%I #26 May 15 2022 07:32:24
%S 1,6,45,378,3402,32076,312741,3127410,31899582,330595668,3471254514,
%T 36848701764,394807518900,4263921204120,46370143094805,
%U 507343918566690,5580783104233590,61682339573108100,684673969261499910,7629224228913856140,85308598196036755020
%N G.f.: c(3x)^2, c(x) the g.f. of A000108.
%H Samuele Giraudo, <a href="http://arxiv.org/abs/1603.01394">Pluriassociative algebras II: The polydendriform operad and related operads</a>, arXiv:1603.01394 [math.CO], 2016.
%F G.f.: 4/(1+sqrt(1-12*x))^2.
%F a(n) = 3^n * A000108(n+1).
%F (n+2)*a(n) -6*(2*n+1)*a(n-1)=0. - _R. J. Mathar_, Nov 15 2011
%F O.g.f. A(x) = 1/x*series reversion( x/(1 + 3*x)^2 ). 1 + x*A'(x)/A(x) = 1/sqrt(1 - 12*x) is the o.g.f. for A098658. - _Peter Bala_, Jul 17 2015
%F From _Amiram Eldar_, May 15 2022: (Start)
%F Sum_{n>=0} 1/a(n) = 87/121 + 648*arcsin(1/(2*sqrt(3)))/(121*sqrt(11)).
%F Sum_{n>=0} (-1)^n/a(n) = 93/169 + 648*arcsinh(1/(2*sqrt(3)))/(169*sqrt(13)). (End)
%p Z[0]:=1: for k to 30 do Z[k]:=simplify(1/(1-3*z*Z[k-1])) od: g:=sum((Z[j]-Z[j-1]), j=1..30): gser:=series(g, z=0, 27): seq(coeff(gser, z, n)/3, n=1..19); # _Zerinvary Lajos_, May 21 2008
%t a[n_] := 3^n * CatalanNumber[n + 1]; Array[a, 20, 0] (* _Amiram Eldar_, May 15 2022 *)
%Y Cf. A050159, A101601, A101602, A098658.
%K easy,nonn
%O 0,2
%A _Paul Barry_, Dec 08 2004