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A071900 1/4 times the number of n X n 0..7 matrices with MM' mod 8 = I, where M' is the transpose of M and I is the n X n identity matrix. 5
1, 16, 1536, 786432, 2013265920 (list; graph; refs; listen; history; text; internal format)



Table of n, a(n) for n=1..5.

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.


From Petros Hadjicostas, Dec 18 2019: (Start)

For n = 2, the 4*a(2) = 64 n X n matrices M with elements in 0..7 that satisfy MM' mod 8 = I can be classified into four categories:

(a) Matrices M with 1 = det(M) mod 8. These form the abelian group SO(2, Z_8). See the comments for sequence A060968.

(b) Matrices M with 3 = det(M) mod 8. These are the elements of the left coset A*SO(2, Z_8) = {AM: M in SO(2, Z_8)}, where A = [[3,0],[0,1]].

(c) Matrices M with 5 = det(M) mod 8. These are the elements of the left coset B*SO(2, Z_8) = {BM: M in SO(2, Z_8)}, where B = [[5,0],[0,1]].

(d) Matrices M with 7 = det(M) mod 8. These are the elements of the left coset C*SO(2, Z_8) = {CM: M in SO(2, Z_8)}, where C= [[7,0],[0,1]].

All four classes of matrices have the same number of elements, that is, 16 each.

Note that for n = 3 we have 4*a(3) = 4*1536 = 6144 = A264083(8). (End)


Cf. A060968, A071302, A071304, A071305, A071306, A071307, A071308, A071309, A071310, A071900, A087784, A208895, A264083.

Sequence in context: A221613 A266156 A054947 * A321247 A145406 A307924

Adjacent sequences:  A071897 A071898 A071899 * A071901 A071902 A071903




R. H. Hardin, Jun 12 2002



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