This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A007239 M2562 #18 Feb 14 2022 07:31:39 %S A007239 3,6,12,24,54,138,378,1080,3186,9642,29784,93552,297966,960294, %T A007239 3126408,10268688,33989388,113277582,379833906,1280618784,4339003044, %U A007239 14767407522,50464951224,173099580168,595786322292,2057106617226,7123467773790,24734460619704 %N A007239 Energy function for hexagonal lattice. %C A007239 The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice. %D A007239 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 A007239 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A007239 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 A007239 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 A007239 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 A007239 See Eq. (32) of Sykes for the g.f. U(v). - _Andrey Zabolotskiy_, Feb 14 2022 %Y A007239 Cf. A002908. %K A007239 nonn %O A007239 1,1 %A A007239 _Simon Plouffe_ %E A007239 Terms a(21) and beyond from _Andrey Zabolotskiy_, Feb 14 2022