Hi,

$\displaystyle _* \ $is a binary operation in E,

C is a set defined as:

$\displaystyle C={ y \in E/ (\forall x \in E): y_*x=x_*y } $

I must determine the specifications of $\displaystyle (C,_*)$: associativity, commutativity, neutral element, inverse element...

I found that $\displaystyle _*$ is commutative but for the others i don't know how to do it.