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 A247836 #21 Jul 17 2025 12:25:16 %S A247836 13,31,2801,3541,5113,10303,17293,30941,88741,292561,732541,1191373, %T A247836 1395943,2922391,3281533,7450171,7781311,12190573,15916111,22292563, %U A247836 28792661,67757593,93886411,103052953,112137511,124802413,135059263,181427431 %N A247836 Primes p of the form sigma(2q-1) where q is a prime. %C A247836 Subsequence of A247837. %e A247836 Prime 13 is in sequence because there is prime 5 such that sigma(2*5-1) = sigma(9) = 13. %t A247836 Select[DivisorSigma[1,2Prime[Range[3*10^6]]-1],PrimeQ] (* The program generates the first 23 terms of the sequence. *) (* _Harvey P. Dale_, Jul 17 2025 *) %o A247836 (Magma) Sort([a: p in PrimesUpTo(5000000) | IsPrime(a) where a is SumOfDivisors(2*p-1)]); // corrected by _Jaroslav Krizek_, Nov 16 2014 %o A247836 (PARI) forprime(p=1,10^9,if(isprime(sigma(2*p-1)),print1(sigma(2*p-1),", "))) \\ _Derek Orr_, Sep 25 2014 %Y A247836 Cf. A000203, A247837, A247820. %K A247836 nonn %O A247836 1,1 %A A247836 _Jaroslav Krizek_, Sep 24 2014