%I
%S 1,8,156,4896,212520,11793600,797448960,63606090240,5846743244160,
%T 608588457523200,70758332701056000,9088747467351552000,
%U 1278179579224720972800,195333707771834926694400
%N Number of labeled simplyrooted 2trees with n labeled vertices (i.e., n+2 vertices altogether; a simplyrooted 2tree is an externally rooted 2tree whose root edge belongs to exactly one triangle).
%D E. M. Palmer and R. C. Read, On the number of plane 2trees, J. London Math. Soc. (2), 6, 1973, 583592.
%F a(n) = (5n2)!/(4n1)!.
%F E.g.f. T = T(x) satisfies T(1T)^4 = x.
%p seq((5*n2)!/(4*n1)!,n=1..16);
%K nonn
%O 1,2
%A _Emeric Deutsch_, Jun 24 2006
