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 A209991 Number of 2 X 2 matrices with all elements in {0,1,...,n} and determinant in {0,1}. 1
 1, 13, 38, 79, 136, 209, 302, 407, 536, 681, 846, 1015, 1240, 1441, 1678, 1951, 2240, 2505, 2854, 3151, 3552, 3945, 4326, 4687, 5216, 5657, 6110, 6615, 7192, 7649, 8342, 8831, 9472, 10105, 10702, 11407, 12272, 12857, 13526, 14279, 15224 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS See A210000 for a guide to related sequences. LINKS FORMULA A059306 + A171503. MATHEMATICA a = 0; b = n; z1 = 40; t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]] c[n_, k_] := c[n, k] = Count[t[n], k] c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, 1}] Table[c1[n, 1], {n, 0, z1}]    (* A209991 *) CROSSREFS Cf. A059306, A171503, A210000. Sequence in context: A044090 A044471 A285525 * A299055 A158647 A283123 Adjacent sequences:  A209988 A209989 A209990 * A209992 A209993 A209994 KEYWORD nonn AUTHOR Clark Kimberling, Mar 18 2012 STATUS approved

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Last modified January 26 17:27 EST 2020. Contains 331280 sequences. (Running on oeis4.)