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 A026376 a(n) is the number of integer strings s(0),...,s(n) counted by array T in A026374 that have s(n)=2; also a(n) = T(2n,n-1). 12
 1, 6, 30, 144, 685, 3258, 15533, 74280, 356283, 1713690, 8263596, 39938616, 193419915, 938430990, 4560542550, 22195961280, 108171753355, 527816696850, 2578310320610, 12607504827600, 61706212037295, 302275142049870, 1481908332595625, 7270432009471224 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Number of Schroeder paths (i.e., consisting of steps U=(1,1), D=(1,-1) and H=(2,0) and never going below the x-axis) from (0,0) to (2n+2,0), with exactly one peak at an even level. E.g., a(2)=6 because we have UUDDH, HUUDD, UDUUDD, UUDDUD, UUDHD and UHUDD. - Emeric Deutsch, Dec 28 2003 Number of left steps in all skew Dyck paths of semilength n+1. 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. Example: a(2)=6 because in the 10 (=A002212(3)) skew Dyck paths of semilength 3 ( namely UDUUDL, UUUDLD, UUDUDL, UUUDDL, UUUDLL and five Dyck paths that have no left steps) we have altogether 6 left steps. - Emeric Deutsch, Aug 05 2007 From Gary W. Adamson, May 17 2009: (Start) Equals A026378 (1, 4, 17, 75, ...) convolved with A007317 (1, 2, 5, 15, 51, ...). Equals A081671 (1, 3, 11, 45, ...) convolved with A002212 (1, 3, 10, 36, 137, ...). (End) LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..200 E. Deutsch, E. Munarini, S. Rinaldi, Skew Dyck paths, J. Stat. Plann. Infer. 140 (8) (2010) 2191-2203. FORMULA E.g.f.: exp(3x)*I_1(2x), where I_1 is Bessel function. - Michael Somos, Sep 09 2002 G.f.: (1 - 3*z - t)/(2*z*t) where t = sqrt(1-6*z+5*z^2). - Emeric Deutsch, May 25 2003 a(n) = [t^(n+1)](1+3t+t^2)^n. a := n -> Sum_{j=ceiling((n+1)/2)..n} 3^(2j-n-1)*binomial(n, j)*binomial(j, n+1-j). - Emeric Deutsch, Jan 30 2004 a(n) = Sum_{k=0..n} binomial(n, k)*binomial(2k, k+1). - Paul Barry, Sep 20 2004 a(n) = n*A002212(n). - Emeric Deutsch, Aug 05 2007 Conjecture: (n+1)*a(n) - 9*n*a(n-1) + (23*n-27)*a(n-2) + 15*(-n+2)*a(n-3) = 0. - R. J. Mathar, Dec 02 2012 a(n) ~ 5^(n+1/2) / (2*sqrt(Pi*n)). - Vaclav Kotesovec, Feb 13 2014 a(n) = n*hypergeometric([1, 3/2, 1-n],[1, 3],-4). - Peter Luschny, Sep 16 2014 a(n) = GegenbauerC(n-1, -n, -3/2). - Peter Luschny, May 09 2016 MAPLE a := n -> simplify(GegenbauerC(n-1, -n, -3/2)): seq(a(n), n=1..24); # Peter Luschny, May 09 2016 MATHEMATICA Rest[CoefficientList[Series[(1-3*x-Sqrt[1-6*x+5*x^2])/(2*x*Sqrt[1-6*x+5*x^2]), {x, 0, 20}], x]] (* Vaclav Kotesovec, Feb 13 2014 *) PROG (PARI) a(n)=if(n<0, 0, polcoeff((1+3*x+x^2)^n, n-1)) (Sage) A026376 = lambda n : n*hypergeometric([1, 3/2, 1-n], [1, 3], -4) [round(A026376(n).n(100)) for n in (1..24)] # Peter Luschny, Sep 16 2014 (Sage) # Recurrence: def A026376():     x, y, n = 1, 1, 1     while True:         x, y = y, ((6*n + 3)*y - (5*n - 5)*x) / (n + 2)         yield n*x         n += 1 a = A026376() [next(a) for i in (1..24)] # Peter Luschny, Sep 16 2014 CROSSREFS Cf. A006318, A002212, A081671, A007317, A026378. Sequence in context: A221397 A082134 A030192 * A026899 A135160 A276022 Adjacent sequences:  A026373 A026374 A026375 * A026377 A026378 A026379 KEYWORD nonn AUTHOR STATUS approved

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Last modified April 18 02:38 EDT 2021. Contains 343072 sequences. (Running on oeis4.)