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A036675 G.f. satisfies A(x) = 1 + x*A(x)^2*A(x^2). 2
1, 1, 2, 6, 18, 59, 198, 690, 2450, 8878, 32632, 121518, 457262, 1736526, 6646340, 25613086, 99298674, 387021728, 1515594560, 5960406102, 23530528512, 93216984177, 370450977206, 1476458287082, 5900150928510, 23635544130948 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Vaclav Kotesovec, Table of n, a(n) for n = 0..1000

FORMULA

G.f.: 1/(1-z/(1-z/(1-z/(...)))) where z=x/(1-x^2/(1-x^2)) (continued fraction); more generally g.f. C(x/(1-x^2/(1-x^2))) where C(x) is the g.f. for the Catalan numbers (A000108). [Joerg Arndt, Mar 18 2011]

a(n) ~ c * d^n / n^(3/2), where d = 4.250770453055989899189676464071962617426..., c = 0.600960911911396921862654605015399962... . - Vaclav Kotesovec, Aug 10 2014

a(n) = T(2*n+1,1), where T(n,m) = sum(i=1..n-m, (m*binomial(m+2*i-1,i))/(m+i)*((1+(-1)^(n-m))/2)*T((n-m)/2,i)), n>m, T(n,n)=1. - Vladimir Kruchinin, Mar 18 2015

MAPLE

A := 1; f := proc(n) global A; coeff(series( 1+x*(A*subs(x=x^2, A)), x, n+1), x, n); end; for n from 1 to 50 do A := series(A+f(n)*x^n, x, n +1); od: A;

MATHEMATICA

terms = 26; A[_] = 0; Do[A[x_] = 1 + x*A[x]^2*A[x^2] + O[x]^terms // Normal, terms]; CoefficientList[A[x], x] (* Jean-Fran├žois Alcover, Jan 15 2018 *)

PROG

(PARI) a(n)=local(A, m); if(n<0, 0, m=2; A=1+O(x); while(m<=n+1, m*=2; A=2/(1+sqrt(1-4*x*subst(A, x, x^2)))); polcoeff(A, n))

(Maxima)

T(n, m):=if m=n then 1 else sum((m*binomial(m+2*i-1, i))/(m+i)*((1+(-1)^(n-m))/2)*T((n-m)/2, i), i, 1, n-m);

makelist(T(2*n+1, 1), n, 0, 30); /* Vladimir Kruchinin, Mar 18 2015 */

CROSSREFS

Cf. A000621, A101913

Sequence in context: A150041 A190790 A150042 * A227373 A121320 A148460

Adjacent sequences:  A036672 A036673 A036674 * A036676 A036677 A036678

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified May 28 17:37 EDT 2020. Contains 334684 sequences. (Running on oeis4.)