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A101168
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Lengths of successive words (starting with a) under the substitution: {a -> aab, b -> aac, c -> a}.
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1
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1, 3, 9, 25, 71, 201, 569, 1611, 4561, 12913, 36559, 103505, 293041, 829651, 2348889, 6650121, 18827671, 53304473, 150914409, 427265435, 1209664161, 3424773601, 9696140959, 27451493281, 77720042081, 220039211683, 622970000809, 1763738467065, 4993456147431
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = 2*a(n-1) + 2*a(n-2) + a(n-3).
G.f.: (1+x+x^2) / (1-2*x-2*x^2-x^3).
a(n-1) = sum(k=1..n, sum(m=0..n-k, (sum(j=0..k, binomial(j, n-m-3*k+2*j) *binomial(k, j))) *sum(i=ceiling(m/2)..m, binomial(i, m-i)*binomial(k+i-1, k-1)))). - Vladimir Kruchinin, May 05 2011
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EXAMPLE
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a => aab => aabaabaac => aabaabaacaabaabaacaabaaba, thus a(0) = 1, a(1) = 3, a(2) = 9, a(3) = 25.
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MAPLE
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a:= n-> (<<0|1|0>, <0|0|1>, <1|2|2>>^n. <<1, 3, 9>>)[1, 1]:
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MATHEMATICA
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Length/@SubstitutionSystem[{a->{a, a, b}, b->{a, a, c}, c->a}, {a}, 15] (* The program generates the first 16 terms of the sequence. To generate more, increase the final ("15") constant. *) (* Harvey P. Dale, Sep 05 2022 *)
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PROG
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(Maxima) a(n):=b(n+1);
b(n):= sum(sum((sum(binomial(j, n+1-m-3*k+2*j) *binomial(k, j), j, 0, k)) *sum(binomial(i, m-i) *binomial(k+i-1, k-1), i, ceiling(m/2), m), m, 0, n+1-k), k, 1, n+1); /* Vladimir Kruchinin, May 05 2011 */
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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