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A227327
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Number of non-equivalent ways to choose two points in an equilateral triangle grid of side n.
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15
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0, 1, 4, 10, 22, 41, 72, 116, 180, 265, 380, 526, 714, 945, 1232, 1576, 1992, 2481, 3060, 3730, 4510, 5401, 6424, 7580, 8892, 10361, 12012, 13846, 15890, 18145, 20640, 23376, 26384, 29665, 33252, 37146, 41382, 45961, 50920, 56260, 62020, 68201, 74844
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OFFSET
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1,3
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COMMENTS
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LINKS
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FORMULA
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a(n) = (n^4 + 2*n^3 + 8*n^2 - 8*n )/48; if n even.
a(n) = (n^4 + 2*n^3 + 8*n^2 - 2*n - 9)/48; if n odd.
G.f.: -x^2*(x^3-x^2+x+1) / ((x-1)^5*(x+1)^2). - Colin Barker, Jul 12 2013
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EXAMPLE
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for n = 3 there are the following 4 choices of 2 points (X) (rotations and reflections being ignored):
X X X .
X . . . . . X X
. . . X . . . X . . . .
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MATHEMATICA
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Table[b = n^4 + 2*n^3 + 8*n^2; If[EvenQ[n], c = b - 8*n, c = b - 2*n - 9]; c/48, {n, 43}] (* T. D. Noe, Jul 09 2013 *)
CoefficientList[Series[-x (x^3 - x^2 + x + 1) / ((x - 1)^5 (x + 1)^2), {x, 0, 50}], x] (* Vincenzo Librandi, Aug 02 2013 *)
LinearRecurrence[{3, -1, -5, 5, 1, -3, 1}, {0, 1, 4, 10, 22, 41, 72}, 50] (* Harvey P. Dale, May 11 2019 *)
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CROSSREFS
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Corresponding questions about the number of ways in a square grid are treated by A083374 (2 points) and A178208 (3 points).
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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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