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A160117 Number of "ON" cells after n-th stage in simple 2-dimensional cellular automaton (see Comments for precise definition). 12
0, 1, 9, 13, 41, 49, 101, 113, 189, 205, 305, 325, 449, 473, 621, 649, 821, 853, 1049, 1085, 1305, 1345, 1589, 1633, 1901, 1949, 2241, 2293, 2609, 2665, 3005, 3065, 3429, 3493, 3881, 3949, 4361, 4433, 4869, 4945, 5405, 5485, 5969, 6053, 6561, 6649, 7181, 7273 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Define "peninsula cell" to be the "ON" cell connected to the structure by exactly one of its vertices.

Define "bridge cell" to be the "ON" cell connected to two cells of the structure by exactly consecutive two of its vertices.

On the infinite square grid, we start at stage 0 with all cells in OFF state. At stage 1, we turn ON a single cell, in the central position.

In order to construct this sequence we use the following rules:

- If n is even, we turn "ON" the cells around the cells turned "ON" at the generation n-1.

- If n is odd, we turn "ON" the possible bridge cells and the possible peninsula cells.

- Everything that is already ON remains ON.

A160411, the first differences, gives the number of cells turned "ON" at n-th stage.

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..1000

David Applegate, Omar E. Pol and N. J. A. Sloane, The Toothpick Sequence and Other Sequences from Cellular Automata

N. J. A. Sloane, Catalog of Toothpick and Cellular Automata Sequences in the OEIS

Index entries for sequences related to cellular automata

FORMULA

a(2n) = 5 + 2n(7n-5) for n>=1, a(2n+1) = 5 + 2n(7n-3) for n>=1. - Nathaniel Johnston, Nov 06 2010

G.f.: x*(x^2+1)*(4*x^3+x^2+8*x+1)/((x+1)^2*(1-x)^3). - Alois P. Heinz, Sep 16 2011

EXAMPLE

If we label the generations of cells turned ON by consecutive numbers we get the cell pattern shown below:

9...9...9...9...9

.888.888.888.888.

.878.878.878.878.

.886668666866688.

9..656.656.656..9

.886644464446688.

.878.434.434.878.

.886644222446688.

9..656.212.656..9

.886644222446688.

.878.434.434.878.

.886644464446688.

9..656.656.656..9

.886668666866688.

.878.878.878.878.

.888.888.888.888.

9...9...9...9...9

At the first generation, only the central "1" is ON, so a(1) = 1. At the second generation, we turn ON eight cells around the central cell, leading to a(2) = a(1)+8 = 9. At the third generation, we turn ON four peninsula cells, so a(3) = a(2)+4 = 13. At the fourth generation, we turn ON the cells around the cells turned ON at the third generation, so a(4) = a(3)+28 = 41. At the 5th generation, we turn ON four peninsula cells and four bridge cells, so a(5) = a(4)+8 = 49.

MAPLE

a:= proc(n) local r;

      r:= irem(n, 2);

      `if`(n<2, n, 5+(n-r)*((7*n-3*r)/2-5))

    end:

seq(a(n), n=0..80);  # Alois P. Heinz, Sep 16 2011

MATHEMATICA

a[0] = 0; a[1] = 1; a[n_] := If[EvenQ[n], (7n^2 - 10n + 10)/2, (7n^2 - 20n + 23)/2]; Table[a[n], {n, 0, 80}] (* Jean-Fran├žois Alcover, Jul 16 2015, after Nathaniel Johnston *)

CROSSREFS

Cf. A139250, A139251, A147562, A160118, A160119, A160379.

Sequence in context: A200537 A160118 A188343 * A068984 A146016 A146128

Adjacent sequences:  A160114 A160115 A160116 * A160118 A160119 A160120

KEYWORD

nonn

AUTHOR

Omar E. Pol, May 05 2009, May 15 2009

EXTENSIONS

a(10) - a(27) from Nathaniel Johnston, Nov 06 2010

a(28) - a(47) from Alois P. Heinz, Sep 16 2011

STATUS

approved

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Last modified October 17 19:36 EDT 2018. Contains 316293 sequences. (Running on oeis4.)