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%I M2562 #18 Feb 14 2022 07:31:39
%S 3,6,12,24,54,138,378,1080,3186,9642,29784,93552,297966,960294,
%T 3126408,10268688,33989388,113277582,379833906,1280618784,4339003044,
%U 14767407522,50464951224,173099580168,595786322292,2057106617226,7123467773790,24734460619704
%N Energy function for hexagonal lattice.
%C The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice.
%D C. Domb, Ising model, in Phase Transitions and Critical Phenomena, vol. 3, ed. C. Domb and M. S. Green, Academic Press, 1974; p. 386.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H C. Domb, <a href="/A007239/a007239.pdf">Ising model</a>, Phase Transitions and Critical Phenomena 3 (1974), 257, 380-381, 384-387, 390-391, 412-423. (Annotated scanned copy)
%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>
%H M. F. Sykes, <a href="https://doi.org/10.1063/1.1724212">Some counting theorems in the theory of the Ising problem and the excluded volume problem</a>, J. Math. Phys., 2 (1961), 52-62.
%F See Eq. (32) of Sykes for the g.f. U(v). - _Andrey Zabolotskiy_, Feb 14 2022
%Y Cf. A002908.
%K nonn
%O 1,1
%A _Simon Plouffe_
%E Terms a(21) and beyond from _Andrey Zabolotskiy_, Feb 14 2022
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