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A007746
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Number of ways for n-3 nonintersecting loops to cross a line 2n times.
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2
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42, 640, 5894, 42840, 271240, 1569984, 8536890, 44346456, 222516030, 1086685600, 5193298110, 24384586200, 112831907760, 515709552000, 2332549535400, 10455495457248, 46500885666900, 205401168733824, 901819865269180, 3938266773556720, 17116175702216624
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OFFSET
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4,1
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LINKS
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FORMULA
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a(n) = 4 * (2*n)! * (n^4+20*n^3+107*n^2-107*n+15) / ( 3*(n-4)! * (n+6)! ).
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MATHEMATICA
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Table[4 (2 n)!/(3 (n - 4)! (n+6)!) (n^4 + 20 n^3 + 107 n^2 - 107 n + 15), {n, 4, 30}] (* Vincenzo Librandi, Nov 23 2015 *)
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PROG
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(Magma) [4*Factorial(2*n)/(3*Factorial(n-4)*Factorial(n+6))* (n^4+20*n^3+107*n^2-107*n+15): n in [4..25]]; // Vincenzo Librandi, Nov 23 2015
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Philippe Di Francesco (philippe(AT)amoco.saclay.cea.fr)
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STATUS
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approved
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