The Toothpick Sequence and Other Sequences from Cellular Automata
Abstract
A two-dimensional arrangement of toothpicks is constructed by the following iterative procedure. At stage 1, place a single toothpick of length 1 on a square grid, aligned with the y-axis. At each subsequent stage, for every exposed toothpick end, place an orthogonal toothpick centered at that end. The resulting structure has a fractal-like appearance. We will analyze the toothpick sequence, which gives the total number of toothpicks after n steps. We also study several related sequences that arise from enumerating active cells in cellular automata. Some unusual recurrences appear: a typical example is that instead of the Fibonacci recurrence, which we may write as a(2+i) = a(i) + a(i+1), we set n = 2^k+i (0 <= i < 2^k), and then a(n)=a(2^k+i)=2a(i)+a(i+1). The corresponding generating functions look like Prod{k >= 0} (1+x^{2^k-1}+2x^{2^k}) and variations thereof.
- Publication:
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arXiv e-prints
- Pub Date:
- April 2010
- DOI:
- 10.48550/arXiv.1004.3036
- arXiv:
- arXiv:1004.3036
- Bibcode:
- 2010arXiv1004.3036A
- Keywords:
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- Mathematics - Combinatorics;
- 11B85
- E-Print:
- 28 pages, 21 figures. Minor improvements Oct 2, 2010, now 36 pages