Let $$ A=\{1,2,\ldots,p-1\},\qquad B=\{1,2,\ldots,q-1\} $$ where $p,q$ are primes not necessarily distinct. Is there any elementary way to find the cardinal of the following set $$ AB=\{ab:\ a\in A,\ b\in B\} $$ and also the distribution of them. Thanks!
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3$\begingroup$ This seems related to the question mathoverflow.net/questions/31663/… $\endgroup$– Johan WästlundCommented Oct 4, 2015 at 7:50
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