A247788 Primes p such that sigma(2p-1) = 3*(p-1).

3, 17, 131, 193, 449, 13469, 23297, 581150417

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