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 A201243 Number of ways to place 2 non-attacking ferses on an n X n board. 7
 0, 4, 28, 102, 268, 580, 1104, 1918, 3112, 4788, 7060, 10054, 13908, 18772, 24808, 32190, 41104, 51748, 64332, 79078, 96220, 116004, 138688, 164542, 193848, 226900, 264004, 305478, 351652, 402868, 459480, 521854, 590368, 665412, 747388, 836710, 933804, 1039108 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Fers is a leaper [1,1] LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 V. Kotesovec, Non-attacking chess pieces, 4ed, p.233 Index to sequences with linear recurrences with constant coefficients, signature (5,-10,10,-5,1). FORMULA a(n) = 1/2*(n-1)*(n^3 + n^2 - 4n + 4) by C. Poisson, 1990. G.f.: 2x^2*(x+1)*(x^2-2x-2)/(x-1)^5. 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^3 + n^2 - 4 n + 4) / 2, {n, 100}] (* Vincenzo Librandi, Apr 30 2013 *) PROG (MAGMA) I:=[0, 4, 28, 102, 268]; [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^3+n^2-4*n+4)/2: n in [1..40]]; // Vincenzo Librandi, Apr 30 2013 CROSSREFS Cf. A172123, A201244, A201245, A201246, A201247, A201248. Sequence in context: A187452 A173296 A077595 * A092712 A202964 A183469 Adjacent sequences:  A201240 A201241 A201242 * A201244 A201245 A201246 KEYWORD nonn,easy AUTHOR Vaclav Kotesovec, Nov 28 2011 STATUS approved

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