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 A072461 #19 Jun 26 2024 04:21:39 %S A072461 1,9,49,50,81,100,121,200,338,361,400,441,450,529,578,625,676,729,800, %T A072461 900,961,1089,1156,1352,1600,1682,1800,1849,2209,2312,2401,2450,2704, %U A072461 2738,3042,3200,3249,3362,3364,3481,3600,3969,4050,4225,4489,4624,4761 %N A072461 Numbers k such that sigma(k) == 1 (mod 4). %H A072461 Ray Chandler, <a href="/A072461/b072461.txt">Table of n, a(n) for n = 1..10000</a> (first 1000 terms from Harvey P. Dale) %t A072461 Select[Range[5000],Mod[DivisorSigma[1,#],4]==1&] (* _Harvey P. Dale_, Oct 24 2016 *) %o A072461 (PARI) isok(n) = ((sigma(n) % 4) == 1); %Y A072461 Cf. A000203, A028982, A072462, A081339. %Y A072461 Positions of 1's in A105824. %K A072461 nonn %O A072461 1,2 %A A072461 _Labos Elemer_, Jun 19 2002 %E A072461 Name edited by _Michel Marcus_, Dec 19 2013 and _Amiram Eldar_, Jun 26 2024