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 A209411 Number of 2 x 2 matrices M such that M*transpose(M) == 0 mod n. 2
 1, 4, 1, 16, 49, 4, 1, 64, 81, 196, 1, 16, 337, 4, 49, 256, 577, 324, 1, 784, 1, 4, 1, 64, 1825, 1348, 81, 16, 1681, 196, 1, 1024, 1, 2308, 49, 1296, 2737, 4, 337, 3136, 3361, 4, 1, 16, 3969, 4, 1, 256, 2401, 7300, 577, 5392, 5617, 324, 49, 64, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Joerg Arndt, Table of n, a(n) for n = 1..500 MATHEMATICA 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}] CROSSREFS Sequence in context: A002568 A111661 A072651 * A093035 A126791 A052179 Adjacent sequences:  A209408 A209409 A209410 * A209412 A209413 A209414 KEYWORD nonn AUTHOR José María Grau Ribas, Mar 08 2012 STATUS approved

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