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 A008657 #17 Oct 19 2018 08:06:08 %S A008657 1,36,540,4356,20556,60696,137916,325152,658476,1023012,1999080, %T A008657 3112560,4446828,7207992,10755936,13150296,20963052,27538056,33706908, %U A008657 47989008,64050696,70696224,103079952,124752096,142308684,189312156,237450312,248276484,344385504,397677816 %N A008657 Theta series of direct sum of 6 copies of hexagonal lattice. %C A008657 The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice. %D A008657 J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, p. 110. %H A008657 Seiichi Manyama, <a href="/A008657/b008657.txt">Table of n, a(n) for n = 0..10000</a> %H A008657 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> %t A008657 terms = 23; s = ((EllipticTheta[3, 0, q]^3 + EllipticTheta[3, Pi/3, q]^3 + EllipticTheta[3, 2 Pi/3, q]^3)/(3*EllipticTheta[3, 0, q^3]))^6 + O[q]^(2 terms); CoefficientList[s, q^2] (* _Jean-François Alcover_, Jul 08 2017, from LatticeData(A2) *) %Y A008657 Cf. A004016. %K A008657 nonn,easy %O A008657 0,2 %A A008657 _N. J. A. Sloane_ %E A008657 More terms from _Seiichi Manyama_, Oct 19 2018