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A168344 G.f. A(x) satisfies: A(x) = G(x*A(x)) where A(x/G(x)) = G(x) = g.f. of A006664, which is the number of irreducible systems of meanders. 9
1, 1, 3, 15, 99, 773, 6743, 63591, 635307, 6634599, 71759983, 798563065, 9098321475, 105733563393, 1249676348391, 14986826364311, 182027688352427, 2235713532561779, 27732857308708571, 347064951865766607 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Number of b^* n-strand braids of length at most 2, see the Biane/Dehornoy reference. - Joerg Arndt, Jul 08 2014

LINKS

Table of n, a(n) for n=0..19.

Philippe Biane, Patrick Dehornoy, Dual Garside structure of braids and free cumulants of products, arXiv:1407.1604 [math.CO], (7-July-2014)

FORMULA

G.f.: A(x) = F(x/A(x)) where A(x*F(x)) = F(x) = g.f. of A001246, which is the squares of Catalan numbers.

G.f.: A(x) = x/Series_Reversion(x*F(x)) where F(x) = g.f. of A001246.

G.f.: A(x) = (1/x)*Series_Reversion(x/G(x)) where G(x) = g.f. of A006664.

EXAMPLE

G.f.: A(x) = 1 + x + 3*x^2 + 15*x^3 + 99*x^4 + 773*x^5 + 6743*x^6 +...

A(x) satisfies: A(x*F(x)) = F(x) = g.f. of A001246:

F(x) = 1 + x + 4*x^2 + 25*x^3 + 196*x^4 + 1764*x^5 + 17424*x^6 +...+ A000108(n)^2*x^n +...

A(x) satisfies: A(x/G(x)) = G(x) = g.f. of A006664:

G(x) = 1 + x + 2*x^2 + 8*x^3 + 46*x^4 + 322*x^5 + 2546*x^6 +...

MATHEMATICA

F[x_] = (Hypergeometric2F1[-1/2, -1/2, 1, 16x] - 1)/(4x);

A[x_] = x/InverseSeries[x F[x] + O[x]^21, x];

CoefficientList[A[x], x] (* Jean-Fran├žois Alcover, Jul 21 2018, from 2nd formula *)

PROG

(PARI) {a(n)=local(C_2=vector(n+1, m, (binomial(2*m-2, m-1)/m)^2)); polcoeff(x/serreverse(x*Ser(C_2)), n)}

CROSSREFS

Cf. A006664, A001246, A000108.

Cf. A168450 (variant). [From Paul D. Hanna, Nov 29 2009]

Sequence in context: A199416 A046635 A208426 * A091713 A156106 A111546

Adjacent sequences:  A168341 A168342 A168343 * A168345 A168346 A168347

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Nov 23 2009

EXTENSIONS

Typo in formula corrected by Paul D. Hanna, Nov 24 2009

STATUS

approved

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Last modified August 3 05:29 EDT 2020. Contains 336197 sequences. (Running on oeis4.)