login
a(n) = floor( (2n-1)!!/(2n) ).
1

%I #6 Oct 10 2018 10:31:08

%S 0,0,2,13,94,866,9652,126689,1914412,32736453,624968662,13176422634,

%T 304071291562,7623501667031,206342778454312,5996836998828457,

%U 186254702081260312,6156752652130549218,215810382437839251562,7995774669321944270390,312215963278285442939062

%N a(n) = floor( (2n-1)!!/(2n) ).

%C Asymptotic estimate for A007769(n).

%H A. Khruzin, <a href="http://arXiv.org/abs/math.CO/0008209">Enumeration of chord diagrams</a>

%t Table[Floor[(2n-1)!!/(2n)],{n,20}] (* _Harvey P. Dale_, Nov 12 2012 *)

%K nonn

%O 1,3

%A _Ralf Stephan_, Mar 02 2005