OFFSET
1,2
REFERENCES
F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Cambridge, 1998, p. 113 (2.4.41).
FORMULA
Double e.g.f.: A(x, y) = Sum_{i, j>=0} (x^i*y^j/(i!*j!)) = log((1-x*y)^2/(1-x*y*(3+x+y))).
EXAMPLE
1 2 0 0 0 ...
2 4 36 24 0 ...
0 36 336 1296 2160 ...
0 24 1296 14832 80640 ...
0 0 2160 80640 1082880 ...
CROSSREFS
KEYWORD
nonn,tabl
AUTHOR
Christian G. Bower, Jan 12 2004
STATUS
approved