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%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_.
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