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A054480
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Number of different positive braids with n crossings of 4 strands.
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1
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1, 3, 8, 19, 43, 94, 202, 429, 905, 1901, 3983, 8332, 17413, 36370, 75938, 158519, 330862, 690523, 1441082, 3007369, 6275917, 13096737, 27330418, 57033206, 119016808, 248363751, 518283950, 1081551220, 2256972626, 4709831964
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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REFERENCES
| P. Xu, Growth of the positive braids semigroups, Journal of Pure and Applied Algebra, 1992.
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FORMULA
| G.f.: 1/(1-3x+x^2+2x^3-x^6)
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EXAMPLE
| a(3)=19 =#{111,112,113,121,122,123,132,133,211,213,221,222,223,232,233,321,322,332,333}
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CROSSREFS
| Cf. A000071, A054761.
Sequence in context: A161993 A008466 A102712 * A191787 A121551 A189391
Adjacent sequences: A054477 A054478 A054479 * A054481 A054482 A054483
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KEYWORD
| nonn,easy
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AUTHOR
| Serge Burckel (burckel(AT)univ-reunion.fr), Apr 26 2000
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EXTENSIONS
| More terms from Benoit Cloitre (benoit7848c(AT)orange.fr), Mar 22 2002
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