The Toothpick Sequence and Other Sequences from Cellular Automata
Abstract
A twodimensional 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 yaxis. At each subsequent stage, for every exposed toothpick end, place an orthogonal toothpick centered at that end. The resulting structure has a fractallike 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^k1}+2x^{2^k}) and variations thereof.
 Publication:

arXiv eprints
 Pub Date:
 April 2010
 arXiv:
 arXiv:1004.3036
 Bibcode:
 2010arXiv1004.3036A
 Keywords:

 Mathematics  Combinatorics;
 11B85
 EPrint:
 28 pages, 21 figures. Minor improvements Oct 2, 2010, now 36 pages