WILFPLUS Example:
321-hexagon-avoiding permutations

The 321-hexagon-avoiding permutations, introduced by Billey and Warrington in their article "Kazhdan-Lusztig polynomials for 321-hexagon-avoiding permutations" were first counted by Stankova and West in their article "Explicit enumeration of 321, hexagon-avoiding permutations." These are the permutations that avoid

321, 46718235, 46781235, 56718234, and 56781234.

The first few terms of the sequence are

1, 2, 5, 14, 42, 132, 429, 1426, 4806, 16329, 55740, 190787, 654044, 2244153, 7704047, 26455216, 90860572, 312090478, 1072034764, 3682565575, 12650266243, 43456340025, 149282561256, 512821712570, 1761669869321, 6051779569463

(This is sequence A058094 in the On-Line Encyclopedia of Integer Sequences.)

WILFPLUS finds the following enumeration scheme for this class.

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