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 A209982 Number of 2 X 2 matrices having all elements in {-n,...,n} and determinant 1. 4
 0, 20, 52, 116, 180, 308, 372, 564, 692, 884, 1012, 1332, 1460, 1844, 2036, 2292, 2548, 3060, 3252, 3828, 4084, 4468, 4788, 5492, 5748, 6388, 6772, 7348, 7732, 8628, 8884, 9844, 10356, 10996, 11508, 12276, 12660, 13812, 14388, 15156 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS See A210000 for a guide to related sequences. LINKS Andrew Howroyd, Table of n, a(n) for n = 0..1000 FORMULA From Andrew Howroyd, May 05 2020: (Start) a(n) = 8*A196227(n) + 4*(4*n + 1) = 8*A171503(n) - 4 for n > 0. a(n) = -12 + 32*Sum_{k=1..n} phi(k) for n > 0. (End) MATHEMATICA (See the Mathematica section at A209981.) PROG (PARI) a(n)=if(n<1, 0, 32*sum(k=1, n, eulerphi(k)) - 12) \\ Andrew Howroyd, May 05 2020 CROSSREFS Cf. A171503, A196227, A210000. Sequence in context: A350115 A211143 A183047 * A205220 A188250 A329627 Adjacent sequences: A209979 A209980 A209981 * A209983 A209984 A209985 KEYWORD nonn AUTHOR Clark Kimberling, Mar 17 2012 STATUS approved

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Last modified April 1 10:36 EDT 2023. Contains 361689 sequences. (Running on oeis4.)