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%I #3 Mar 30 2012 16:49:30
%S 18,1800,352800,114307200,55324684800,37399486924800,
%T 33659538232320000,38910426196561920000,56186655427835412480000,
%U 99113260174701667614720000,209723658529668728672747520000,524309146324171821681868800000000
%N (2*n)!*binomial(2*n,n)/8.
%H Michel Bousquet, Gilbert Labelle and Pierre Leroux, <a href="http://www.lacim.uqam.ca/~leroux/articles/cartes2f.ps.gz">Enumeration of planar 2-face maps</a>, Discrete Mathematics, 222 (2000), 1-25.
%K nonn
%O 2,1
%A _N. J. A. Sloane_, Aug 02 2002
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