What does it mean to demonstrate "por doble inclusión" in Spanish?

Question

What does it mean to demonstrate "por doble inclusión" in Spanish? "By double-counting?" And what is that? thanks

For context, see, e.g., this.

Answer 1

I don't know Spanish, but based on similarities with Portuguese I'd say it means "by double inclusion". I infer it's a problem that you need to prove $A = B$ where $A,B$ are sets. Therefore you prove $A \subseteq B$ and $B \subseteq A$.

Answer 2

When you want to prove an equality of two sets, say $X=Y$, you prove $X\subseteq Y$ and $Y\subseteq X$. Thatś what it means.