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 A275667 Number of ON cells after n generations in a 2-dimensional "Odd-Rule" cellular automaton on triangular tiling. 1
 1, 3, 7, 9, 7, 21, 25, 27, 7, 21, 49, 63, 25, 75, 103, 81, 7, 21, 49, 63, 49, 147, 175, 189, 25, 75, 175, 225, 103, 309, 409, 243, 7, 21, 49, 63, 49, 147, 175, 189, 49, 147, 343, 441, 175, 525, 721, 567, 25, 75, 175, 225, 175, 525, 625, 675, 103, 309, 721 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Each triangular tile has 3 neighbors. A cell is ON in a given generation if and only if there was an odd number of ON cells among the three nearest neighbors in the preceding generation. At the initial moment there is a single ON cell. Given pattern replicates after a number of generations which is a power of 2 when a(n) = 7. Number of cells on each even step minus one is divisible by 6. By analogy with the Ekhad, Sloane, Zeilberger link, one may suppose that using ternary expansion of n, recurrence relations for a(n) can be obtained and proved. From Andrey Zabolotskiy, Aug 04 2016: (Start) If the first conjecture from the Formula section is true then the fact that the right border of the triangle (see Example) gives A000244 follows directly from it. If the second conjecture is true then the numbers just before the right border give A102900. Since the 7 cells which are ON at the beginning of every row are farther and farther away from each other, the n-th term of a row (with offset 0) is a(n)*7 for not very large n. See also comments to A247666. (End) LINKS Kovba Alexey, Illustration for n = 0..5 Shalosh B. Ekhad, N. J. A. Sloane, and Doron Zeilberger, A Meta-Algorithm for Creating Fast Algorithms for Counting ON Cells in Odd-Rule Cellular Automata, arXiv:1503.01796 [math.CO], 2015. FORMULA a(0) = 1. Conjecture: a(2*t+1) = 3*a(t). Conjectures: a(8*t+6) = 3*a(4*t+2) + 4*a(2*t), a(8*t+2) = 3*a(4*t) + 4*a(2*t), a(4*t) = a(2*t). These conjectured formulas together give recurrent relations for a(n) for any n. Also, obviously a(2*n) = A247666(n). - Andrey Zabolotskiy, Aug 04 2016 EXAMPLE From Omar E. Pol, Aug 04 2016: (Start) Written as an irregular triangle in which the row lengths are the terms of A011782 the sequence begins: 1; 3; 7, 9; 7, 21, 25, 27; 7, 21, 49, 63, 25, 75, 103, 81; 7, 21, 49, 63, 49, 147, 175, 189, 25, 75, 175, 225, 103, 309, 409, 243; ... It appears that the right border gives A000244. (End) CROSSREFS Cf. A160239 (square tiling analog), A247640, A247666 (hexagonal tiling analogs). Cf. A000244, A011782, A102900. Sequence in context: A005534 A113014 A102520 * A122490 A179021 A096910 Adjacent sequences:  A275664 A275665 A275666 * A275668 A275669 A275670 KEYWORD nonn,tabf AUTHOR Kovba Alexey, Aug 04 2016 STATUS approved

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Last modified October 22 20:57 EDT 2018. Contains 316502 sequences. (Running on oeis4.)