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%I #74 Dec 03 2019 07:15:35
%S 0,1,3,7,11,15,23,33,41,45,53,63,75,89,111,133,149,153,161,171,183,
%T 197,219,241,261,275,299,327,361,403,463,511,547,551,559,569,581,595,
%U 617,639,659,673,697,725,759,801,861,909,949,967,995,1029,1075,1125,1183,1233,1281,1321,1389,1465,1549,1657
%N "Concave pentagon" toothpick sequence (see Comments for precise definition).
%C This arises from a hybrid cellular automaton on a triangular grid formed of I-toothpicks (A160164) and V-toothpicks (A161206).
%C The surprising fact is that after 2^k stages the structure looks like a concave pentagon, which is formed essentially by an equilateral triangle (E) surrounded by two quadrilaterals (Q1 and Q2), both with their largest sides in vertical position, as shown below:
%C .
%C * *
%C * * * *
%C * * * *
%C * * *
%C * Q1 * Q2 *
%C * * * *
%C * * * *
%C * * * *
%C * * * *
%C * * E * *
%C * * * *
%C * * * *
%C ** **
%C * * * * * * * * * *
%C .
%C Note that for n >> 1 both quadrilaterals look like right triangles.
%C Every polygon has a slight resemblance to Sierpinsky's triangle, but here the structure is much more complex.
%C For the construction of the sequence the rules are as follows:
%C On the infinite triangular grid at stage 0 there are no toothpicks, so a(0) = 0.
%C At stage 1 we place an I-toothpick formed of two single toothpicks in vertical position, so a(1) = 1.
%C For the next n generation we have that:
%C If n is even then at every free end of the structure we add a V-toothpick, formed of two single toothpicks, with its central vertex directed upward, like a gable roof.
%C If n is odd then we add I-toothpicks in vertical position (see the example).
%C a(n) gives the total number of I-toothpicks and V-toothpicks in the structure after the n-th stage.
%C A327331 (the first differences) gives the number of elements added at the n-th stage.
%C 2*a(n) gives the total number of single toothpicks of length 1 after the n-th stage.
%C The structure contains many kinds of polygonal regions, for example: triangles, trapezes, parallelograms, regular hexagons, concave hexagons, concave decagons, concave 12-gons, concave 18-gons, concave 20-gons, and other polygons.
%C The structure is almost identical to the structure of A327332, but a little larger at the upper edge.
%C The behavior seems to suggest that this sequence can be calculated with a formula, in the same way as A139250, but that is only a conjecture.
%C The "word" of this cellular automaton is "ab". For more information about the word of cellular automata see A296612.
%C For another version, very similar, starting with a V-toothpick, see A327332, which it appears that shares infinitely many terms with this sequence.
%H N. J. A. Sloane, <a href="/wiki/Catalog_of_Toothpick_and_CA_Sequences_in_OEIS">Catalog of Toothpick and Cellular Automata Sequences in the OEIS</a>
%H <a href="/index/Ce#cell">Index entries for sequences related to cellular automata</a>
%H <a href="/index/To#toothpick">Index entries for sequences related to toothpick sequences</a>
%F Conjecture: a(2^k) = A327332(2^k), k >= 0.
%e Illustration of initial terms:
%e .
%e | /|\ |/|\|
%e | | | | |
%e / \ |/ \|
%e | |
%e n : 0 1 2 3
%e a(n): 0 1 3 7
%e After three generations there are five I-toothpicks and two V-toothpicks in the structure, so a(3) = 5 + 2 = 7 (note that in total there are 2*a(3) = 2*7 = 14 single toothpicks of length 1).
%Y First differs from A231348 at a(11).
%Y Cf. A047999, A139250 (normal toothpicks), A160164 (I-toothpicks), A160722 (a concave pentagon with triangular cells), A161206 (V-toothpicks), A296612, A323641, A323642, A327331 (first differences), A327332 (another version).
%Y For other hybrid cellular automata, see A194270, A194700, A220500, A289840, A290220, A294020, A294962, A294980, A299770, A323646, A323650.
%K nonn
%O 0,3
%A _Omar E. Pol_, Sep 01 2019