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 A247823 #15 Sep 08 2022 08:46:09 %S A247823 7,8191,131071,524287,2147483647,2305843009213693951, %T A247823 618970019642690137449562111 %N A247823 Mersenne primes p such that there is a number k with sigma(sigma(2k-1)) = p. %C A247823 Mersenne primes p such that there is a number m such that sigma(sigma(m)) = p. %C A247823 Distinct values attained by the A247822(n) function, in ascending order. %C A247823 Mersenne primes p such that there are a numbers n and m such that sigma(sigma(2n-1)) = sigma(sigma(2*A247821(n)-1)) = A000203(A000203(2*A247821(n)-1)) = A051027(2*A247821(n)-1) = sigma(sigma(A247838(m))) = A000203(A000203(A247838(m))) = A051027(A247838(m)) where m = 2n-1. %C A247823 The Mersenne prime 7 is the only prime p such that there is a prime q with sigma(sigma(q)) = p. %e A247823 Mersenne prime 8191 is in sequence because there are numbers n = 1334 and 1969 with sigma(sigma(2*n-1)) = 8191. %o A247823 (Magma) Set(Sort([SumOfDivisors(SumOfDivisors(n)): n in [1..10000000] | IsPrime(SumOfDivisors(SumOfDivisors(n)))])) // _Jaroslav Krizek_, Mar 25 2015 %Y A247823 Cf. A000203, A008438, A247790, A247791, A247820, A247821, A247822, A247954. %Y A247823 Cf. A000668 (Mersenne primes). %K A247823 nonn,more %O A247823 1,1 %A A247823 _Jaroslav Krizek_, Sep 28 2014 %E A247823 a(5)-a(7) from _Jaroslav Krizek_, Mar 25 2015