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 A081325 #12 May 03 2025 07:45:39 %S A081325 1,3,1,3,3,3,1,3,1,1,1,3,3,3,3,3,3,3,1,1,1,3,1,3,1,1,1,3,3,1,1,3,1,1, %T A081325 3,3,3,3,3,1,3,3,1,3,3,3,1,3,1,3,3,1,3,3,3,3,1,1,1,1,3,3,1,3,1,3,1,1, %U A081325 1,1,1,3,3,1,1,3,1,1,1,1,1,1,1,3,1,3,3,3,3,1,3,3,1,3,3,3,3,3,1,3,3,1,1,1,3 %N A081325 a(n) = sigma(n^2) modulo 4. %H A081325 Amiram Eldar, <a href="/A081325/b081325.txt">Table of n, a(n) for n = 1..10000</a> %F A081325 From _Amiram Eldar_, May 03 2025: (Start) %F A081325 a(n) = A010873(A065764(n)). %F A081325 a(A081339(n)) = 1. %F A081325 a(A081354(n)) = 3. (End) %t A081325 a[n_] := Mod[DivisorSigma[1, n^2], 4]; Array[a, 100] (* _Amiram Eldar_, May 03 2025 *) %o A081325 (PARI) a(n) = sigma(n^2) % 4; \\ _Amiram Eldar_, May 03 2025 %Y A081325 Cf. A000203, A010873, A065764, A081334, A081339, A081354. %K A081325 nonn,easy %O A081325 1,2 %A A081325 _Benoit Cloitre_, Apr 20 2003