So... I've decided to study some algebra, to see if I can handle it... lol
I've come accross this (very basic) question, which may rather be set theory, but the original question is algebra :
Let G be a group. Let H be a subgroup containing a subset P.
So basically, I have to prove that
It is clear that
So it is very logical to have the equality
But I didn't succeed in finding a formal proof