A very elementary question:

> 
> Let $|A|=a$, and $|B|=b$. What's $|\mathcal P(\mathcal P(\mathcal P(A \times B \times \varnothing)))|$?
> 
> 
> 

So how I got stuck on it: I'm not sure what's the number of pairs that a generic $A \times B$ gives, and what's $A \times B \times \varnothing$? Is it $A$ times $B$ times empty set? Sorry, I'm kinda confused on all of this subject and don't have a good source of knowledge (terrible lecturer and no textbook...).

P.S. Is there a good online source, or an eBook or whatever for discrete mathematics?

Thanks in advance for any help, tips etc.