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

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

Offset: 1

Author

Jaroslav Krizek, Sep 24 2014

Keywords

• nonn
• more

Comments

Primes p such that A247787(p) = A000203(A076274(p)) = 3*(p-1).

If a(9) exists it must be bigger than 10^10.

Examples

			Prime 17 is in sequence because sigma(2*17-1) = sigma(33) = 48 = 3*(17-1).
		

Crossrefs

Cf. A000203, A076274, A247787.

Programs

Magma

[p: p in PrimesUpTo(20000)| SumOfDivisors(2*p-1) eq 3*p-3]

Mathematica

Select[Prime@ Range[10^5], DivisorSigma[1, 2 # - 1] == 3 (# - 1) &] (* Michael De Vlieger, Jan 03 2017 *)

PARI

forprime(p=1,10^7,if(sigma(2*p-1)==3*(p-1),print1(p,", "))) \\ Derek Orr, Sep 25 2014

Extensions

a(8) from Matthew Campbell, Jan 03 2017