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A135017
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a(n) is number of strings of length n that can be obtained by starting with abc and repeatedly doubling any substring in place and then discarding any string that contains two successive equal letters.
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4
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0, 0, 1, 0, 2, 1, 3, 5, 7, 15, 20, 48, 60, 156, 205, 489, 761, 1572, 2796, 5357, 10174, 19021, 37272, 69375, 137759, 258444, 513696, 976890, 1934900, 3727164, 7358675, 14316861, 28217028, 55288907, 108942267, 214462953, 422973649
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,5
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COMMENTS
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These strings may be regarded as the "primitive" strings among those enumerated by A135473.
Equals the inverse binomial transform of A135473.
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LINKS
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FORMULA
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Empirically, grows like 2^n.
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EXAMPLE
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n=3: abc
n=4: -
n=5: ababc, abcbc
n=6: abcabc
n=7: abababc, ababcbc, abcbcbc
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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