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Search: id:A109033
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| A109033 |
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Number of permutations in S_n avoiding the patterns 1342 and 2143. |
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+0
4
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| 1, 1, 2, 6, 22, 88, 368, 1584, 6968, 31192, 141656, 651136, 3023840, 14166496, 66876096, 317809216, 1519163456, 7299577216, 35237444736, 170812433536, 831127053696, 4057858988416, 19873611712896, 97609555091456
(list; graph; listen)
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OFFSET
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0,3
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COMMENT
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Also number of permutations in S_n avoiding the patterns 3142 and 2341. Partial sums of A109034.
Hankel transform is 2^floor(n^2/3) (see A134751). [From Paul Barry (pbarry(AT)wit.ie), Dec 15 2008]
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REFERENCES
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Ian Le, Wilf classes of pairs of permutations of length 4, The Electronic J. of Combinatorics, 12, 2005, R25.
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FORMULA
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G.f.=[1-sqrt(1-8x+16x^2-8x^3)]/[4x(1-x)]
Contribution from Paul Barry (pbarry(AT)wit.ie), Dec 15 2008: (Start)
G.f.: (1-x)c(2x(1-x)^2), c(x) the g.f. of A000108;
a(n):=sum{k=0..n, (-1)^(n-k)*C(2k+1,n-k)*2^k*A000108(k)}; (End)
G.f.: 1/(1-x/(1-x/(1-2x/(1-x/(1-x/(1-2x/(1-x/(1-x/(1-2x...... (continued fraction). [From Paul Barry (pbarry(AT)wit.ie), Dec 15 2008]
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EXAMPLE
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a(4)=22 because all permutations of 1234 qualify with the exception of 1342 and 2143.
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MAPLE
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G:=(1-sqrt(1-8*x+16*x^2-8*x^3))/4/x/(1-x): Gser:=series(G, x=0, 30): 1, seq(coeff(Gser, x^n), n=1..27);
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CROSSREFS
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Cf. A109034.
Sequence in context: A165537 A165538 A165539 this_sequence A049135 A049127 A049137
Adjacent sequences: A109030 A109031 A109032 this_sequence A109034 A109035 A109036
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KEYWORD
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nonn
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AUTHOR
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Emeric Deutsch (deutsch(AT)duke.poly.edu), Jun 16 2005
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