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A097405
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Number of different rectangles created when a square sheet of paper is folded n times, the first time by one of the diagonals of the square and after by the median of the triangle.
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1
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0, 0, 8, 17, 108, 265, 1461, 4011, 21211, 62135, 322423, 977647, 5025263, 15510495, 79345631, 247115711, 1261100991, 3945447295, 20110344063, 63059984127, 321227980543, 1008422616575, 5135350103551, 16130465856511, 82131231439871
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OFFSET
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1,3
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COMMENTS
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There are two types of rectangles: (1) those whose edges are parallel to the edges of the initial square and (2) those whose edges are diagonal to the edges of the initial square. These rectangles are enumerated by the p(x) and d(x) functions.
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LINKS
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FORMULA
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Let p(x) = x^2 (x+1)^2/4 and d(x) = (x^4 - x^2 - 6 x)/24. Then, for n>1, a(n) = -1 + p(2^ceiling(n/2-1)) + d(2^floor(n/2))
G.f.: x^3*(256*x^7+880*x^6-360*x^5-706*x^4+113*x^3+149*x^2-9*x-8) / ((x-1)*(2*x-1)*(2*x+1)*(4*x-1)*(4*x+1)*(2*x^2-1)*(8*x^2-1)). [Colin Barker, Nov 23 2012]
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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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