Cannot become ring because distribution law does not hold

abelian-groupsabstract-algebragroup-theorymonoidring-theory

Commutative ring with unit is defined as $(R,+,\times)$, where $(R,+)$ is abelian group and $(R,\times)$ is commutative multiplicative monoid with $1$ and $+$ and $\times$ satisfies distributive law.

Could you give me an example $(R,+,\times)$ cannnot be a ring because $+$ and $\times$ does not satisfy distributive law although $(R,+)$ is abelian group and $(R,\times)$ is commutative multiplicative monoid with $1$.

Best Answer

Here is a "dumb" example. Let $R=\mathbb Z$, and let $\times=+$, i.e., addition and multiplication are the same thing. Now $(R,\times)$ is a commutative monoid, with a $1$ (i.e, $0\in R$). This is clearly not distributive: $1\times(1+1)=3\neq1\times 1+1\times 1=4$.