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 A253851 #13 Jun 22 2025 21:31:59 %S A253851 7,127,8191,2147483647,170141183460469231731687303715884105727 %N A253851 Mersenne primes (A000668) of the form 2^sigma(n) - 1 for some n. %C A253851 Numbers n such that 2^sigma(n) - 1 is a Mersenne primes are given in A253849. %C A253851 Sequence of corresponding values of sigma(n) are given in A253850 and each term of this sequence must be a prime from the sequence of Mersenne exponents (A000043). %C A253851 If a(6) exists, it must be bigger than A000668(43) = 2^30402457-1. %e A253851 Mersenne prime 2147483647 is in the sequence because there are two numbers n (16 and 25) with 2^sigma(n) - 1 = 2^31 - 1 = 2147483647. %o A253851 (Magma) Set(Sort([(2^SumOfDivisors(n))-1: n in[1..10000] | IsPrime((2^SumOfDivisors(n))-1)])); %Y A253851 Cf. A000043, A000203, A000668, A023195, A253849, A253850. %K A253851 nonn %O A253851 1,1 %A A253851 _Jaroslav Krizek_, Jan 16 2015