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 A124881 #13 Feb 16 2025 08:33:03
%S A124881 2,3,9,15,25,27,33,45,55,67,70,93,94,97,112,113,125,137,189,193,212,
%T A124881 232,262,273,281,381,453,528,670,677,742,743,827,996,1257,1349,1402,
%U A124881 1645,1683,2110,2217,2408,2480,2623,3208,3517,3637,3665,4571,4730
%N A124881 Numbers k such that A124837(k) is prime.
%H A124881 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HarmonicNumber.html">Harmonic Number</a>.
%e A124881 A124837(n) begins {1, 7, 47, 57, 459, 341, 3349, 3601, 42131, 44441, ...}.
%e A124881 Thus a(1) = 2, a(2) = 3, a(3) = 9.
%t A124881 s=3/2;Do[s=s+1/n;f=Numerator[n*(n-1)/2*(s-3/2)]; If[PrimeQ[f],Print[{n-2,f}]],{n,3,1000}]
%Y A124881 A124837 are the numerators of third-order harmonic numbers H(n, (3)).
%Y A124881 Corresponding primes in A124837 are listed in A124880.
%Y A124881 Cf. A001008, A002805, A067657, A056903, A027612, A124878, A124879, A124837, A124880.
%K A124881 hard,nonn
%O A124881 1,1
%A A124881 _Alexander Adamchuk_, Nov 11 2006
%E A124881 More terms from _Stefan Steinerberger_, May 09 2007
%E A124881 Crossrefs edited by _Michel Marcus_, Jul 14 2018