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 A281312 Numbers n such that sigma(4*(n-1)) is prime. 2
 2, 5, 17, 1025, 16385, 65537, 268435457, 288230376151711745, 77371252455336267181195265, 20282409603651670423947251286017, 21267647932558653966460912964485513217 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjecture: the next terms are: 288230376151711745, 77371252455336267181195265, 20282409603651670423947251286017, 21267647932558653966460912964485513217. Conjecture: prime terms are in A258429: 2, 5, 17, 65537. Conjecture: corresponding primes p are Mersenne primes (A000668) > 3. Sigma is multiplicative, and sigma(m) > 1 for all m > 1, so sigma(m) can be prime only if m is a prime power. Hence all n in this sequence are of the form 2^m + 1 for some m >= 0. This proves the above conjectures and leads to an explicit formula (q.v.) in terms of the Mersenne numbers. - Charles R Greathouse IV, Mar 01 2017 LINKS FORMULA a(n) = 2^(A000043(n+1)-3) + 1. - Charles R Greathouse IV, Mar 01 2017 PROG (MAGMA) [n: n in [2..100000] | IsPrime(SumOfDivisors(4*(n-1)))] (PARI) isok(n) = isprime(sigma(4*(n-1))); \\ Michel Marcus, Jan 21 2017 CROSSREFS Cf. A000203, A000668, A193553, A258429. Sequence in context: A041455 A081465 A128000 * A182313 A124374 A113617 Adjacent sequences:  A281309 A281310 A281311 * A281313 A281314 A281315 KEYWORD nonn AUTHOR Jaroslav Krizek, Jan 19 2017 EXTENSIONS a(7) = 268435457 confirmed by Jon E. Schoenfield, Jan 20 2017 a(8)-a(11) from Charles R Greathouse IV, Mar 01 2017 STATUS approved

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Last modified March 31 15:40 EDT 2020. Contains 333151 sequences. (Running on oeis4.)