%I A094047
%S A094047 0,0,2,12,312,9600,416880,23879520,1749363840,159591720960,
%T A094047 17747520940800,2363738855385600,371511874881100800,
%U A094047 68045361697964851200,14367543450324474009600,3464541314885011705344000
%N A094047 a(n) is the number of arrangements of n couples around a round table 
               so that each person sits between two members of the opposite sex 
               and no couple is seated together.
%C A094047 Also, the number of Hamiltonian directed circuits in the crown graph 
               of order n.
%H A094047 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               CrownGraph.html">Crown Graph</a>
%H A094047 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               HamiltonianCircuit.html">Hamiltonian Circuit</a>
%F A094047 For n>1, a(n) = (-1)^n * 2 * (n-1)! + n! * SUM[j=0..n-1] (-1)^j * (n-j-1)! 
               * binomial(2*n-j-1,j).
%Y A094047 Cf. A114939, A137729.
%Y A094047 Sequence in context: A012425 A012422 A122767 this_sequence A091472 A156518 
               A012727
%Y A094047 Adjacent sequences: A094044 A094045 A094046 this_sequence A094048 A094049 
               A094050
%K A094047 nonn
%O A094047 1,3
%A A094047 Matthijs Coster (matthijs(AT)coster.demon.nl), Apr 29 2004
%E A094047 Better definition from Joel Lewis (jblewis(AT)post.harvard.edu), Jun 
               30 2007
%E A094047 Formula and further terms from Max Alekseyev (maxale(AT)gmail.com), Feb 
               10 2008