Confusing subtlety

[bbposter]

"Let p and q be odd primes. If p divides M sub q, then p = 1 (mod q) and p = +/-1 (mod 8)."

What is M sub q? It doesn't elucidate on this at all in my book. I'm assuming this notation implies that a number M has a factor of q? Is anyone aware of a place I can look up common mathematical notations such as this? It's one thing to not understand a theorm, but another to not even be able to get past the ambiguous notation. Thanks.

[leas5040]

I believe the notation implies that M sub q has a factor of q. Obviously, if p divides M as well, then M has factors of p and q. What exactly is the question asking? Are you supposed to find p or M or q, or all of them? You will see this notation quite a bit when you are dealing with points in three-space and in dealing with augmented matrices.

[Togra]

Replace "M sub q" by "2^q - 1".
M stands for Mersenne, M_q is the q-th Mersenne number (not necessarily a prime!).