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 A197168 Number of 2 X 2 integer matrices with elements from {1,...,n} whose determinant is 2. 6
 0, 0, 2, 6, 18, 30, 50, 70, 98, 118, 162, 198, 226, 270, 338, 366, 426, 486, 554, 622, 682, 726, 842, 926, 986, 1062, 1202, 1270, 1362, 1470, 1562, 1678, 1802, 1878, 2066, 2158, 2250, 2390, 2602, 2694, 2818, 2974, 3114, 3278, 3434, 3526, 3786, 3966, 4090, 4254, 4490 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS It is also the number of 2 X 2 integer matrices with elements from {1,...,n} whose determinant is -2. LINKS Chai Wah Wu, Table of n, a(n) for n = 0..10000 FORMULA a(n) = A209973(n) - 4*n - 2 for n >= 2. - Chai Wah Wu, Nov 28 2016 MATHEMATICA Table[cnt = 0; Do[If[a*d-b*c == 2, cnt++], {a, n}, {b, n}, {c, n}, {d, n}]; cnt, {n, 50}] (* T. D. Noe, Oct 11 2011 *) CROSSREFS Sequence in context: A185382 A257065 A066286 * A277200 A277324 A034881 Adjacent sequences:  A197165 A197166 A197167 * A197169 A197170 A197171 KEYWORD nonn,changed AUTHOR Aldo González Lorenzo, Oct 10 2011 EXTENSIONS a(0) added by Chai Wah Wu, Nov 27 2016 STATUS approved

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