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A172123 Number of ways to place 2 nonattacking bishops on an n X n board. 17
0, 4, 26, 92, 240, 520, 994, 1736, 2832, 4380, 6490, 9284, 12896, 17472, 23170, 30160, 38624, 48756, 60762, 74860, 91280, 110264, 132066, 156952, 185200, 217100, 252954, 293076, 337792, 387440, 442370, 502944, 569536, 642532 (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

V. Kotesovec, Number of ways of placing non-attacking queens and kings on boards of various sizes

Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).

FORMULA

a(n) = n(n - 1)(3n^2 - n + 2)/6.

G.f.: -2x^2*(x+1)(x+2)/(x-1)^5. - 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 29 2013

MATHEMATICA

CoefficientList[Series[-2 x (x+1)(x+2)/(x-1)^5, {x, 0, 80}], x] (* Vincenzo Librandi, Apr 29 2013 *)

PROG

(MAGMA) [n*(n-1)*(3*n^2-n+2)/6: n in [1..40]]; /* or */  I:=[0, 4, 26, 92, 240]; [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]]; // Vincenzo Librandi, Apr 29 2013

(PARI) a(n)=n*(n-1)*(3*n^2-n+2)/6 \\ Charles R Greathouse IV, Jun 16 2015

CROSSREFS

Cf. A036464.

Sequence in context: A247194 A102198 A100207 * A245457 A118285 A101166

Adjacent sequences:  A172120 A172121 A172122 * A172124 A172125 A172126

KEYWORD

nonn,easy

AUTHOR

Vaclav Kotesovec, Jan 26 2010

STATUS

approved

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Last modified May 27 11:14 EDT 2017. Contains 287204 sequences.