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 A154895 #6 Jul 29 2020 03:18:21
%S A154895 6,28,8128,137438691328,2305843008139952128
%N A154895 Perfect numbers whose number of proper divisors is prime.
%C A154895 The next term is too large to include in the data section. If there are no odd perfect numbers then the next term is a(6) = 2^606 * (2^607 - 1) = 1.410... * 10^365. - _Amiram Eldar_, Jul 29 2020
%F A154895 a(n) = A006516(A172461(n)), assuming that odd perfect numbers do not exist. - _Amiram Eldar_, Jul 29 2020
%e A154895 28 is member because the number of proper divisors of 28 is 5, a prime number.
%t A154895 Table[2^(p-1)*(2^p-1), {p, Select[MersennePrimeExponent[Range[8]], PrimeQ[2# - 1] &]}] (* _Amiram Eldar_, Jul 29 2020 *)
%Y A154895 Cf. A000396, A006516, A032741, A133033, A154896, A172461.
%K A154895 nonn
%O A154895 1,1
%A A154895 _Omar E. Pol_, Jan 25 2009