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A161830 Y-toothpick triangle (see Comments lines for definition). 9
0, 1, 3, 5, 9, 11, 15, 19, 27, 31, 35, 39, 47, 53, 61, 71, 89, 99, 103 (list; graph; refs; listen; history; text; internal format)



Y-toothpick sequence starting at the corner of an infinite hexagon in which its vertex touch an endpoint of the initial Y-toothpick and the two other endpoints are equidistant from the nearest sides of the hexagon.

The sequence gives the number of Y-toothpicks in the structure after n rounds. A161831 (the first differences) gives the number added at the n-th round.

See the Y-toothpick sequence A160120 for more information about the recursive, fractal-like structure.


Table of n, a(n) for n=0..18.

David Applegate, Omar E. Pol and N. J. A. Sloane, The Toothpick Sequence and Other Sequences from Cellular Automata, Congressus Numerantium, Vol. 206 (2010), 157-191. [There is a typo in Theorem 6: (13) should read u(n) = 4.3^(wt(n-1)-1) for n >= 2.]

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


Cf. A139250, A160120, A160121, A160406, A160407, A161831, A161832, A161833.

Sequence in context: A216091 A002731 A006046 * A151922 A233762 A104635

Adjacent sequences:  A161827 A161828 A161829 * A161831 A161832 A161833




Omar E. Pol, Jun 20 2009



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Last modified January 18 20:57 EST 2019. Contains 319282 sequences. (Running on oeis4.)