A186996 G.f. satisfies: A(x) = 1 + x*A(x) + x^2*A(x)^4.

1, 1, 2, 6, 20, 72, 273, 1073, 4333, 17869, 74937, 318601, 1370113, 5949201, 26046727, 114857599, 509669295, 2274146599, 10197234215, 45925646367, 207656685443, 942302814363, 4289903653615, 19588180438263, 89685571667763

Offset

0,3

Comments

Compare to a g.f. C(x) of Catalan numbers: C(x) = 1 + x*C(x) + x^2*C(x)^3.

Links

Robert Israel, Table of n, a(n) for n = 0..1000

Formula

G.f.: A(x) = (1/x)*Series_Reversion( 2*x^3/(1 - sqrt(1-4*x^2-4*x^3)) ).

G.f. satisfies: A(x) = (1 + x*A(x)) * (1 + x*A(x)^3) / (1 + x*A(x)^2).

Recurrence: 3*(n-2)*n*(2*n-5)*(3*n-2)*(3*n+2)*a(n) = 3*(2*n-5)*(2*n-1)*(18*n^3 - 54*n^2 + 36*n - 5)*a(n-1) + (2*n-3)*(94*n^4 - 564*n^3 + 1222*n^2 - 1128*n + 331)*a(n-2) + 3*(2*n - 5)*(2*n - 1)*(18*n^3 - 108*n^2 + 198*n - 103)*a(n-3) - 3*(n-3)*(n-1)*(2*n-1)*(3*n-11)*(3*n-7)*a(n-4). - Vaclav Kotesovec, Sep 10 2013

a(n) ~ c*d^n/n^(3/2), where d = (9 + 8*sqrt(3) + 4*sqrt(12 + 9*sqrt(3)))/9 = 4.874032512954972962... is the root of the equation 27 - 108*d - 94*d^2 - 108*d^3 + 27*d^4 = 0 and c = sqrt(11/(8*(-44 + sqrt(748 + 891*sqrt(3)))*Pi)) = 0.336422381089368230542882135982348331566666028... - Vaclav Kotesovec, Sep 10 2013, updated Apr 27 2015

a(n) = sum(k=0..n, binomial(3n-2k+1, k)*binomial(k,n-k)*1/(3n-2k+1)). - Michael D. Weiner, Apr 23 2015

G.f.: A(x) = hypergeom([1/4, 1/2, 3/4], [2/3, 4/3], (256/27)*x^2/(x-1)^4)/(1-x). - Robert Israel, Apr 28 2015

Example

G.f.: A(x) = 1 + x + 2*x^2 + 6*x^3 + 20*x^4 + 72*x^5 + 273*x^6 +...
Related expansions:
A(x)^2 = 1 + 2*x + 5*x^2 + 16*x^3 + 56*x^4 + 208*x^5 + 806*x^6 +...
A(x)^3 = 1 + 3*x + 9*x^2 + 31*x^3 + 114*x^4 + 438*x^5 + 1739*x^6 +...
A(x)^4 = 1 + 4*x + 14*x^2 + 52*x^3 + 201*x^4 + 800*x^5 + 3260*x^6 +...

Maple

S:= series(RootOf(y - 1 - x*y - x^2*y^4, y), x, 101):
seq(coeff(S, x, i), i=0..100); # Robert Israel, Apr 23 2015

Mathematica

nmax=20; aa=ConstantArray[0, nmax]; aa[[1]]=1; Do[AGF=1+Sum[aa[[n]]*x^n, {n, 1, j-1}]+koef*x^j; sol=Solve[Coefficient[1 + x*AGF + x^2*AGF^4 - AGF, x, j]==0, koef][[1]]; aa[[j]]=koef/.sol[[1]], {j, 2, nmax}]; Flatten[{1, aa}] (* Vaclav Kotesovec, Sep 10 2013 *)
Table[Sum[Binomial[3n-2k+1, k]*Binomial[k, n-k]/(3n-2k+1), {k, 0, n}], {n, 0, 20}]  (* Vaclav Kotesovec, Apr 27 2015 after Michael D. Weiner *)

Prog

(PARI) {a(n)=local(A=1+x); for(i=1, n, A=1+x*A+x^2*(A+x*O(x^n))^4); polcoeff(A, n)}
(PARI) {a(n)=polcoeff((1/x)*serreverse(2*x^3/(1 - sqrt(1-4*x^2-4*x^3 +x^3*O(x^n)))), n)}

Crossrefs

Cf. A182454.

Sequence in context: A380211 A071356 A141200 * A186576 A272485 A122737

Adjacent sequences: A186993 A186994 A186995 * A186997 A186998 A186999

Keyword

nonn

Author

Paul D. Hanna, Mar 01 2011

Status

approved