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 A181647 #21 Apr 16 2025 04:31:28
%S A181647 42,51,123,141,204,371,497,708,923,992,1034,1343,1391,1484,1595,1691,
%T A181647 1826,3266,3317,5015,5152,7367,8003,9132,9287,9494,11078,13223,15458,
%U A181647 15833,17975,18752,19428,20120,20915,21251,21566,24119,24503,25787,28000,29726,29795
%N A181647 Numbers m having the same sum of divisors as m+20 has.
%D A181647 Jean-Marie De Koninck, Those Fascinating Numbers, Amer. Math. Soc., 2009, page 16.
%H A181647 Donovan Johnson, <a href="/A181647/b181647.txt">Table of n, a(n) for n = 1..1000</a> (first 100 terms from Reinhard Zumkeller)
%H A181647 Andreas Weingartner, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL14/Weingartner/wein3.html">On the Solutions of sigma(n) = sigma(n+k)</a>, Journal of Integer Sequences, Vol. 14 (2011), Article 11.5.5.
%F A181647 A000203(a(n)) = A000203(a(n) + 20).
%e A181647 a(1) = 42, divisors(42) = {1,2,3,6,7,14,21,42}, divisors(42+20) = {1,2,31,62}: 1+2+3+6+7+14+21+42 = 1+2+31+62.
%t A181647 Select[Range[30000], Equal @@ DivisorSigma[1, # + {0, 20}] &] (* _Amiram Eldar_, Apr 16 2025 *)
%o A181647 (PARI) isok(n) = sigma(n) == sigma(n+20); \\ _Michel Marcus_, Feb 06 2016
%Y A181647 Cf. A000203, A002961, A007373, A015861, A015863, A015865, A015866, A015867, A015876, A015877, A015880, A015881, A015882, A015883.
%K A181647 nonn
%O A181647 1,1
%A A181647 _Reinhard Zumkeller_, Nov 03 2010