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A266249
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Number of ON (black) cells in the n-th iteration of the "Rule 9" elementary cellular automaton starting with a single ON (black) cell.
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1
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1, 0, 2, 2, 4, 5, 4, 9, 4, 13, 4, 17, 4, 21, 4, 25, 4, 29, 4, 33, 4, 37, 4, 41, 4, 45, 4, 49, 4, 53, 4, 57, 4, 61, 4, 65, 4, 69, 4, 73, 4, 77, 4, 81, 4, 85, 4, 89, 4, 93, 4, 97, 4, 101, 4, 105, 4, 109, 4, 113, 4, 117, 4, 121, 4, 125, 4, 129, 4, 133, 4, 137
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OFFSET
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0,3
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REFERENCES
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S. Wolfram, A New Kind of Science, Wolfram Media, 2002; p. 55.
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LINKS
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FORMULA
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Conjectures from Colin Barker, Dec 28 2015 and Apr 14 2019: (Start)
a(n) = (-2*(-1)^n*n+2*n+9*(-1)^n-1)/2 for n>3.
a(n) = 2*a(n-2)-a(n-4) for n>7.
G.f.: (1+2*x^3+x^4+x^5-2*x^6+x^7) / ((1-x)^2*(1+x)^2).
(End)
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MATHEMATICA
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rule=9; rows=20; ca=CellularAutomaton[rule, {{1}, 0}, rows-1, {All, All}]; (* Start with single black cell *) catri=Table[Take[ca[[k]], {rows-k+1, rows+k-1}], {k, 1, rows}]; (* Truncated list of each row *) Table[Total[catri[[k]]], {k, 1, rows}] (* Number of Black cells in stage n *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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