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%I M4222 #14 Feb 02 2022 16:06:29
%S 1,6,36,216,1260,7206,40650,227256,1262832,6983730,38470224,211220814,
%T 1156489782,6317095284,34435495872,187380150468,1018035642054
%N Walks on hexagonal lattice using each point at most twice.
%C The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice.
%C Guttmann et al. has incorrect a(10) = 38470220, a(11) = 211220800, a(12) = 1156490000. - _Sean A. Irvine_, Dec 02 2017
%D A. J. Guttmann, C. Byrnes and N. E. Frankel, A generalized self-avoiding walk, J. Phys. A 17 (1984), L457-L461.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H G. Nebe and N. J. A. Sloane, <a href="http://www.math.rwth-aachen.de/~Gabriele.Nebe/LATTICES/A2.html">Home page for hexagonal (or triangular) lattice A2</a>
%Y Cf. A001334, A007275.
%K nonn,walk
%O 0,2
%A _Simon Plouffe_
%E Title improved, a(10)-a(12) corrected, and a(15)-a(16) added by _Sean A. Irvine_, Dec 02 2017