OFFSET
1,1
REFERENCES
I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
C. G. Wagner, Enumeration of generalized weak orders. Arch. Math. (Basel) 39 (1982), no. 2, 147-152.
LINKS
T. D. Noe, Table of n, a(n) for n = 1..100
C. G. Wagner, Enumeration of generalized weak orders, Preprint, 1980. [Annotated scanned copy]
C. G. Wagner and N. J. A. Sloane, Correspondence, 1980
FORMULA
E.g.f. : 1/(2-exp(x)*exp(exp(x)-1)).
MATHEMATICA
With[{nn=20}, Rest[CoefficientList[Series[1/(2-Exp[x]Exp[Exp[x]-1]), {x, 0, nn}], x] Range[0, nn]!]] (* Harvey P. Dale, Nov 05 2011 *)
CROSSREFS
KEYWORD
nonn,nice,easy
AUTHOR
EXTENSIONS
Formula and more terms from Vladeta Jovovic, Mar 27 2001
STATUS
approved