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 A102604 #20 Aug 12 2024 14:48:28
%S A102604 2,3,7,41,43,79,421
%N A102604 Numbers k such that ((2*k)^k - 1)/(2*k - 1) is prime.
%C A102604 The next k in the sequence is > 4261, if it exists.
%C A102604 Note that (b^k - 1)/(b-1) is prime only if k is prime, so all the elements in this sequence must be primes. - Marco Bodrato (marco2007(AT)bodrato.it), Oct 31 2007
%C A102604 a(8) > 20000, if it exists. - _Michael S. Branicky_, Aug 12 2024
%e A102604 (((2*2)^2) - 1)/(2*2 - 1) = 15/3 = 5 is prime so a(1)=2.
%t A102604 Select[Prime[Range[100]],PrimeQ[((2#)^#-1)/(2#-1)]&] (* _Harvey P. Dale_, Mar 09 2022 *)
%o A102604 (PARI) lista(nn) = {forprime(n = 2, nn, if (isprime(((2*n)^n-1)/(2*n-1)), print1(n, ", ")););} \\ _Michel Marcus_, Feb 05 2014
%Y A102604 Cf. A088790.
%K A102604 more,nonn
%O A102604 1,1
%A A102604 _Pierre CAMI_, Jan 29 2005