OFFSET
0,3
COMMENTS
From Lara Pudwell, Oct 23 2008: (Start)
A permutation p avoids a pattern q if it has no subsequence that is order-isomorphic to q. For example, p avoids the pattern 132 if it has no subsequence abc with a < c < b.
Barred pattern avoidance considers permutations that avoid a pattern except in a special case. Given a barred pattern q, we may form two patterns, q1 = the sequence of unbarred letters of q and q2 = the sequence of all letters of q.
A permutation p avoids barred pattern q if every instance of q1 in p is embedded in a copy of q2 in p. In other words, p avoids q1, except in the special case that a copy of q1 is a subsequence of a copy of q2.
For example, if q = 5{bar 1}32{bar 4}, then q1 = 532 and q2 = 51324. p avoids q if every for decreasing subsequence acd of length 3 in p, one can find letters b and e so that the subsequence abcde of p has b < d < c < e < a. (End)
LINKS
Lara Pudwell, Table of n, a(n) for n = 0..30
David Callan, A Combinatorial Interpretation of the Eigensequence for Composition, Journal of Integer Sequences, Vol. 9 (2006), Article 06.1.4. (Erratum: The rising and falling factorials in the second displayed line on page 12 should be interchanged.)
Lara Pudwell, Enumeration Schemes for Pattern-Avoiding Words and Permutations, Ph. D. Dissertation, Math. Dept., Rutgers University, May 2008.
Lara Pudwell, Enumeration schemes for permutations avoiding barred patterns, El. J. Combinat. 17 (1) (2010) R29.
MATHEMATICA
FallingFactorial[n_, k_] := Product[n - i, {i, 0, k - 1}];
RisingFactorial[n_, k_] := Product[n + i, {i, 0, k - 1}];
Table[(n - 1)! + Sum[FallingFactorial[k, i] RisingFactorial[n - 2 - k, j], {k, 0, n - 2}, {i, 0, k}, {j, 0, k - i}], {n, 15}] (* David Callan, Nov 21 2011 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lara Pudwell, Apr 25 2008
EXTENSIONS
a(0)=1 prepended by Alois P. Heinz, Jul 10 2023
STATUS
approved