# bilinearity and commutative rings

We show that a bilinear map $b:U\times V\to W$ is almost always definable only for commutative rings. The exceptions lie only where non-trivial commutators act trivially on one of the three modules.

###### Lemma 1.

Let $R$ be a ring and $U,V$ and $W$ be $R$-modules. If $b:U\times V\to W$ is $R$-bilinear then $b$ is also $R$-middle linear.

###### Proof.

Given $r\in R$, $u\in U$ and $v\in V$ then $b(ru,v)=rb(u,v)$ and $b(u,rv)=rb(u,v)$ so $b(ru,v)=b(u,rv)$. ∎

###### Theorem 2.

Let $R$ be a ring and $U,V$ and $W$ be faithful  $R$-modules. If $b:U\times V\to W$ is $R$-bilinear and (left or right) non-degenerate, then $R$ must be commutative   .

###### Proof.

We may assume that $b$ is left non-degenerate. Let $r,s\in R$. Then for all $u\in U$ and $v\in V$ it follows that

 $\displaystyle b((sr)u,v)=sb(ru,v)=sb(u,rv)=b(su,rv)=b((rs)u,v).$

Therefore $b([s,r]u,v)=0$, where $[s,r]=sr-rs$. This makes $[s,r]u$ an element of the left radical  of $b$ as it is true for all $v\in V$. However $b$ is non-degenerate so the radical is trivial and so $[s,r]u=0$ for all $u\in U$. Since $U$ is a faithful $R$-module this makes $[s,r]=0$ for all $s,r\in R$. That is, $R$ is commutative. ∎

Alternatively we can interpret the result in a weaker fashion as:

###### Corollary 3.

Let $R$ be a ring and $U,V$ and $W$ be $R$-modules. If $b:U\times V\to W$ is $R$-bilinear with $W=\langle b(U,V)\rangle$ then every element $[R,R]$ acts trivially on one of the three modules $U$, $V$ or $W$.

###### Proof.

Suppose $[r,s]\in[R,R]$, $[r,s]U\neq 0$ and $[r,s]V\neq 0$. Then we have shown $0=b([r,s]u,v)=[r,s]b(u,v)$ for all $u\in U$ and $v\in V$. As $W=\langle b(U,V)\rangle$ it follows that $[r,s]W=0$. ∎

Whenever a non-commutative ring is required for a biadditive map $U\times V\to W$ it is therefore often preferable to use a scalar map instead.

Title bilinearity and commutative rings BilinearityAndCommutativeRings 2013-03-22 17:24:19 2013-03-22 17:24:19 Algeboy (12884) Algeboy (12884) 5 Algeboy (12884) Theorem msc 13C99