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%I M4223 #15 Dec 05 2017 10:14:05
%S 1,6,36,216,1296,7776,46440,276054,1633848,9633366,56616132,331847118,
%T 1940715960,11327957196,66010769382,384094025382,2231978658906
%N Walks on hexagonal lattice using each point at most three times.
%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) = 56616140, a(11) = 331847200, and a(12) = 1940717000. - _Sean A. Irvine_, Dec 03 2017
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A. J. Guttmann, C. Byrnes and N. E. Frankel, <a href="https://doi.org/10.1088/0305-4470/17/9/001">A generalized self-avoiding walk</a>, J. Phys. A 17 (1984), L457-L461.
%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, A007274.
%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 03 2017