%I A137749
%S A137749 1,30,13560,27785520,162030637440,2156625389318400,56857271240920550400,
%T A137749 2686506065987036477184000,211180868835057744408834048000,
%U A137749 26072812428113877344085395644416000,4829206317935252350431489703482654720000
%N A137749 Number of circular permutations of the multiset {1,1,2,2,...,2n,2n} (up 
               to rotations) with even distances between equal elements.
%F A137749 a(n) = ((2n)!^3 / n!^2 / 2^(2n) + (2n)!) / (4n).
%F A137749 a(n) = A137737(2n)
%Y A137749 Cf. A114938, A137729, A137730, A137737
%Y A137749 Sequence in context: A087216 A059049 A056071 this_sequence A007741 A089037 
               A116076
%Y A137749 Adjacent sequences: A137746 A137747 A137748 this_sequence A137750 A137751 
               A137752
%K A137749 nonn
%O A137749 1,2
%A A137749 Max Alekseyev (maxale(AT)gmail.com), Feb 09, 2008