

A111004


Number of permutations avoiding a consecutive 132 pattern.


12



1, 1, 2, 5, 16, 63, 296, 1623, 10176, 71793, 562848, 4853949, 45664896, 465403791, 5108121216, 60069714207, 753492215808, 10042248398625, 141712039383552, 2110880441637045, 33097631526180864, 544903371859138335, 9398216812334008320, 169463659008217238055
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OFFSET

0,3


COMMENTS

a(n) is the number of permutations on [n] that avoid the consecutive pattern 132 (pattern entries must occur consecutively in the permutation).
In the Mathematica code below, a[n, k] is the number of such permutations with first entry k and they are counted recursively by the length, say ell, of the longest increasing left factor L. (For ell >= 2 the first entry following L must be < the penultimate entry of L or else a consecutive 132 would occur.) The first sum counts ell = 1, the second ell = 2, the third ell >= 3; m is the penultimate entry of L and j is the first entry in the (reduced) subpermutation following L. Note that j is indexed from 0 to cover the case when L is the entire permutation.
Asymptotically, a(n)/n! ~ c/r^n where r = 1.2755477364172... is the unique positive root of Integrate[exp(t^2/2), {t,0,r}] = 1 and c = exp(r^2/2)/r = 1.7685063678958....


LINKS

Ray Chandler, Table of n, a(n) for n = 0..200
A. Baxter, B. Nakamura, and D. Zeilberger. Automatic generation of theorems and proofs on enumerating consecutive Wilfclasses
Sergi Elizalde, Asymptotic enumeration of permutations avoiding generalized patterns, arXiv:math/0505254 [math.CO], 2005.
S. Elizalde and M. Noy, Consecutive patterns in permutations, Adv. Appl. Math. 30 (2003), 110125.
M. E. Jones and J. B. Remmel, Pattern matching in the cycle structures of permutations, Pure Math. Appl. (PU.M.A.) 22 (2011), 173208.


FORMULA

E.g.f.: Sum_{n >= 0} a(n) x^n/n! = 1/( 1  (Pi/2)^(1/2)*Erf(x/2^(1/2)) ).
a(n) = A197365(n,0).  Peter Bala, Oct 14 2011
From Sergei N. Gladkovskii, Nov 28 2011: (Start)
E.g.f.: A(x) = 1/( 1  (Pi/2)^(1/2)*erf(x/2^(1/2)) ) = (1 + (x^3)/(2*(x1)*W(0) (x^2)))/(1  x) with
W(k) = 2*(k^2) + (5  4*(x^2))*k + 3  2*(x^2) + 2*(x^2)*(k+1)*((2*k + 3)^2)/W(k+1) (continued fraction). (End)


EXAMPLE

The first 3 entries of 2431 form a consecutive 132 pattern.
The 4!a(4) = 8 permutations on [4] that DO contain a consecutive 132 pattern are 1243, 1324, 1423, 1432, 2143, 2431, 3142, 4132. Also, for example, 1342 contains a scattered 132 pattern but not a consecutive 132.


MATHEMATICA

Clear[a]; a[0, 0] = a[0] = 1; a[n_, 0]/; n>=1 := 0; a[n_, k_]/; k>n := 0; a[n_, k_]/; 1<=k<=n<=2 := 1; a[n_, k_]/; n>=3 := a[n, k] = Sum[a[n1, j], {j, k1}] + (nk)Sum[a[n2, j], {j, k1}] + Sum[(nm) Binomial[mk1, ell3]a[nell, j], {ell, 3, nk+1}, {m, k+ell2, n1}, {j, 0, mell+1}]; a[n_]/; n>=1 := a[n] = Sum[a[n, k], {k, n}]; Table[a[n], {n, 0, 15}]
(* or, faster *) ExpGfToList[f_, n_, x_] := CoefficientList[Normal[Series[f, {x, 0, n}]] /. x^(pwr_) > pwr!*x^pwr, x]; ExpGfToList[1/( 1(Pi/2)^(1/2)*Erf[z/2^(1/2)] ), 25, z]


CROSSREFS

Cf. A049774, A197365, A324130, A324134.
Row m = 0 of A327722.
Sequence in context: A105072 A022494 A136127 * A079566 A059685 A000112
Adjacent sequences: A111001 A111002 A111003 * A111005 A111006 A111007


KEYWORD

nonn,changed


AUTHOR

David Callan, Oct 01 2005


STATUS

approved



