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 A383921 #19 May 22 2025 09:39:19 %S A383921 1,2,6,24,91,841,13861,360361 %N A383921 Least integer k for which sigma(k - x) + sigma(k + x) >= n*k has at least one solution. %C A383921 The corresponding x are: 0, 0, 0, 0, 89, 839, 13859, 360359, ... %C A383921 a(10) <= 61261201 via sigma(61261201 - 61261199) + sigma(61261201 + 61261199) >= 10*61261201. - _David A. Corneth_, May 16 2025 %C A383921 a(11) <= 20951330401 via sigma(20951330401 - 20951330399) + sigma(20951330401 + 20951330399) >= 11*20951330401. - _Michel Marcus_, May 22 2025 %o A383921 (PARI) isok(k,n) = my(v=vector(k, x, (sigma(k - x + 1) + sigma(k + x - 1))/k)); my(vk = select(t->(t>=n), v, 1)); if (#vk != 0, v[1]); %o A383921 a(n) = my(k=1); while (!isok(k, n), k++); k; %Y A383921 Cf. A000203, A004394, A383758, A383920. %K A383921 nonn,more %O A383921 2,2 %A A383921 _Michel Marcus_, May 15 2025 %E A383921 a(9) from _Jean-Marc Rebert_, May 16 2025