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 A209411 #18 Aug 06 2018 12:37:17
%S A209411 1,4,1,16,49,4,1,64,81,196,1,16,337,4,49,256,577,324,1,784,1,4,1,64,
%T A209411 1825,1348,81,16,1681,196,1,1024,1,2308,49,1296,2737,4,337,3136,3361,
%U A209411 4,1,16,3969,4,1,256,2401,7300,577,5392,5617,324,49,64,1
%N A209411 Number of 2 x 2 matrices M such that M*transpose(M) == 0 mod n.
%H A209411 Joerg Arndt, <a href="/A209411/b209411.txt">Table of n, a(n) for n = 1..500</a>
%t A209411 gg[n_] := gg[n] = Flatten[Table[{{x, y}, {z, t}}, {x, n}, {y, n}, {t, n}, {z, n}], 3]; nil[n_] := nil[n] = Length@gg[n][[Select[Range[Length[gg[n]]], Mod[gg[n][[#]].Transpose[gg[n][[#]]], n] == {{0, 0}, {0, 0}} &]]]; Table[Print[nil[n]]; nil[n], {n, 1, 22}]
%Y A209411 Cf. A182039.
%K A209411 nonn,mult
%O A209411 1,2
%A A209411 _José María Grau Ribas_, Mar 08 2012
%E A209411 Keyword:mult added by _Andrew Howroyd_, Aug 02 2018