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A279111
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Number of non-equivalent ways to place 2 non-attacking kings on an n X n board.
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10
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0, 0, 4, 13, 37, 75, 147, 246, 406, 610, 910, 1275, 1779, 2373, 3157, 4060, 5212, 6516, 8136, 9945, 12145, 14575, 17479, 20658, 24402, 28470, 33202, 38311, 44191, 50505, 57705, 65400, 74104, 83368, 93772, 104805, 117117, 130131, 144571, 159790, 176590, 194250, 213654
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OFFSET
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1,3
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COMMENTS
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Rotations and reflections of placements are not counted. If they are to be counted, see A061995.
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LINKS
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FORMULA
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a(n) = (n^4 - 2*n^2 - 4*n + IF(MOD(n, 2) = 1, 2*n^2 - 4*n + 7))/16.
a(n) = (2*n^4 - 2*n^2 - 12*n + 7 - (2*n^2 - 4*n + 7)*(-1)^n)/32. - Bruno Berselli, Dec 07 2016
a(n) = 2*a(n-1) + 2*a(n-2) - 6*a(n-3) + 6*a(n-5) - 2*a(n-6) - 2*a(n-7) + a(n-8).
a(n) = n*(n - 2)*(n^2 + 2*n + 2)/16 for n even.
a(n) = (n - 1)*(n^3 + n^2 + n - 7)/16 for n odd.
G.f.: x^3*(4 + 5*x + 3*x^2 - x^3 + x^4) / ((1 - x)^5*(1 + x)^3).
(End)
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EXAMPLE
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There are 4 non-equivalent ways to place 2 non-attacking kings on a 3 X 3 board:
K.K K.. K.. .K.
... ..K ... ...
... ... ..K .K.
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MATHEMATICA
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Table[(n^4 - 2 n^2 - 4 n + Boole[OddQ@ n] (2 n^2 - 4 n + 7))/16, {n, 43}] (* or *)
Rest@ CoefficientList[Series[x^3*(4 + 5 x + 3 x^2 - x^3 + x^4)/((1 - x)^5*(1 + x)^3), {x, 0, 43}], x] (* Michael De Vlieger, Dec 08 2016 *)
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PROG
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(PARI) concat(vector(2), Vec(x^3*(4 + 5*x + 3*x^2 - x^3 + x^4) / ((1 - x)^5*(1 + x)^3) + O(x^60))) \\ Colin Barker, Dec 07 2016
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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