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A118311
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Number of dissimilar squarefree quaternary words of length n.
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1
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1, 1, 2, 4, 11, 29, 77, 202, 532, 1395, 3664, 9605, 25192, 66047, 173183, 453998, 1190259, 3120294, 8180124, 21444290, 56217025, 147373441, 386342414, 1012799936, 2655067412
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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COMMENTS
| Sherman Stein and A006156 count ordered squarefree(twin-free) ternary words. A060688 counts the dissimilar cases essentially by dividing by 3! (the number of ways to permute a,b,c). A051041 counts ordered squarefree quaternary words. A118311 counts the dissimilar cases (beginning with the 4th term) by dividing A051041 by 4!
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REFERENCES
| Sherman Stein, How The Other Half Thinks, 2001, page 149
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EXAMPLE
| a(1) = 1 because a,b,c and d are similar.
a(2) = 1 because aa is not squarefree; so ab is the only valid case.
a(3) = 2 counting aba and abc
a(4) = 4 counting abac, abca, abcb and abcd
a(5) = 11 counting abaca,abacb,abcab,abcac,abcba,abacd,abcad,abcbd,abcda,abcdb and abcdc
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CROSSREFS
| Cf. A006156, A060688, A051041.
Sequence in context: A148139 A061860 A093960 * A132836 A148140 A148141
Adjacent sequences: A118308 A118309 A118310 * A118312 A118313 A118314
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KEYWORD
| nonn
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AUTHOR
| Alford Arnold (Alford1940(AT)aol.com), Apr 22 2006
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EXTENSIONS
| a(16)-a(25) from Max Alekseyev (maxale(AT)gmail.com), Jul 03 2006
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