%I M4043 #21 Aug 07 2020 02:11:20
%S 1,0,0,6,0,30,24,168,288,1170,2760,9504,25776,84006,244104,779970,
%T 2356800,7487310,23167656,73639326,231348000,737718534,2341772136,
%U 7498417956,23982456384,77119604400,248088123192,800978437674,2588744704560,8388692108016,27217694697000
%N Number of 2n-step polygons on honeycomb.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H M. E. Fisher and M. F. Sykes, <a href="http://dx.doi.org/10.1103/PhysRev.114.45">Excluded-volume problem and the Ising model of ferromagnetism</a>, Phys. Rev. 114 (1959), 45-58.
%F a(n) = 2 * n * A006774(n), n > 1. - _Sean A. Irvine_, Aug 06 2020
%Y Cf. A001335.
%K nonn
%O 2,4
%A _N. J. A. Sloane_.
%E a(11)-a(21) from _Sean A. Irvine_, Jun 12 2016
%E a(0)-a(1) inserted and more terms from _Sean A. Irvine_, Aug 06 2020