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A108136
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a(1)=1; a(2)=1; a(3)=1; a(n) = 3*a(n-1) + 2*a(n-2) + a(n-3).
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0
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1, 1, 1, 6, 21, 76, 276, 1001, 3631, 13171, 47776, 173301, 628626, 2280256, 8271321, 30003101, 108832201, 394774126, 1431989881, 5194350096, 18841804176, 68346102601, 247916266251, 899282808131, 3262027059496, 11832563061001
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OFFSET
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1,4
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COMMENTS
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Length of steps in the 3-symbol substitution 1->{2}, 2->{3}, 3->{1,2,2,3,3,3} with characteristic polynomial: x^3 - 3*x^2 - 2*x - 1.
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LINKS
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FORMULA
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G.f.: x*(1 - 2*x - 4*x^2)/(1 - 3*x - 2*x^2 - x^3).
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MATHEMATICA
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LinearRecurrence[{3, 2, 1}, {1, 1, 1}, 30]
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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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STATUS
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approved
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