Past exam of the mathematics course of the University of Oxford / 2021 / Prelims / Mathematics I / 5 / b / iii

For any $x$, choose $z$ with $x\ast z=e$. By ii, $z\ast x=e$ as well. Associativity and the right identity element property now giveThus $e$ is also a left identity element. The given binary operation is closed on $X$ and associative, $e$ is a two-sided identity element, and each $x$ has the two-sided inverse element $z$. All group axioms hold, so $(X,\ast )$ is a group. This is the one-sided group axioms criterion.