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 A027307 Number of paths from (0,0) to (3n,0) that stay in first quadrant (but may touch horizontal axis) and where each step is (2,1), (1,2) or (1,-1). 53

%I

%S 1,2,10,66,498,4066,34970,312066,2862562,26824386,255680170,

%T 2471150402,24161357010,238552980386,2375085745978,23818652359682,

%U 240382621607874,2439561132029314,24881261270812490,254892699352950850

%N Number of paths from (0,0) to (3n,0) that stay in first quadrant (but may touch horizontal axis) and where each step is (2,1), (1,2) or (1,-1).

%C Equals row sums of triangle A104978 which has g.f. F(x,y) that satisfies: F = 1 + x*F^2 + x*y*F^3. - _Paul D. Hanna_, Mar 30 2005

%C a(n) counts ordered complete ternary trees with 2*n-1 leaves, where the internal vertices come in two colors and such that each vertex and its rightmost child have different colors. See [Drake, Example 1.6.9]. An example is given below. - _Peter Bala_, Sep 29 2011

%H Vincenzo Librandi, <a href="/A027307/b027307.txt">Table of n, a(n) for n = 0..200</a>

%H Gi-Sang Cheon, S.-T. Jin, L. W. Shapiro, <a href="http://dx.doi.org/10.1016/j.laa.2015.03.015">A combinatorial equivalence relation for formal power series</a>, Linear Algebra and its Applications, Available online 30 March 2015.

%H Emeric Deutsch, <a href="http://www.jstor.org/stable/2589192">Problem 10658</a>, American Math. Monthly, 107, 2000, 368-370.

%H B. Drake, <a href="http://people.brandeis.edu/~gessel/homepage/students/drakethesis.pdf">An inversion theorem for labeled trees and some limits of areas under lattice paths (Example 1.6.9)</a>, A dissertation presented to the Faculty of the Graduate School of Arts and Sciences of Brandeis University.

%H J. Winter, M. M. Bonsangue and J. J. M. M. Rutten, <a href="http://oai.cwi.nl/oai/asset/21313/21313A.pdf">Context-free coalgebras</a>, 2013.

%H Anssi Yli-Jyrä and Carlos Gómez-Rodríguez, <a href="https://arxiv.org/abs/1706.03357">Generic Axiomatization of Families of Noncrossing Graphs in Dependency Parsing</a>, arXiv:1706.03357 [cs.CL], 2017.

%F G.f.: (2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3. a(n)=(Sum_{i=0..n-1} 2^(i+1)*binomial(2*n, i)*binomial(n, i+1))/n, n>0.

%F a(n) = 2*A034015(n-1), n>0.

%F a(n) = Sum_{k=0..n} C(2*n+k, n+2*k)*C(n+2*k, k)/(n+k+1). - _Paul D. Hanna_, Mar 30 2005

%F Given g.f. A(x), y=A(x)x satisfies 0=f(x, y) where f(x, y)=x(x-y)+(x+y)y^2 . - _Michael Somos_, May 23 2005

%F Series reversion of x(Sum_{k>=0} a(k)x^k) is x(Sum_{k>=0} A085403(k)x^k).

%F G.f. A(x) satisfies A(x)=A006318(x*A(x)). - _Vladimir Kruchinin_, Apr 18 2011

%F The function B(x) = x*A(x^2) satisfies B(x) = x+x*B(x)^2+B(x)^3 and hence B(x) = compositional inverse of x*(1-x^2)/(1+x^2) = x+2*x^3+10*x^5+66*x^7+.... Let f(x) = (1+x^2)^2/(1-4*x^2+x^4) and let D be the operator f(x)*d/dx. Then a(n) equals 1/(2*n+1)!*D^(2*n)(f(x)) evaluated at x = 0. For a refinement of this sequence see A196201. - _Peter Bala_, Sep 29 2011

%F Recurrence: 2*n*(2*n+1)*a(n) = (46*n^2-49*n+12)*a(n-1) - 3*(6*n^2-26*n+27)*a(n-2) - (n-3)*(2*n-5)*a(n-3). - _Vaclav Kotesovec_, Oct 08 2012

%F a(n) ~ sqrt(50+30*sqrt(5))*((11+5*sqrt(5))/2)^n/(20*sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Oct 08 2012

%e a(3) = 10. Internal vertices colored either b(lack) or w(hite); 5 uncolored leaf vertices shown as o.

%e ........b...........b.............w...........w.....

%e ......./|\........./|\.........../|\........./|\....

%e ....../.|.\......./.|.\........./.|.\......./.|.\...

%e .....b..o..o.....o..b..o.......w..o..o.....o..w..o..

%e ..../|\............/|\......../|\............/|\....

%e .../.|.\........../.|.\....../.|.\........../.|.\...

%e ..o..o..o........o..o..o....o..o..o........o..o..o..

%e ....................................................

%e ........b...........b.............w...........w.....

%e ......./|\........./|\.........../|\........./|\....

%e ....../.|.\......./.|.\........./.|.\......./.|.\...

%e .....w..o..o.....o..w..o.......b..o..o.....o..b..o..

%e ..../|\............/|\......../|\............/|\....

%e .../.|.\........../.|.\....../.|.\........../.|.\...

%e ..o..o..o........o..o..o....o..o..o........o..o..o..

%e ....................................................

%e ........b...........w..........

%e ......./|\........./|\.........

%e ....../.|.\......./.|.\........

%e .....o..o..w.....o..o..b.......

%e ........../|\........./|\......

%e ........./.|.\......./.|.\.....

%e ........o..o..o.....o..o..o....

%e ...............................

%t a[n_] := ((n+1)*(2n)!*Hypergeometric2F1[-n, 2n+1, n+2, -1]) / (n+1)!^2; Table[a[n], {n, 0, 19}] (* _Jean-François Alcover_, Nov 14 2011, after Pari *)

%o (PARI) a(n)=if(n<1,n==0,sum(i=0,n-1,2^(i+1)*binomial(2*n,i)*binomial(n,i+1))/n)

%o (PARI) a(n)=sum(k=0,n,binomial(2*n+k,n+2*k)*binomial(n+2*k,k)/(n+k+1)) \\ _Paul D. Hanna_

%o (PARI) a(n)=sum(k=0,n, binomial(n,k)*binomial(2*n+k+1,n)/(2*n+k+1) ) /* _Michael Somos_, May 23 2005 */

%Y Cf. A104978. A196201.

%K nonn

%O 0,2

%A _Emeric Deutsch_

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Last modified September 16 18:45 EDT 2019. Contains 327117 sequences. (Running on oeis4.)