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A252696
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Number of strings of length n over a 3-letter alphabet that do not begin with a nontrivial palindrome.
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9
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0, 3, 6, 12, 30, 78, 222, 636, 1878, 5556, 16590, 49548, 148422, 444630, 1333254, 3997884, 11991774, 35969766, 107903742, 323694636, 971067318, 2913152406, 8739407670, 26218074588, 78654075342, 235961781396, 707884899558, 2123653365420, 6370958763006
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OFFSET
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0,2
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COMMENTS
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A nontrivial palindrome is of length at least 2.
3 divides a(n) for all n.
lim n -> infinity a(n)/3^n ~ 0.278489919882115 is the probability that a random, infinite string over a 3-letter alphabet does not begin with a palindrome.
This sequence gives the number of walks on K_3 with loops that do not begin with a palindromic sequence.
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LINKS
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FORMULA
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a(2n) = k*a(2n-1) - a(n) for n >= 1; a(2n+1) = k*a(2n) - a(n+1) for n >= 1. - Jeffrey Shallit, Jun 09 2019
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EXAMPLE
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For n = 3, the first 10 of the a(3) = 12 solutions are (in lexicographic order) 011, 012, 021, 022, 100, 102, 120, 122, 200, 201.
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MATHEMATICA
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b[0] = 0; b[1] = 0; b[n_] := b[n] = 3*b[n-1] + 3^Ceiling[n/2] - b[Ceiling[n/2]]; a[n_] := 3^n - b[n]; a[0] = 0; Table[a[n], {n, 0, 28}] (* Jean-François Alcover, Jan 19 2015 *)
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PROG
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(Ruby) seq = [1, 0]; (2..N).each { |i| seq << 3 * seq[i-1] + 3**((i+1)/2) - seq[(i+1)/2] }; seq = seq.each_with_index.collect { |a, i| 3**i - a }
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CROSSREFS
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A248122 gives the number of strings of length n over a 3 letter alphabet that DO begin with a palindrome.
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KEYWORD
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easy,nonn,walk
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AUTHOR
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STATUS
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approved
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