This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A247788 #18 Sep 08 2022 08:46:09 %S A247788 3,17,131,193,449,13469,23297,581150417 %N A247788 Primes p such that sigma(2p-1) = 3*(p-1). %C A247788 Primes p such that A247787(p) = A000203(A076274(p)) = 3*(p-1). %C A247788 If a(9) exists it must be bigger than 10^10. %e A247788 Prime 17 is in sequence because sigma(2*17-1) = sigma(33) = 48 = 3*(17-1). %t A247788 Select[Prime@ Range[10^5], DivisorSigma[1, 2 # - 1] == 3 (# - 1) &] (* _Michael De Vlieger_, Jan 03 2017 *) %o A247788 (Magma) [p: p in PrimesUpTo(20000)| SumOfDivisors(2*p-1) eq 3*p-3] %o A247788 (PARI) forprime(p=1,10^7,if(sigma(2*p-1)==3*(p-1),print1(p,", "))) \\ _Derek Orr_, Sep 25 2014 %Y A247788 Cf. A000203, A076274, A247787. %K A247788 nonn,more %O A247788 1,1 %A A247788 _Jaroslav Krizek_, Sep 24 2014 %E A247788 a(8) from _Matthew Campbell_, Jan 03 2017