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A087214
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Expansion of exp(x)/(1-x^2/2).
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5
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1, 1, 2, 4, 13, 41, 196, 862, 5489, 31033, 247006, 1706816, 16302397, 133131649, 1483518128, 13978823146, 178022175361, 1901119947857, 27237392830234, 325091511083548, 5175104637744461
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| a(n) is also the number of permutations in S_n whose prefix transposition distance is tight with respect to Dias and Meidanis' lower bound (proof: see Fortuna). [From Anthony Labarre (alabarre(AT)ulb.ac.be), Feb 16 2009]
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REFERENCES
| Zanoni Dias and Joao Meidanis, Sorting by Prefix Transpositions, Proceedings of the Ninth International Symposium on String Processing and Information Retrieval (SPIRE), 2002, 65-76, vol. 2476 of Lecture Notes in Computer Science, Springer-Verlag [From Anthony Labarre (alabarre(AT)ulb.ac.be), Feb 16 2009]
V. J. Fortuna, Distancias de Transposito entre Genomas, Master's Thesis, Universidade Estadual de Campinas, 2005. [From Anthony Labarre (alabarre(AT)ulb.ac.be), Feb 16 2009]
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FORMULA
| a(n) = Sum_{k=0..floor(n/2)} n!/(n-2*k)!/2^k = hypergeom([1, -n/2, -n/2+1/2], [], 2).
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CROSSREFS
| Sequence in context: A148256 A163136 A118930 * A002771 A050624 A135501
Adjacent sequences: A087211 A087212 A087213 * A087215 A087216 A087217
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KEYWORD
| nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Oct 19 2003
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