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Permutation Pattern


Let F(n,sigma) denote the number of permutations on the symmetric group S_n which avoid sigma in S_k as a subpattern, where "tau contains sigma as a subpattern" is interpreted to mean that there exist 1<=x_1<=x_2<=...<=x_k<=n such that for 1<=i,j<=k,

 tau(x_i)<tau(x_j)

iff sigma(i)<sigma(j).

For example, a permutation contains the pattern (123) iff it has an ascending subsequence of length three. Here, note that members need not actually be consecutive, merely ascending (Wilf 1997). Therefore, of the 3!=6 partitions of {1,2,3}, all but {3,2,1} (i.e., {1,2,3}, {1,3,2}, {2,1,3}, {2,3,1}, and {3,1,2}) contain the pattern (12) (i.e., an increasing subsequence of length two).

The following table gives the numbers of pattern-matching permutations of k, k+1, ..., n numbers for various patterns (a_1...a_k) of length k.

patternOEISnumber of pattern-matching permutations
1A0001421, 2, 6, 24, 120, 720, 5040, ...
12A0333121, 5, 23, 119, 719, 5039, 40319, ...
alpha_3A0569861, 10, 78, 588, 4611, 38890, ...
1234A1580051, 17, 207, 2279, 24553, ...
1324A1580091, 17, 207, 2278, 24527, ...
1342A1580061, 17, 208, 2300, 24835, ...

The following table gives the numbers of pattern-avoiding permutations of {1,...,n} for various sets of patterns.

Wilf classOEISnumber of pattern-avoiding permutations
alpha_3A0001081, 2, 5, 14, 42, 132, ...
123, 132, 213A0000271, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...
132, 231, 321A0000271, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...
123, 132, 3214A0000731, 2, 4, 7, 13, 24, 44, 81, 149, ...
123, 132, 3241A0000711, 2, 7, 12, 20, 33, 54, 88, 143, ...
123, 132, 3412A0001241, 2, 4, 7, 11, 16, 22, 29, 37, 46, ...
123, 231, alpha_4^((1))A0042751, 2, 4, 6, 8, 10, 12, 14, 16, 18, ...
123, 231, alpha_4^((2))A0001241, 2, 4, 7, 11, 16, 22, 29, 37, 46, ...
123, 231, 43211, 2, 4, 6, 3, 1, 0, ...
132, 213, 1234A0000731, 2, 4, 7, 13, 24, 44, 81, 149, ...
213, 231, alpha_4^((3))A0001241, 2, 4, 7, 11, 16, 22, 29, 37, 46, ...

Abbreviations used in the above table are summarized below.

abbreviationpatterns in class
alpha_3123, 132, 213, 232, 312, 321
alpha_4^((1))1432, 2143, 3214, 4132, 4213, 4312
alpha_4^((2))1234, 1243, 1324, 1342, 1423, 2134, 2314, 2341, 2413, 2431, 3124, 3142, 3241, 3412, 3421, 4123, 4231
alpha_4^((3))1234, 1243, 1423, 1432

See also

Contained Pattern, Order Isomorphic, Permutation, Stanley-Wilf Conjecture, Wilf Class, Wilf Equivalent

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References

Arratia, R. "On the Stanley-Wilf Conjecture for the Number of Permutations Avoiding a Given Pattern." Electronic J. Combinatorics 6, No. 1, N1, 1-4, 1999. http://www.combinatorics.org/Volume_6/Abstracts/v6i1n1.html.Billey, S.; Jockusch, W.; and Stanley, R. P. "Some Combinatorial Properties of Schubert Polynomials." J. Alg. Combin. 2, 345-374, 1993.Guibert, O. "Permutations sans sous séquence interdite." Mémoire de Diplô me d'Etudes Approfondies de L'Université Bordeaux I. 1992.Mansour, T. "Permutations Avoiding a Pattern from S_k and at Least Two Patterns from S_3." 31 Jul 2000. http://arxiv.org/abs/math.CO/0007194.Simon, R. and Schmidt, F. W. "Restricted Permutations." Europ. J. Combin. 6, 383-406, 1985.Sloane, N. J. A. Sequences A000027/M0472, A000071/M1056, A000073/M1074, A000108/M1459, A000124/M1041, A000142/M1675, A004275, A033312, and A056986, A158005, and A158006 in "The On-Line Encyclopedia of Integer Sequences."Stankova, Z. E. "Forbidden Subsequences." Disc. Math. 132, 291-316, 1994.West, J. "Generating Trees and Forbidden Subsequences." Disc. Math. 157, 363-372, 1996.Wilf, H. "On Crossing Numbers, and Some Unsolved Problems." In Combinatorics, Geometry, and Probability: A Tribute to Paul Erdős. Papers from the Conference in Honor of Erdős' 80th Birthday Held at Trinity College, Cambridge, March 1993 (Ed. B. Bollobás and A. Thomason). Cambridge, England: Cambridge University Press, pp. 557-562, 1997.

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Permutation Pattern

Cite this as:

Weisstein, Eric W. "Permutation Pattern." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PermutationPattern.html

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