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 A124878 #11 Feb 16 2025 08:33:03
%S A124878 5,13,223,4861,197698279,25472027467,6975593267347,218572480850557,
%T A124878 1592457339642613,2955634782407818711841368777079578319,
%U A124878 2950127241932882597818337002939124083061,232242878286351670588710938679161483012314573381293769
%N A124878 Primes in A027612.
%H A124878 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HarmonicNumber.html">Harmonic Number</a>.
%F A124878 a(n) = A027612(A124879(n)).
%e A124878 A027612(n) begins {1, 5, 13, 77, 87, 223, 481, 4609, 4861, ...}.
%e A124878 Thus a(1) = 5, a(2) = 13, a(3) = 223, a(4) = 4861.
%t A124878 s=1;Do[s=s+1/(n+1);f=Numerator[(n+1)*(s-1)]; If[PrimeQ[f],Print[{n,f}]],{n,1,1942}]
%o A124878 (PARI) lista(nn) = {for (n=1, nn, if (isprime(p=numerator(sum(k=1, n, k/(n-k+1)))), print1(p, ", ")););} \\ _Michel Marcus_, Jul 14 2018
%Y A124878 A027612(n) are the numerators of second order harmonic numbers H(n, (2)).
%Y A124878 Corresponding numbers n such that A027612(n) is prime are listed in A124879.
%Y A124878 Cf. A001008, A002805, A067657, A056903, A027612, A124879, A124837, A124880, A124881.
%K A124878 nonn
%O A124878 1,1
%A A124878 _Alexander Adamchuk_, Nov 11 2006
%E A124878 a(12) from, and crossrefs edited by _Michel Marcus_, Jul 14 2018