions are Commutative

## Theorem

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

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

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

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

Then $\struct {G, +_G, \circ'}$ is a left module over $\struct {R, +_R, \times_R}$ if and only if:

$\forall \lambda, \mu \in R: \forall x \in G: x \circ \paren{ \lambda \times_R \mu} = x \circ \paren {\mu \times_R \lambda}$

## Proof

### Necessary Condition

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

Then:

 $\ds x \circ \paren {\lambda \times_R \mu}$ $=$ $\ds \paren {\lambda \times_R \mu} \circ' x$ Definition of $\circ'$ $\ds$ $=$ $\ds \lambda \circ' \paren {\mu \circ' x}$ Left Module Axiom $\text M 3$: Associativity $\ds$ $=$ $\ds \paren {x \circ \mu} \circ \lambda$ Definition of $\circ'$ $\ds$ $=$ $\ds x \circ \paren {\mu \times_R \lambda}$ Right Module Axiom $\text {RM} 3$: Associativity

$\Box$

### Sufficient Condition

Let the scalar multiplication $\circ$ satisfy:

$\forall \lambda, \mu \in R: \forall x \in G: x \circ \paren {\lambda \times_R \mu} = x \circ \paren {\mu \times_R \lambda}$

It needs to be shown that $\struct {G, +_G, \circ'}$ satisfies the left module axioms.

#### Left Module Axiom $\text M 1$: (Left) Distributivity over Module Addition

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

Then:

 $\ds \lambda \circ' \paren {x +_G y}$ $=$ $\ds \paren {x +_G y} \circ \lambda$ Definition of $\circ'$ $\ds$ $=$ $\ds x \circ \lambda +_G y \circ \lambda$ Right Module Axiom $\text {RM} 1$: (Right) Distributivity over Module Addition on $\circ$ $\ds$ $=$ $\ds \lambda \circ' x +_G \lambda \circ' y$ Definition of $\circ'$

$\Box$

#### Left Module Axiom $\text M 2$: (Right) Distributivity over Scalar Addition

Let $\lambda \in R, x, y \in G$.

Then:

 $\ds \paren{\lambda +_R \mu} \circ' x$ $=$ $\ds x \circ \paren{\lambda +_R \mu}$ Definition of $\circ'$ $\ds$ $=$ $\ds x \circ \lambda +_G x \circ \mu$ Right Module Axiom $\text {RM} 2$: (Left) Distributivity over Scalar Addition on $\circ$ $\ds$ $=$ $\ds \lambda \circ' x +_G \mu \circ' x$ Definition of $\circ'$

$\Box$

#### Left Module Axiom $\text M 3$: Associativity

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

Then:

 $\ds \paren {\lambda \times_R \mu} \circ' x$ $=$ $\ds x \circ \paren {\lambda \times_R \mu}$ Definition of $\circ'$ $\ds$ $=$ $\ds x \circ \paren {\mu \times_R \lambda}$ Assumption $\ds$ $=$ $\ds \paren {x \circ \mu} \circ \lambda$ Right Module Axiom $\text {RM} 3$: Associativity on $\circ$ $\ds$ $=$ $\ds \lambda \circ' \paren {\mu \circ' x}$ Definition of $\circ'$

$\blacksquare$