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 A124401 #21 Oct 24 2024 09:26:08
%S A124401 3,5,8,9,11,15,21,39,50,63,83,95,99,173,350,854,1308,1769,2903,5250,
%T A124401 5345,5639,6195,7239,21368,41669,47684,58619,63515,69468,70539,133508,
%U A124401 134993,187160,493095
%N A124401 Indices where 2 occurs in A124151.
%C A124401 Does 2 occur infinitely often in A124151?
%C A124401 The sum in A124151 is 1+n if k=1, and 1+k*(k^(2n)-1)/(k^2-1) if k>1. The indices of A124151(n)=2 are where k=1 is avoided, but where k=2 leads to a prime, i.e., where 1+n is not prime but 1+2*(4^n-1)/3 = (2^(2n+1)+1)/3 is prime. Therefore this sequence here is constructed by taking all n=(A000978(i)-1)/2 (the members of A127936), and eliminating cases with 1+n in A000040. - _R. J. Mathar_, Feb 03 2010
%F A124401 A127936 \ A006093. - _R. J. Mathar_, Feb 03 2010
%t A124401 f[n_] := Block[{k = 1}, While[ !PrimeQ[ Sum[k^(2j - 1), {j, n}] + 1] && k < 3, k++ ]; k]; lst = {}; Do[ If[f@n == 2, Print[n]; AppendTo[lst, n]], {n, 9250}]; lst (* _Robert G. Wilson v_, Dec 17 2006 *)
%o A124401 (PARI) is(n) = !isprime(n+1) && isprime(1 + 2*(4^n-1)/3); \\ _Amiram Eldar_, Oct 24 2024
%Y A124401 Cf. A006093, A124151, A124154, A124163, A124164, A124178, A124181, A124185-A124187, A124189, A124200, A124205-A124209.
%K A124401 nonn,more
%O A124401 1,1
%A A124401 _Artur Jasinski_, Dec 14 2006
%E A124401 More terms from _Robert G. Wilson v_, Dec 17 2006
%E A124401 a(24)-a(35) from _R. J. Mathar_, Feb 03 2010