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A166073 Triangle read by rows: a(n,k)=number of permutations in S_n which avoid the pattern 123 and have exactly k descents. 2
1, 1, 1, 1, 0, 4, 1, 0, 2, 11, 1, 0, 0, 15, 26, 1, 0, 0, 5, 69, 57, 1, 0, 0, 0, 56, 252, 120, 1, 0, 0, 0, 14, 364, 804, 247, 1, 0, 0, 0, 0, 210, 1800, 2349, 502, 1, 0, 0, 0, 0, 42, 1770, 7515, 6455, 1013, 1, 0, 0, 0, 0, 0, 792, 11055, 27940, 16962, 2036, 1, 0, 0, 0, 0, 0, 132, 8217 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,6

COMMENTS

Also number of Dyck paths of semi-length n for which the number of valleys added to the number of triple falls is k.

Apparently deletion of zeros and row-reversal maps A166073 to A091156. - R. J. Mathar, Oct 08 2009

The trivariate o.g.f. G=G(t,s,x), where t marks triple falls, s marks valleys, and x marks semilength is given by G=1+x[1+xg+t(G-1-xg)]g, where g = s(G-1)+1. Letting t=s=y, yields the given o.g.f. - Emeric Deutsch, Nov 03 2009

Apparently a variant of A126222, zeros moved to the start of each row [J. Gardiner, seqfan list, Aug 19 2010] [From R. J. Mathar, Aug 30 2010]

REFERENCES

M. Barnabei, F. Bonetti and M. Silimbani, The descent statistic on 123 avoiding permutations, preprint

LINKS

Table of n, a(n) for n=0..73.

FORMULA

O.g.f. E(x,y)=(-1+2xy+2x^2y-2xy^2-4x^2y^2+2x^2y^3+Sqrt[1-4xy-4x^2y+4*x^2*y^2])/ (2xy^2(xy-1-x)).

EXAMPLE

For example, for n=4 and k=1 whe have the 2 permutations 3412 and 2413.

Triangle begins:

1

1

1,1

0,4,1

0,2,11,1

0,0,15,26,1

0,0,5,69,57,1

0,0,0,56,252,120,1

0,0,0,14,364,804,247,1

0,0,0,0,210,1800,2349,502,1

0,0,0,0,42,1770,7515,6455,1013,1

0,0,0,0,0,792,11055,27940,16962,2036,1

0,0,0,0,0,132,8217,57035,95458,43086,4083,1

0,0,0,0,0,0,3003,62062,257257,305812,106587,8178,1

0,0,0,0,0,0,429,37037,381381,1049685,931385,258153,16369,1

0,0,0,0,0,0,0,11440,328328,2022384,3962140,2723280,614520,32752,1

0,0,0,0,0,0,0,1430,163592,2341976,9591764,14051660,7699800,1441928,65519,1

0,0,0,0,0,0,0,0,43758,1665456,14275716,41666184,47352820,21167312,3342489, 131054,1

0,0,0,0,0,0,0,0,4862,712062,13527852,77161980,168567444,152915748,56818743, 7667883,262125,1

...

MAPLE

G := (-1+2*x*y+2*x^2*y-2*x*y^2-4*x^2*y^2+2*x^2*y^3+sqrt(1-4*x*y-4*x^2*y+4*x^2*y^2))/(2*x*y^2*(x*y-1-x)): Gser := simplify(series(G, x = 0, 17)): for n from 0 to 12 do P[n] := sort(expand(coeff(Gser, x, n))) end do: for n from 0 to 12 do seq(coeff(P[n], y, k), k = 0 .. n-1) end do; # yields sequence in triangular form # Emeric Deutsch, Oct 30 2009

MATHEMATICA

E[x_, y_]:=(-1+2*x*y+2*x^2*y-2*x*y^2-4*x^2*y^2+2*x^2*y^3+Sqrt[1-4*x*y-4*x^2*y+4*x^2*y^2])/(2*x*y^2*(x*y-1-x))

CROSSREFS

Cf. A001263. Row sums given by A000108.

Cf. A091156, A126222.

Sequence in context: A062862 A206799 A084119 * A283879 A216178 A122899

Adjacent sequences:  A166070 A166071 A166072 * A166074 A166075 A166076

KEYWORD

nonn,tabf

AUTHOR

Matteo Silimbani (silimban(AT)dm.unibo.it), Oct 06 2009, Oct 08 2009

EXTENSIONS

Extended by Emeric Deutsch, Oct 30 2009

STATUS

approved

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Last modified May 29 13:14 EDT 2017. Contains 287247 sequences.