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 A071303 1/2 times the number of n X n 0..3 matrices M with MM' mod 4 = I, where M' is the transpose of M and I is the n X n identity matrix. 4
 1, 8, 192, 12288, 1966080, 1509949440 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS It seems that a(n) = n! * 2^(binomial(n+1,2) - 1) for n = 1, 2, 3, 4, 5, while for n = 6, a(n) is twice this number. The number n! * 2^(binomial(n+1,2) - 1) appears in Proposition 6.1 in Eriksson and Linusson (2000) as an upper bound to the number of three-dimensional permutation arrays of size n (see column k = 3 of A330490). - Petros Hadjicostas, Dec 16 2019 LINKS Kimmo Eriksson and Svante Linusson A combinatorial theory of higher-dimensional permutation arrays, Adv. Appl. Math. 25(2) (2000a), 194-211; see Proposition 6.1 (p. 210). Jianing Song, Structure of the group SO(2,Z_n). László Tóth, Counting solutions of quadratic congruences in several variables revisited, arXiv:1404.4214 [math.NT], 2014. László Tóth, Counting Solutions of Quadratic Congruences in Several Variables Revisited, J. Int. Seq. 17 (2014), #14.11.6. EXAMPLE From Petros Hadjicostas, Dec 16 2019: (Start) For n = 2, here are the 2*a(2) = 16 2 x 2 matrices M with elements in {0,1,2,3} that satisfy MM'  mod 4 = I: (a) With 1 = det(M) mod 4: [[1,0],[0,1]]; [[0,1],[3,0]]; [[0,3],[1,0]]; [[1,2],[2,1]]; [[2,1],[3,2]]; [[2,3],[1,2]]; [[3,0],[0,3]]; [[3,2],[2,3]]. These form the abelian group SO(2, Z_n). See the comments for sequence A060968. (b) With 3 = det(M) mod 4: [[0,1],[1,0]]; [[0,3],[3,0]]; [[1,0],[0,3]];  [[1,2],[2,3]]; [[2,1],[1,2]]; [[2,3],[3,2]]; [[3,0],[0,1]];  [[3,2],[2,1]]. Note that, for n = 3, we have 2*a(3) = 2*192 = 384 = A264083(4). (End) CROSSREFS Cf. A003920, A060968, A071302, A071304, A071305, A071306, A071307, A071308, A071309, A071310, A071900, A087784, A208895, A229136, A264083, A330490. Sequence in context: A058873 A052734 A003435 * A128406 A265269 A003956 Adjacent sequences:  A071300 A071301 A071302 * A071304 A071305 A071306 KEYWORD nonn,more AUTHOR R. H. Hardin, Jun 11 2002 STATUS approved

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Last modified June 22 16:35 EDT 2021. Contains 345388 sequences. (Running on oeis4.)