%I #7 Mar 30 2012 16:47:39

%S 1,30,2940,612360,221371920,123506903520,98513415000000,

%T 106466143825641600,149822417112092524800,266259578976734741798400,

%U 583256031964344606759091200,1544112027465820312500000000000,4860420955731492909364541975040000

%N Line-labeled 2-trees with n nodes.

%D F. Harary and E. Palmer, Graphical Enumeration, (1973), p. 30, Problem 1.13(c).

%H <a href="/index/Tra#trees">Index entries for sequences related to trees</a>

%p [ seq((2*n-3)!*(2*n-3)^(n-4)/(2*(n-2)!), n=3..20) ];

%K nonn

%O 3,2

%A _N. J. A. Sloane_.