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 A069519 #22 Apr 27 2025 09:08:31
%S A069519 1,2,4,8,12,16,24,32,48,56,64,96,112,120,128,192,224,240,256,384,448,
%T A069519 480,512,528,672,768,896,960,992,1024,1056,1344,1456,1536,1792,1920,
%U A069519 1984,2048,2112,2160,2208,2688,2912,3072,3584,3840,3968,4096,4224,4320
%N A069519 Numbers k such that 1/(Sum_{d|k} (-1)^d/d) is an integer.
%H A069519 Amiram Eldar, <a href="/A069519/b069519.txt">Table of n, a(n) for n = 1..4907</a> (terms below 10^11)
%F A069519 a(1)=1, a(2)=2, a(n) = A066192(n-2) for n > 2.
%t A069519 q[n_] := IntegerQ[1/DivisorSum[n, (-1)^# / # &]]; Select[Range[4500], q] (* _Amiram Eldar_, Apr 27 2025 *)
%o A069519 (PARI) isok(k) = denominator(1/sumdiv(k, d, (-1)^d/d)) == 1; \\ _Amiram Eldar_, Apr 27 2025
%Y A069519 Cf. A066192.
%K A069519 nonn
%O A069519 1,2
%A A069519 _Benoit Cloitre_, Apr 16 2002
%E A069519 Name corrected by _Amiram Eldar_, Apr 27 2025