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A127848

Series reversion of x/(1+6x+5x^2).

0, 1, 6, 41, 306, 2426, 20076, 171481, 1500666, 13386206, 121267476, 1112674026, 10318939956, 96572168916, 910896992856, 8650566601401, 82644968321226, 793753763514806, 7659535707782916, 74225795172589006, 722042370787826076

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OFFSET

0,3

COMMENTS

Hankel transform is -A127849(n)=-5^C(n,2)*(5^n-1)/4; a(n+1) counts (6,5)-Motzkin paths of length n, where there are 6 colors available for the H(1,0) steps and 5 for the U(1,1) steps. See A078009 for more information.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..200

FORMULA

G.f.: (1-6x-sqrt(1-12x+16x^2))/(10x); a(n)=sum{k=0..n-1, (1/n)*C(n,k)C(n,k+1)5^k}; a(n+1)=sum{k=0..floor(n/2), C(n, 2k)C(k)6^(n-2k)*5^k};

Recurrence: (n+1)*a(n) = 6*(2*n-1)*a(n-1) - 16*(n-2)*a(n-2). - Vaclav Kotesovec, Oct 20 2012

a(n) ~ sqrt(10+6*sqrt(5))*(6+2*sqrt(5))^n/(10*sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Oct 20 2012. Equivalently, a(n) ~ 2^(2*n) * phi^(2*n + 1) / (5^(3/4) * sqrt(Pi) * n^(3/2)), where phi = A001622 is the golden ratio. - Vaclav Kotesovec, Dec 07 2021

a(n) = A078009(n) for n>0. - Philippe Deléham, Apr 03 2013

MATHEMATICA

CoefficientList[ InverseSeries[ Series[ x/(1+6x+5x^2), {x, 0, 20}], x], x] (* Jean-François Alcover, May 24 2012 *)

CROSSREFS

Cf. A078009.

Sequence in context: A152107 A143023 A078009 * A113573 A083161 A077147

Adjacent sequences: A127845 A127846 A127847 * A127849 A127850 A127851

KEYWORD

easy,nonn

AUTHOR

Paul Barry, Feb 02 2007

STATUS

approved

URL: https://oeis.org/A127848

⇱ A127848 - OEIS