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Search: id:A115224
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| A115224 |
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Number of 3 X 3 symmetric matrices over Z(n) having determinant 1. |
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+0
2
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| 1, 28, 234, 896, 3100, 6552, 16758, 28672, 56862, 86800, 160930, 209664, 371124, 469224, 725400, 917504, 1419568, 1592136, 2475738, 2777600, 3921372, 4506040, 6435814, 6709248, 9687500, 10391472, 13817466, 15015168, 20510308, 20311200
(list; graph; listen)
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OFFSET
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1,2
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FORMULA
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a(1)=1 because the matrix of all zeros has determinant 0, but 0=1 (mod 1). For prime n, a(n)=(n^3-1)n^2. Multiplicative with a(p^e)=(p^3-1)*p^(5e-3). a(n)=A011785(n)/A000056(n).
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MATHEMATICA
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Table[cnt=0; Do[m={{a, b, c}, {b, d, e}, {c, e, f}}; If[Det[m, Modulus->n]==1, cnt++ ], {a, 0, n-1}, {b, 0, n-1}, {c, 0, n-1}, {d, 0, n-1}, {e, 0, n-1}, {f, 0, n-1}]; cnt, {n, 2, 20}]
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CROSSREFS
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Cf. A000056 (order of the group SL(2, Z_n)), A011785 (number of 3 X 3 matrices whose determinant is 1 mod n, i.e. order of SL(3, Z_n)).
Sequence in context: A042524 A125365 A126523 this_sequence A135497 A138405 A024015
Adjacent sequences: A115221 A115222 A115223 this_sequence A115225 A115226 A115227
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KEYWORD
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mult,nonn
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AUTHOR
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T. D. Noe (noe(AT)sspectra.com), Jan 16 2006
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