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 A249761 Primes p such that there is prime q with sigma(q-1) = p. 2
 3, 7, 31, 131071 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(n) = corresponding values of sigma(n) of numbers n from A249760(n); a(n) = A000203(A249760(n)). Corresponding values of primes q are in A249759. Conjectures: 1) sequence is finite; 2) (a(n)+3)/2 is a Fermat prime (A019434); 3) a(n) is subsequence of Mersenne primes (A000668); 4) a(n) = 2*A249759(n)-3. LINKS EXAMPLE Prime 31 is in the sequence because there is prime 17 with sigma(17-1) = sigma(16) = 31. PROG (MAGMA) [a: p in PrimesUpTo(5000000) | IsPrime(a) where a is  SumOfDivisors(p-1)] CROSSREFS Cf. A000203, A000668, A019434, A249759, A249760. Sequence in context: A070231 A263049 A167917 * A163081 A096239 A074699 Adjacent sequences:  A249758 A249759 A249760 * A249762 A249763 A249764 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Nov 13 2014 STATUS approved

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Last modified October 15 04:33 EDT 2019. Contains 328026 sequences. (Running on oeis4.)