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 A249761 #10 Sep 08 2022 08:46:10 %S A249761 3,7,31,131071 %N A249761 Primes p such that there is prime q with sigma(q-1) = p. %C A249761 a(n) = corresponding values of sigma(n) of numbers n from A249760(n); a(n) = A000203(A249760(n)). %C A249761 Corresponding values of primes q are in A249759. %C A249761 Conjectures: 1) sequence is finite; 2) (a(n)+3)/2 is a Fermat prime (A019434); 3) a(n) is subsequence of Mersenne primes (A000668); 4) a(n) = 2*A249759(n)-3. %e A249761 Prime 31 is in the sequence because there is prime 17 with sigma(17-1) = sigma(16) = 31. %o A249761 (Magma) [a: p in PrimesUpTo(5000000) | IsPrime(a) where a is SumOfDivisors(p-1)] %Y A249761 Cf. A000203, A000668, A019434, A249759, A249760. %K A249761 nonn,more %O A249761 1,1 %A A249761 _Jaroslav Krizek_, Nov 13 2014