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 A247788 Primes p such that sigma(2p-1) = 3*(p-1). 2
 3, 17, 131, 193, 449, 13469, 23297, 581150417 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Primes p such that A247787(p) = A000203(A076274(p)) = 3*(p-1). If a(9) exists it must be bigger than 10^10. LINKS Table of n, a(n) for n=1..8. EXAMPLE Prime 17 is in sequence because sigma(2*17-1) = sigma(33) = 48 = 3*(17-1). MATHEMATICA Select[Prime@ Range[10^5], DivisorSigma[1, 2 # - 1] == 3 (# - 1) &] (* Michael De Vlieger, Jan 03 2017 *) PROG (Magma) [p: p in PrimesUpTo(20000)| SumOfDivisors(2*p-1) eq 3*p-3] (PARI) forprime(p=1, 10^7, if(sigma(2*p-1)==3*(p-1), print1(p, ", "))) \\ Derek Orr, Sep 25 2014 CROSSREFS Cf. A000203, A076274, A247787. Sequence in context: A321587 A089815 A328402 * A355598 A006759 A073513 Adjacent sequences: A247785 A247786 A247787 * A247789 A247790 A247791 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Sep 24 2014 EXTENSIONS a(8) from Matthew Campbell, Jan 03 2017 STATUS approved

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Last modified February 22 10:17 EST 2024. Contains 370250 sequences. (Running on oeis4.)