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 A124880 #10 Feb 16 2025 08:33:03
%S A124880 7,47,42131,23763863,192066102203,5733412167187,34745876421709,
%T A124880 185813891783454008069,171312804637561107990389,
%U A124880 29207630124216024960052176833,6300447575454970515437116064749
%N A124880 Primes in A124837.
%H A124880 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HarmonicNumber.html">Harmonic Number</a>.
%e A124880 A124837(n) begins {1, 7, 47, 57, 459, 341, 3349, 3601, 42131, 44441, ...}.
%e A124880 Thus a(1) = 7, a(2) = 47, a(3) = 42131.
%t A124880 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,3,125}]
%Y A124880 A124837 are the numerators of third-order harmonic numbers H(n, (3)).
%Y A124880 Corresponding numbers n such that A124837(n) is prime are listed in A124881.
%Y A124880 Cf. A001008, A002805, A067657, A056903, A027612, A124878, A124879, A124837, A124881.
%K A124880 nonn
%O A124880 1,1
%A A124880 _Alexander Adamchuk_, Nov 11 2006
%E A124880 Crossrefs edited by _Michel Marcus_, Jul 14 2018