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Let p be an odd prime and gcd(a,p) = 1. Let n be the number of residues of the set S = {a,2a,3a,...,(p-1)/2*a} (only half the multiples) which are greater than p/2. Then (a/p) = (-1)^n.

But what if a=2 Is there an analogous lemma?

EDIT: I was right the first time

But what if a=2 Is there an analogous lemma?

EDIT: I was right the first time

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