

A247666


Number of ON cells after n generations of "OddRule" cellular automaton on hexagonal lattice based on 7celled neighborhood.


5



1, 7, 7, 25, 7, 49, 25, 103, 7, 49, 49, 175, 25, 175, 103, 409, 7, 49, 49, 175, 49, 343, 175, 721, 25, 175, 175, 625, 103, 721, 409, 1639, 7, 49, 49, 175, 49, 343, 175, 721, 49, 343, 343, 1225, 175, 1225, 721, 2863, 25, 175, 175, 625
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OFFSET

0,2


COMMENTS

The neighborhood of a cell consists of the cell itself together with its six surrounding cells. A cell is ON at generation n iff an odd number of its neighbors were ON at the previous generation. We start with one ON cell.
This is the Run Length Transform of the sequence 1,7,25,103,409,1639,26215,... (almost certainly A102900).
This appears to be the same as the number of ON cells in a certain 2D CA on the square grid in which the neighborhood of a cell is defined by f = 1/(x*y)+1/x+1/x*y+1/y+x/y+x+x*y, and in which a cell is ON iff there was an odd number of ON cells in the neighborhood at the previous generation. Here is the neighborhood:
[X, 0, X]
[X, 0, X]
[X, X, X]
which contains a(1) = 7 ON cells.
This is the oddrule cellular automaton defined by OddRule 557 (see EkhadSloaneZeilberger "OddRule Cellular Automata on the Square Grid" link).
Furthermore, this is also the number of ON cells in the 2D CA on the square grid in which the neighborhood of a cell is defined by f = 1/(x*y)+1/x+1/y+1+y+x+x*y, with the same rule. Here is the neighborhood:
[0, X, X]
[X, X, X]
[X, X, 0]
 N. J. A. Sloane, Feb 19 2015
This is the oddrule cellular automaton defined by OddRule 376 (see EkhadSloaneZeilberger "OddRule Cellular Automata on the Square Grid" link).
The partial sums are in A253767 in which the structure looks like an irregular step pyramid, apparently with a likehexagonal base.  Omar E. Pol, Jan 29 2015


LINKS

Table of n, a(n) for n=0..51.
Shalosh B. Ekhad, N. J. A. Sloane, and Doron Zeilberger, A MetaAlgorithm for Creating Fast Algorithms for Counting ON Cells in OddRule Cellular Automata, arXiv:1503.01796 [math.CO], 2015; see also the Accompanying Maple Package.
Shalosh B. Ekhad, N. J. A. Sloane, and Doron Zeilberger, OddRule Cellular Automata on the Square Grid, arXiv:1503.04249 [math.CO], 2015.
N. J. A. Sloane, Illustrations of generations 0 to 4
N. J. A. Sloane, On the No. of ON Cells in Cellular Automata, Video of talk in Doron Zeilberger's Experimental Math Seminar at Rutgers University, Feb. 05 2015: Part 1, Part 2
N. J. A. Sloane, On the Number of ON Cells in Cellular Automata, arXiv:1503.01168 [math.CO], 2015.
Index entries for sequences related to cellular automata


FORMULA

a(n) = number of terms in expansion of f^n mod 2, where f = 1+1/x+x+1/y+y+1/(x*y)+x*y (mod 2);


EXAMPLE

From Omar E. Pol, Jan 29 2015: (Start)
May be arranged into blocks of sizes A011782:
1;
7;
7, 25;
7, 49, 25, 103;
7, 49, 49, 175, 25, 175, 103, 409;
7, 49, 49, 175, 49, 343, 175, 721, 25, 175, 175, 625, 103, 721, 409, 1639;
7, 49, 49, 175, 49, 343, 175, 721, 49, 343, 343, 1225, 175, 1225, 721, 2863, 25, 175, 175, 625, ...
It appears that right border gives A102900 without repetitions, see Comments section. [This is just a restatement of the fact that this sequence is the run length transform of what is presumably A102900.  N. J. A. Sloane, Feb 06 2015]
(End)
From Omar E. Pol, Mar 19 2015: (Start)
Also, the sequence can be written as an irregular tetrahedron T(s,r,k) as shown below:
1;
..
7;
..
7;
25;
.........
7, 49;
25;
103;
...................
7, 49, 49, 175;
25, 175;
103;
409;
......................................
7, 49, 49, 175, 49, 343, 175, 721;
25, 175, 175, 625;
103, 721;
409;
1639;
...
Apart from the initial 1, we have that T(s,r,k) = T(s+1,r,k).
(End)


MAPLE

C := f>`if`(type(f, `+`), nops(f), 1);
f := 1+1/x+x+1/y+y+1/(x*y)+x*y;
g := n>expand(f^n) mod 2;
[seq(C(g(n)), n=0..100)];


MATHEMATICA

A247666[n_] := Total[CellularAutomaton[{170, {2, {{1, 1, 0}, {1, 1, 1}, {0, 1, 1}}}, {1, 1}}, {{{1}}, 0}, {{{n}}}], 2]; Array[A247666, 52, 0] (* JungHwan Min, Sep 01 2016 *)
A247666L[n_] := Total[#, 2] & /@ CellularAutomaton[{170, {2, {{1, 1, 0}, {1, 1, 1}, {0, 1, 1}}}, {1, 1}}, {{{1}}, 0}, n]; A247666L[51] (* JungHwan Min, Sep 01 2016 *)


CROSSREFS

Cf. A102900, A071053, A160239, A247640.
Sequence in context: A289378 A289409 A290520 * A255279 A230496 A295733
Adjacent sequences: A247663 A247664 A247665 * A247667 A247668 A247669


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Sep 22 2014


STATUS

approved



