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A128732
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Number DL's in all skew Dyck paths of semilength n.
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2
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0, 0, 1, 5, 23, 106, 493, 2312, 10917, 51840, 247319, 1184557, 5692517, 27434578, 132547877, 641789941, 3113487683, 15130119784, 73637665027, 358883327591, 1751237017413, 8555108199294, 41836182269267, 204779733440086
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OFFSET
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0,4
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COMMENTS
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A skew Dyck path is a path in the first quadrant which begins at the origin, ends on the x-axis, consists of steps U=(1,1)(up), D=(1,-1)(down) and L=(-1,-1)(left) so that up and left steps do not overlap. The length of the path is defined to be the number of its steps.
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LINKS
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E. Deutsch, E. Munarini, S. Rinaldi, Skew Dyck paths, J. Stat. Plann. Infer. 140 (8) (2010) 2191-2203
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FORMULA
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G.f.: z*(1 - z - sqrt(1 - 6*z + 5*z^2))/(1 - 6*z + 5*z^2 +(1+z)*sqrt(1 - 6*z + 5*z^2)).
Conjecture: +2*n*(3*n-1)*a(n) -n*(39*n-37)*a(n-1) +4*(12*n^2-22*n-15)*a(n-2) -5*(3*n+2)*(n-3)*a(n-3)=0. - R. J. Mathar, Jun 17 2016
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EXAMPLE
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a(3)=5 because we have UDUUDL, UUUDLD, UUDUDL, UUUDDL and UUUDLL (the remaining 5 paths are Dyck paths which, obviously, contain no DL's).
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MAPLE
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G:=z*(1-z-sqrt(1-6*z+5*z^2))/(1-6*z+5*z^2+(1+z)*sqrt(1-6*z+5*z^2)): Gser:=series(G, z=0, 30): seq(coeff(Gser, z, n), n=0..26);
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MATHEMATICA
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CoefficientList[Series[x*(1-x-Sqrt[1-6*x+5*x^2])/(1-6*x+5*x^2+(1+x)*Sqrt[1-6*x+5*x^2]), {x, 0, 20}], x] (* Vaclav Kotesovec, Mar 20 2014 *)
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PROG
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(PARI) z='z+O('z^50); concat([0, 0], Vec(z*(1-z-sqrt(1-6*z+5*z^2))/(1-6*z+5*z^2 +(1+z)*sqrt(1-6*z+5*z^2)))) \\ G. C. Greubel, Mar 20 2017
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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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