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 A128723 Number of skew Dyck paths of semilength n having no peaks at level 1. 2
 1, 0, 2, 6, 22, 84, 334, 1368, 5734, 24480, 106086, 465462, 2063658, 9231084, 41610162, 188820726, 861891478, 3954732384, 18230522422, 84390187986, 392120098258, 1828220666844, 8550445900442, 40103716079436 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS 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 a path is defined to be the number of steps in it. LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 E. Deutsch, E. Munarini, S. Rinaldi, Skew Dyck paths, J. Stat. Plann. Infer. 140 (8) (2010) 2191-2203 FORMULA a(n) = A128722(n,0). a(n) = 2*A117641(n) for n>=1. G.f.: (3-3*z-sqrt(1-6*z+5*z^2))/(1+3*z+sqrt(1-6*z+5*z^2)). a(n) ~ 5^(n+3/2)/(16*sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Mar 20 2014 Conjecture: +3*(n+1)*a(n) +(-17*n+10)*a(n-1) +9*(n-3)*a(n-2) +5*(n-2)*a(n-3)=0. - R. J. Mathar, Jun 17 2016 EXAMPLE a(3)=6 because we have UUDUDD, UUUDDD, UUUDLD, UUDUDL, UUUDDL and UUUDLL. MAPLE G:=(3-3*z-sqrt(1-6*z+5*z^2))/(1+3*z+sqrt(1-6*z+5*z^2)): Gser:=series(G, z=0, 30): seq(coeff(Gser, z, n), n=0..27); MATHEMATICA CoefficientList[Series[(3-3*x-Sqrt[1-6*x+5*x^2])/(1+3*x+Sqrt[1-6*x+5*x^2]), {x, 0, 20}], x] (* Vaclav Kotesovec, Mar 20 2014 *) PROG (PARI) x='x+O('x^50); Vec((3-3*z-sqrt(1-6*z+5*z^2)) /(1+3*z +sqrt(1-6*z+5*z^2))) \\ G. C. Greubel, Mar 19 2017 CROSSREFS Cf. A117641, A128722. Sequence in context: A164870 A121686 A245904 * A150244 A151288 A150245 Adjacent sequences:  A128720 A128721 A128722 * A128724 A128725 A128726 KEYWORD nonn AUTHOR Emeric Deutsch, Mar 31 2007 STATUS approved

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Last modified January 18 16:27 EST 2020. Contains 331011 sequences. (Running on oeis4.)