%I M2257 #22 Sep 07 2022 14:55:59
%S 1,0,0,1,0,3,2,12,18,65,138,432,1074,3231,8718,25999,73650,220215,
%T 643546,1937877,5783700,17564727,53222094,163009086,499634508,
%U 1542392088,4770925446,14832934031,46227584010,144632622552,453628244950
%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 I. Jensen, <a href="/A006774/b006774.txt">Table of n, a(n) for n = 0..79</a>
%H I. G. Enting and A. J. Guttmann, <a href="https://www.researchgate.net/publication/230900049_Polygons_on_the_honeycomb_lattice">Polygons on the honeycomb lattice</a>, J. Phys. A 22 (1989), 1371-1384.
%H I. Jensen, <a href="http://www.ms.unimelb.edu.au/~iwan/polygons/series/tri.perim.rgs.ser">More terms</a>
%F a(n) = A005396(n) / (2*n), n > 1. - _Sean A. Irvine_, Aug 06 2020
%K nonn
%O 0,6
%A _N. J. A. Sloane_
%E a(0)-a(2) inserted by _Sean A. Irvine_, Aug 06 2020