For any subgroup of the group $G$, let $H^2$ denote the product $H^2=HH$. Prove that $H^2=H$.
This question seems simple but I do not know how can I prove.
For any subgroup of the group $G$, let $H^2$ denote the product $H^2=HH$. Prove that $H^2=H$.
This question seems simple but I do not know how can I prove.