%I A004121 M2095
%S A004121 2,16,208,3968,109568,4793344,410662912,82657083392,38274970222592,
%T A004121 37590755515826176,75458309991776124928,305873605165090925969408,
%U A004121 2491832958314452159507202048,40704585435508852018947014262784
%N A004121 Generalized weak orders on n points.
%D A004121 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A004121 I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley 
               and Sons, N.Y., 1983.
%D A004121 C. G. Wagner, Enumeration of generalized weak orders. Arch. Math. (Basel) 
               39 (1982), no. 2, 147-152.
%F A004121 E.g.f.: 1/(1-Sum_{i >= 1} 2^binomial(i+1, 2)*x^i/i!).
%Y A004121 Cf. A004122, A004123, A000670 (asymmetric generalized weak orders on 
               n points)
%Y A004121 Sequence in context: A161568 A138429 A087923 this_sequence A114531 A012056 
               A062971
%Y A004121 Adjacent sequences: A004118 A004119 A004120 this_sequence A004122 A004123 
               A004124
%K A004121 nonn,nice,easy
%O A004121 1,1
%A A004121 N. J. A. Sloane (njas(AT)research.att.com).
%E A004121 Formula and more terms from Vladeta Jovovic (vladeta(AT)eunet.rs), Mar 
               27 2001