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 A247791 #35 Feb 26 2025 05:54:19 %S A247791 7,131071,524287 %N A247791 Primes p such that there is a prime q for which sigma(sigma(2*q-1)) = p. %C A247791 The next term, if it exists, must be greater than 5*10^7. %C A247791 Primes p such that there is prime q for which sigma(sigma(2*q-1)) = A247954(q) = A000203(A000203(2*q-1)) = A000203(A008438(q-1)) = A051027(2*q-1) = p. %C A247791 Corresponding values of primes q: 2, 28669, 126961, ... (A247790). %C A247791 Conjecture: Subsequence of Mersenne primes. %C A247791 Conjecture: the next term is 2305843009213693951 when 2305843009213693951 = sigma(sigma(2*500461553802019261-1)) where 500461553802019261 is prime (see comment of Hiroaki Yamanouchi in A247821). - _Jaroslav Krizek_, Oct 08 2014 %e A247791 Prime 7 is in sequence because there is prime 2 such that sigma(sigma(2*2-1)) = sigma(sigma(3)) = sigma(4) = 7. %o A247791 (Magma) [SumOfDivisors(SumOfDivisors(2*n-1)): n in [A247790(n)]]; %o A247791 (Magma) [SumOfDivisors(SumOfDivisors(2*n-1)): n in[1..1000000] | IsPrime(SumOfDivisors(SumOfDivisors(2*n-1))) and IsPrime(n)]; %o A247791 (PARI) %o A247791 forprime(p=1,10^7,if(ispseudoprime(sigma(sigma(2*p-1))),print1(sigma(sigma(2*p-1)),", "))) \\ _Derek Orr_, Sep 29 2014 %Y A247791 Cf. A000203, A008438, A247790, A247820, A247821, A247822, A247823, A247954. %K A247791 nonn,more,bref %O A247791 1,1 %A A247791 _Jaroslav Krizek_, Sep 28 2014