%I #26 Mar 11 2020 13:24:17
%S 1,6,15,27,39,51,63,75,87,99,111,123,135,147,159,171,183,195,207,219,
%T 231,243,255,267,279,291,303,315,327,339,351,363,375,387,399,411,423,
%U 435,447,459,471,483,495,507,519,531,543,555,567,579,591,603,615,627,639
%N Coordination sequence for a hexavalent node in a chamfered version of the 3^6 triangular tiling of the plane.
%C Let E denote the lattice of Eisenstein integers u + v*w in the plane, with each point joined to its six neighbors. Here u and v are ordinary integers and w = (-1+sqrt(-3))/2 is a complex cube root of unity. Let theta = w - w^2 = sqrt(-3). Then theta*E is a sublattice of E of index 3 (Conway-Sloane, Fig. 7.2). The tiling considered in this sequence is obtained by replacing each node in theta*E by a small hexagon.
%D J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, 3rd. ed., 1993. See Fig. 7.2, page 199.
%H Colin Barker, <a href="/A316320/b316320.txt">Table of n, a(n) for n = 0..1000</a>
%H Rémy Sigrist, <a href="/A316320/a316320.png">Illustration of initial terms</a>
%H N. J. A. Sloane, <a href="/A316319/a316319.png">The graph of the tiling.</a> (The red dots indicate the nodes of the sublattice theta*E.)
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F a(n) = 12*n-9 = A017557(n-1) for n > 1.
%F From _Colin Barker_, Mar 11 2020: (Start)
%F G.f.: (1 + 3*x)*(1 + x + x^2) / (1 - x)^2.
%F a(n) = 2*a(n-1) - a(n-2) for n>3.
%F (End)
%o (PARI) Vec((1 + 3*x)*(1 + x + x^2) / (1 - x)^2 + O(x^50)) \\ _Colin Barker_, Mar 11 2020
%Y See A316319 for trivalent node.
%Y See A250120 for links to thousands of other coordination sequences.
%Y Cf. A017557, A008486.
%K nonn,easy
%O 0,2
%A _Rémy Sigrist_ and _N. J. A. Sloane_, Jul 01 2018
%E Terms a(15) and beyond from _Andrey Zabolotskiy_, Sep 30 2019