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A116842 Number of permutations of length n which avoid the patterns 1234, 2143, 3421. 0
1, 2, 6, 21, 69, 181, 375, 651, 1009, 1449, 1971, 2575, 3261, 4029, 4879, 5811, 6825, 7921, 9099, 10359, 11701, 13125, 14631, 16219, 17889, 19641, 21475, 23391, 25389, 27469 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..30.

D. Callan, T. Mansour, Enumeration of small Wilf classes avoiding 1324 and two other 4-letter patterns, arXiv:1705.00933 (2017), Table 2 No 3.

Lara Pudwell, Systematic Studies in Pattern Avoidance, 2005.

Index entries for linear recurrences with constant coefficients, signature (3,-3,1).

FORMULA

G.f.: A(x) = -{x(18x^6+31x^5+22x^4+8x^3+3x^2-x+1)}/(x-1)^3.

FOr n >= 5, a(n) = 41n^2 - 339n + 739. - Franklin T. Adams-Watters, Sep 16 2006

MAPLE

cn := [1, -2, 2, 2, 8, 22, 31, 18] ;

p := add(cn[i]*x^(i-1), i=1..nops(cn)) ;

q := (1-x)^3 ;

taylor(p/q, x=0, 40) ;

gfun[seriestolist](%) ; # R. J. Mathar, Nov 07 2017

CROSSREFS

Sequence in context: A119098 A294706 A294707 * A294708 A294709 A116774

Adjacent sequences:  A116839 A116840 A116841 * A116843 A116844 A116845

KEYWORD

nonn,easy

AUTHOR

Lara Pudwell, Feb 26 2006

STATUS

approved

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Last modified September 26 16:43 EDT 2020. Contains 337374 sequences. (Running on oeis4.)