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A129777
Number of freely-braided hexagon-avoiding permutations in S_n; the freely-braided hexagon-avoiding permutations are those that avoid 3421, 4231, 4312, 4321, 46718235, 46781235, 56718234 and 56781234.
0
1, 2, 6, 20, 71, 260, 971, 3670, 13968, 53369, 204352, 783408, 3005284, 11533014, 44267854, 169935041, 652385639, 2504613713, 9615798516, 36917689075, 141737959416, 544175811783, 2089262741393, 8021347093432, 30796530585417, 118237818141689, 453953210838465
OFFSET
1,2
COMMENTS
If w is freely-braided and hexagon-avoiding, there are simple explicit formulas for all the Kazhdan-Lusztig polynomials P_{x,w}.
REFERENCES
Jozsef Losonczy, Maximally clustered elements and Schubert varieties, Ann. Comb. 11 (2007), no. 2, 195-212.
FORMULA
G.f.: (-x^7-2x^6+2x^5+x^4-3x^3+4x^2-x) / (x^7-x^6-8x^5+x^4+3x^3-9x^2+6x-1).
EXAMPLE
a(8)=3670 because there are 3670 permutations of size 8 that avoid 3421, 4231, 4312, 4321, 46718235, 46781235, 56718234 and 56781234.
MATHEMATICA
LinearRecurrence[{6, -9, 3, 1, -8, -1, 1}, {1, 2, 6, 20, 71, 260, 971}, 27] (* Jean-François Alcover, Feb 02 2019 *)
PROG
(PARI) lista(nt) = { my(x = 'x + 'x*O('x^nt) ); P = (-x^7-2*x^6+2*x^5+x^4-3*x^3+4*x^2-x) / (x^7-x^6-8*x^5+x^4+3*x^3-9*x^2+6*x-1); print(Vec(P)); } \\ Michel Marcus, Mar 17 2013
CROSSREFS
Sequence in context: A376792 A145138 A000707 * A108600 A274484 A128729
KEYWORD
nonn
AUTHOR
Brant Jones (brant(AT)math.washington.edu), May 17 2007
EXTENSIONS
More terms from Michel Marcus, Mar 17 2013
STATUS
approved