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 A172132 Number of ways to place 2 nonattacking knights on an n X n board. 13
 0, 6, 28, 96, 252, 550, 1056, 1848, 3016, 4662, 6900, 9856, 13668, 18486, 24472, 31800, 40656, 51238, 63756, 78432, 95500, 115206, 137808, 163576, 192792, 225750, 262756, 304128, 350196, 401302, 457800, 520056, 588448, 663366 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES E. Bonsdorff, K. Fabel, O. Riihimaa, Schach und Zahl, 1966, p. 51-63 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1). FORMULA a(n) = (n - 1)(n + 4)(n^2 - 3n + 4)/2 G.f.: 2*(12x^4-39x^3+37x^2-20x+4)/(x-1)^5 [From Vaclav Kotesovec, Mar 25 2010] a(n) = 5*a(n-1)-10*a(n-2)+10*a(n-3)-5*a(n-4)+a(n-5). - Vincenzo Librandi, Apr 30 2013 MATHEMATICA Table[(n - 1) (n + 4) (n^2 - 3 n + 4)/2, {n, 40}] (* Vincenzo Librandi, Apr 30 2013 *) PROG (MAGMA) I:=[0, 6, 28, 96, 252]; [n le 5 select I[n] else 5*Self(n-1)-10*Self(n-2)+10*Self(n-3)-5*Self(n-4)+Self(n-5): n in [1..40]]; /* or */ [(n-1)*(n+4)*(n^2-3*n+4)/2: n in [1..40]]; Vincenzo Librandi, Apr 30 2013 CROSSREFS Cf. A036464, A172123. Column k=2 of A244081. Sequence in context: A125310 A138874 A172141 * A011856 A276041 A134416 Adjacent sequences:  A172129 A172130 A172131 * A172133 A172134 A172135 KEYWORD easy,nonn AUTHOR Vaclav Kotesovec, Jan 26 2010 STATUS approved

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Last modified December 18 20:06 EST 2018. Contains 318245 sequences. (Running on oeis4.)