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%I #18 Sep 08 2022 08:46:09
%S 3,17,131,193,449,13469,23297,581150417
%N Primes p such that sigma(2p-1) = 3*(p-1).
%C Primes p such that A247787(p) = A000203(A076274(p)) = 3*(p-1).
%C If a(9) exists it must be bigger than 10^10.
%e Prime 17 is in sequence because sigma(2*17-1) = sigma(33) = 48 = 3*(17-1).
%t Select[Prime@ Range[10^5], DivisorSigma[1, 2 # - 1] == 3 (# - 1) &] (* _Michael De Vlieger_, Jan 03 2017 *)
%o (Magma) [p: p in PrimesUpTo(20000)| SumOfDivisors(2*p-1) eq 3*p-3]
%o (PARI) forprime(p=1,10^7,if(sigma(2*p-1)==3*(p-1),print1(p,", "))) \\ _Derek Orr_, Sep 25 2014
%Y Cf. A000203, A076274, A247787.
%K nonn,more
%O 1,1
%A _Jaroslav Krizek_, Sep 24 2014
%E a(8) from _Matthew Campbell_, Jan 03 2017
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