



1, 13, 25, 109, 121, 193, 325, 493, 529, 661, 829, 1129, 1189, 1405, 1657, 2101, 2149, 2281, 2533, 3133, 3337, 3709, 4309, 4909, 5065, 5449, 5917, 6757, 6877, 7381, 7873, 8845, 8893, 9025, 9277, 9877, 10165, 10849, 11737
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

0,2


COMMENTS

Also the number of ON cells after n generations in a knight'smove, oneneighbor, accumulative cellular automaton on the hexagonal lattice A_2. Define v(m)=2*sqrt(3)*[cos(m*Pi/3+Pi/6), sin(m*Pi/3+Pi/6)], vL(m)=2*v(m)+v(m+1), vR(m)=2*v(m)+v(m1). The set of "knight's moves", M={vL(m):m=1,2,..6} U {vR(m):m=1,2,..6}, follows from an analogy between Z^2 and A_2. At each generation all ON cells remain ON while an OFF cell turns ON if and only if it has exactly one Mneighbor in the previous generation.
Fractal Structure Theorem (FST). A pair of lattice vectors M={v1,v2} generate a wedge, W = {x*v1 + y*v2 : x>=0, y>=0}. Define WSubsets T_k such that T_{k+1}= T_k U { 2^n*v1 + v : v in T_k } U {2^n*v2 + v : v in T_k}, T_0 = { [0,0] }. The limit set T_{oo} is a fractal, and acquires the topology of a binary tree when points are connected by either v1 or v2. As a tree, T_k has height 2^k1, with 2^k vertices at maximum depth, along a line in the direction v1v2. Assume a oneMneighbor, accumulative cellular automaton on W, where all vertices in T_k are ON. In the next generation, the front F_k={2^k*v1+m*(v2v1) : 0<=m<=2^k} contains only two ON cells, {2^k*v1,2^k*v2}. The spacing, 2^k1, is wide enough to turn ON two copies of T_k, one starting from each of the two ON cells in F_k. Thus T_{k+1} is also ON. Whenever only T_0 is ON as an initial condition, by induction, T_{oo} is ultimately ON.
The FST applies here to 12 distinct wedges: with {v1,v2}={vL(m), vR(m)} or with (v1,v2)={vL(m), vR(m+1)}, and m=1,2,..6. The triangle inequality ensures that paths including other vectors cannot reach the front F_k by generation 2^k. However, other vectors do generate retrogressive growth, which turns ON many additional cells.
The FST applies to a wide range of Cellular Automata. Wolfram's onedimensional rule 90 gives the most elementary example where T_{oo} determines every ON cell. The tree structure T_{oo} also occurs with two dimensional, accumulative, oneneighbor C.A. such as A151723, A319018, A147562. Also try: M={[0,1],[0,1],[2,1],[2,1]}.
According to S. Ulam (Cf. Links), some version of the FST was already known to J. Holladay circa 1960.
The FST implies scale resonance between this cellular automaton and the arrowed half hexagon tiling (Cf. Links).


LINKS

Table of n, a(n) for n=0..38.
Bradley Klee, LogPeriodic Coloring over Arrowed Half Hexagon tiling.
Bradley Klee, LogPeriodic Coloring to Stage 64.
Bradley Klee, T_n Tree Structure, n=1,2,3,4.
Bradley Klee, LimitPeriodic Tilings, Wolfram Demonstrations Project (2015).
S. M. Ulam, On some mathematical problems connected with patterns of growth of figures, pp. 216 of R. E. Bellman, ed., Mathematical Problems in the Biological Sciences, Proc. Sympos. Applied Math., Vol. 14, Amer. Math. Soc., 1962 [Annotated scanned copy]


MATHEMATICA

HexStar=2*Sqrt[3]*{Cos[#*Pi/3+Pi/6], Sin[#*Pi/3+Pi/6]}&/@Range[0, 5];
MoveSet=Join[2*HexStar+RotateRight[HexStar], 2*HexStar+RotateLeft[HexStar]];
Clear@Pts; Pts[0] = {{0, 0}};
Pts[n_]:=Pts[n]=With[{pts=Pts[n1]}, Union[pts, Cases[Tally[Flatten[pts/.{x_, y_}:> Evaluate[{x, y}+#&/@MoveSet], 1]], {x_, 1}:>x]]]; Length[Pts[#]]&/@Range[0, 32]


CROSSREFS

Hexagonal: A151723. Square: A319018, A147562. Tree: A006046, A267700, A038573. A322663.
Sequence in context: A283174 A298037 A283255 * A151776 A301327 A116524
Adjacent sequences: A322659 A322660 A322661 * A322663 A322664 A322665


KEYWORD

nonn


AUTHOR

Bradley Klee, Dec 22 2018


STATUS

approved



