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A191794
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Number of length n left factors of Dyck paths having no UUDD's; here U=(1,1) and D=(1,-1).
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1
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1, 1, 2, 3, 5, 8, 14, 23, 41, 69, 124, 212, 383, 662, 1200, 2091, 3799, 6661, 12122, 21359, 38919, 68850, 125578, 222892, 406865, 724175, 1322772, 2360010, 4313155, 7711148, 14099524, 25252819, 46192483, 82863807, 151628090, 272385447, 498578411, 896774552
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OFFSET
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0,3
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LINKS
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FORMULA
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G.f.: g(z) = 2/(1-2*z+z^4+sqrt(1-4*z^2+2*z^4+z^8)).
D-finite with recurrence (n+1)*a(n) +2*(-1)*a(n-1) +4*(-n+1)*a(n-2) +2*(n-3)*a(n-4) +6*a(n-5) +(n-7)*a(n-8)=0. - R. J. Mathar, Jul 22 2022
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EXAMPLE
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a(4)=5 because we have UDUU, UDUD, UUDU, UUUD, and UUUU, where U=(1,1) and D=(1,-1) (the path UUDD does not qualify).
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MAPLE
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g := 2/(1-2*z+z^4+sqrt(1-4*z^2+2*z^4+z^8)): gser := series(g, z = 0, 42): seq(coeff(gser, z, n), n = 0 .. 37);
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MATHEMATICA
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CoefficientList[Series[2/(1-2x+x^4+Sqrt[1-4x^2+2x^4+x^8]), {x, 0, 40}], x] (* Harvey P. Dale, Jun 19 2011 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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