A143554 - OEIS

%I #24 Feb 08 2024 20:45:16

%S 1,1,1,5,9,55,117,775,1785,12350,29799,211876,527085,3818430,9706503,

%T 71282640,184138713,1366368375,3573805950,26735839650,70625252863,

%U 531838637759,1416298046436,10723307329700,28748759731965,218658647805780,589546754316126

%N G.f. satisfies: A(x) = 1 + x*A(x)^5*A(-x)^4.

%C Number of achiral noncrossing partitions composed of n blocks of size 9. - _Andrew Howroyd_, Feb 08 2024

%H Andrew Howroyd, <a href="/A143554/b143554.txt">Table of n, a(n) for n = 0..500</a>

%H Michel Bousquet and Cédric Lamathe, <a href="https://doi.org/10.46298/dmtcs.420">On symmetric structures of order two</a>, Discrete Math. Theor. Comput. Sci. 10 (2008), 153-176. See Table 1. - From _N. J. A. Sloane_, Jul 12 2011

%F G.f. satisfies: A(x) = [A(x)*A(-x)] + x*[A(x)*A(-x)]^5.

%F G.f. satisfies: A(x)*A(-x) = (A(x) + A(-x))/2 = G(x^2) where G(x) = 1 + x*G(x)^9 is the g.f. of A062994.

%F a(2n) = binomial(9*n,n)/(8*n+1); a(2n+1) = binomial(9*n+4,n)*5/(8*n+5).

%e G.f.: A(x) = 1 + x + x^2 + 5*x^3 + 9*x^4 + 55*x^5 + 117*x^6 + 775*x^7 +...

%e Let G(x) = 1 + x*G(x)^9 be the g.f. of A062994, then

%e G(x^2) = A(x)*A(-x) and A(x) = G(x^2) + x*G(x^2)^5 where

%e G(x) = 1 + x + 9*x^2 + 117*x^3 + 1785*x^4 + 29799*x^5 + 527085*x^6 +...

%e G(x)^5 = 1 + 5*x + 55*x^2 + 775*x^3 + 12350*x^4 + 211876*x^5 +...

%t terms = 25;

%t A[_] = 1; Do[A[x_] = 1 + x A[x]^5 A[-x]^4 + O[x]^terms // Normal, {terms}];

%t CoefficientList[A[x], x] (* _Jean-François Alcover_, Jul 24 2018 *)

%o (PARI) {a(n)=my(A=1+x*O(x^n));for(i=0,n,A=1+x*A^5*subst(A^4,x,-x));polcoef(A,n)}

%o (PARI) {a(n)=my(m=n\2,p=4*(n%2)+1);binomial(9*m+p-1,m)*p/(8*m+p)}

%Y Column k=9 of A369929 and k=10 of A370062.

%Y Cf. A143338, A143546, A143547, A143550, A143551, A143552, A143553, A062994 (bisection).

%K nonn

%O 0,4

%A _Paul D. Hanna_, Aug 24 2008