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 A124879 #13 Feb 16 2025 08:33:03
%S A124879 2,3,6,9,18,25,29,30,39,84,91,125,130,184,195,199,203,241,245,273,281,
%T A124879 378,552,571,653,776,901,1099,1215,1224,1235,1315,1412,1657,1942,2076,
%U A124879 2085,2743,2745,2855,2859,3517,3717,4183,4188,4362,4547,4728,4783
%N A124879 Numbers k such that A027612(k) is prime.
%H A124879 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HarmonicNumber.html">Harmonic Number</a>.
%e A124879 A027612 begins {1, 5, 13, 77, 87, 223, 481, 4609, 4861, ...}.
%e A124879 Thus a(1) = 2, a(2) = 3, a(3) = 6, a(4) = 9.
%t A124879 s=1;Do[s=s+1/(n+1);f=Numerator[(n+1)*(s-1)]; If[PrimeQ[f],Print[{n,f}]],{n,1,1942}]
%o A124879 (PARI) isok(n) = isprime(numerator(sum(k=1, n, k/(n-k+1)))); \\ _Michel Marcus_, Jul 14 2018
%Y A124879 A027612(n) are the numerators of second order harmonic numbers H(n, (2)).
%Y A124879 Corresponding primes in A027612 are listed in A124878.
%Y A124879 Cf. A001008, A002805, A067657, A056903, A027612, A124878, A124837, A124880, A124881.
%K A124879 nonn
%O A124879 1,1
%A A124879 _Alexander Adamchuk_, Nov 11 2006
%E A124879 More terms from _Stefan Steinerberger_, May 29 2007
%E A124879 Crossrefs edited by _Michel Marcus_, Jul 14 2018