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A094218
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Number of permutations of length n with exactly 2 occurrences of the pattern 2-13.
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8
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0, 0, 0, 2, 25, 198, 1274, 7280, 38556, 193800, 937992, 4412826, 20309575, 91861770, 409704750, 1806342720, 7887861960, 34166674800, 146977222320, 628521016500, 2673950235138, 11324837666604, 47773836727540, 200828153398752
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OFFSET
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1,4
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REFERENCES
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R. Lie, Permutations and Patterns, Master's Thesis, Goeteborg, Sweden: Chalmers University of Technology, 2004.
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LINKS
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Alois P. Heinz, Table of n, a(n) for n = 1..500
R. Parviainen, Lattice Path Enumeration of Permutations with k Occurrences of the Pattern 2-13, Journal of Integer Sequences, Vol. 9 (2006), Article 06.3.2.
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FORMULA
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a(n) = n*binomial(2*n,n-4)/2.
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MATHEMATICA
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Table[n Binomial[2 n, n - 4]/2, {n, 30}] (* Vincenzo Librandi, Aug 20 2015 *)
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PROG
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(PARI) a(n)=n*binomial(2*n, n-4)/2
(MAGMA) [n*Binomial(2*n, n-4)/2: n in [1..30]]; // Vincenzo Librandi, Aug 20 2015
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CROSSREFS
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Cf. A094219.
Column k=2 of A263776.
Sequence in context: A024533 A220276 A215298 * A203767 A206392 A074438
Adjacent sequences: A094215 A094216 A094217 * A094219 A094220 A094221
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KEYWORD
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nonn
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AUTHOR
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Benoit Cloitre, May 27 2004
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STATUS
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approved
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