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 A247820 #16 Nov 11 2024 01:57:29 %S A247820 5,13,145,365,841,1201,1741,2521,3961,5101,7813,8581,13945,14281, %T A247820 14965,41761,42925,73345,139921,229165,245701,265721,276025,289561, %U A247820 298765,341965,351961,353641,367225,414961,595141,601705,676285,697381,711625,740545,942565 %N A247820 Numbers k such that sigma(2*k-1) is a prime p. %C A247820 Supersequence of A247789. %C A247820 Corresponding values of primes p for a(n) are A008438(a(n)-1) or A247837. %H A247820 Martin Møller Skarbiniks Pedersen, <a href="/A247820/b247820.txt">Table of n, a(n) for n = 1..1000</a> %e A247820 Number 13 is in sequence because sigma(2*13-1) = sigma(25) = 31 (prime). %o A247820 (Magma) [n: n in [1..10000000] | IsPrime(SumOfDivisors(2*n-1))]; %o A247820 (PARI) for(n=1,10^7,if(isprime(sigma(2*n-1)),print1(n,", "))) \\ _Derek Orr_, Sep 25 2014 %Y A247820 Cf. A000203, A008438, A247789, A247837. %K A247820 nonn %O A247820 1,1 %A A247820 _Jaroslav Krizek_, Sep 24 2014