Let p be prime, prove that if (2^p)-1 is a composite number, then m=(2^p)-1 is a pseudoprime.
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n = 2^p -1 == (denotes congruent) 2^(p-1) == 1modp by fermats little theorem since p is an odd prime. p|(2^(p-1)-1) so p|n-1 n = 2^p -1|2^(n-1) -1 2^(n-1) == 1mod n and 2^n == 2 mod n, so n is a pseudoprime
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