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 A073541 #19 Jun 11 2024 08:13:07 %S A073541 5,14,54,55,56,58,70,87,88,89,114,118,123,125,178,201,204,206,220,246, %T A073541 354,358,417,419,459,478,500,534,549,594,604,620,646,654,663,715,718, %U A073541 753,834,835,838,894,957,1006,1018,1106,1194,1239,1253,1318,1334,1364 %N A073541 Numbers k such that 1/(1/sigma(k) + 1/sigma(k+1)) is an integer. %H A073541 Amiram Eldar, <a href="/A073541/b073541.txt">Table of n, a(n) for n = 1..10000</a> %e A073541 1/sigma(54) + 1/sigma(55) = 1/120 + 1/72 = 1/45 so 54 is in the sequence. %t A073541 Select[Range[1400], IntegerQ[1/(1/DivisorSigma[1, # ]+1/DivisorSigma[1, #+1])]&] %o A073541 (PARI) isok(k) = denominator(1/(1/sigma(k) + 1/sigma(k+1))) == 1; \\ _Michel Marcus_, May 13 2022 %Y A073541 Cf. A000203. %K A073541 easy,nonn %O A073541 1,1 %A A073541 _Benoit Cloitre_, Aug 27 2002 %E A073541 Edited by _Dean Hickerson_, Aug 31 2002