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A091869 Triangle read by rows: T(n,k)=number of Dyck paths of semilength n having k peaks at even height. 6
1, 1, 1, 2, 2, 1, 4, 6, 3, 1, 9, 16, 12, 4, 1, 21, 45, 40, 20, 5, 1, 51, 126, 135, 80, 30, 6, 1, 127, 357, 441, 315, 140, 42, 7, 1, 323, 1016, 1428, 1176, 630, 224, 56, 8, 1, 835, 2907, 4572, 4284, 2646, 1134, 336, 72, 9, 1, 2188, 8350, 14535, 15240, 10710, 5292, 1890, 480, 90, 10, 1 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

Number of ordered trees with n edges having k leaves at even height. Row sums are the Catalan numbers (A000108). T(n,0)=A001006(n-1) (the Motzkin numbers). sum(k*T(n,k),k=0..n-1) = binom(2n-2,n-2)=A001791(n-1). Mirror image of A091187.

T(n,k)= number of Dyck paths of semilength n and having k dud's (here u=(1,1) and d=(1,-1)). Example: T(4,2)=3 because we have uud(du[d)ud], uu(dud)(dud) and uu(du[d)ud]d (the dud's are shown between parentheses).

T(n,k)= number of Dyck paths of semilength n and containing exactly k double rises whose matching down steps form a doublefall. Example: UUUDUDDD has 2 double rises but only the first has matching Ds - the path's last 2 steps - forming a doublefall. (Travel horizontally east from an up step to encounter its matching down step.) - David Callan, Jul 15 2004

T(n,k)=number of ordered trees on n edges containing k edges of outdegree 1. (The outdegree of an edge is the outdegree of its child vertex. Thus edges of outdegree 1 correspond to non-root vertices of outdegree 1.) T(3,2)=2 because

/\.../\.

|.....|.

each have one edge of outdegree 1. - David Callan, Oct 25 2004

Exponential Riordan array [exp(x)*Bessel_I(1,2x)/x, x]. - Paul Barry, Mar 09 2010

LINKS

Alois P. Heinz, Rows n = 1..200, flattened

M. Dziemianczuk, Enumerations of plane trees with multiple edges and Raney lattice paths, Discrete Mathematics 337 (2014): 9-24.

A. Sapounakis, I. Tasoulas and P. Tsikouras, Counting strings in Dyck paths, Discrete Math., 307 (2007), 2909-2924.

Yidong Sun, The statistic "number of udu's" in Dyck paths, Discrete Math., 287 (2004), 177-186.

Chao-Jen Wang, Applications of the Goulden-Jackson cluster method to counting Dyck paths by occurrences of subwords.

FORMULA

T(n, k)=binomial(n-1, k)*sum(binomial(n-k, j)*binomial(n-k-j, j-1), j=0..ceil((n-k)/2))/(n-k) for 0<=k<n; T(n, k)=0 for k>=n. G.f.=G=G(t, z) satisfies zG^2-(1+z-tz)G+1+z-tz=0. T(n, k)=M(n-k-1)*binomial(n-1, k), where M(n)=A001006(n) are the Motzkin numbers.

T(n+1, k+1)=n*T(n, k)/(k+1). - David Callan, Dec 09 2004

G.f.: 1/(1-x-xy-x^2/(1-x-xy-x^2/(1-x-xy-x^2/(1-x-xy-x^2/(1-... (continued fraction). - Paul Barry, Aug 03 2009

E.g.f.: exp(x+xy)*Bessel_I(1,2x)/x. - Paul Barry, Mar 10 2010

EXAMPLE

T(4,1)=6 because we have u(ud)dudud, udu(ud)dud, ududu(ud)d, uuudd(ud)d, u(ud)uuddd and uuu(ud)ddd (here u=(1,1), d=(1,-1) and the peaks at even height are shown between parentheses).

Triangle begins:

[1],

[1, 1],

[2, 2, 1],

[4, 6, 3, 1],

[9, 16, 12, 4, 1],

[21, 45, 40, 20, 5, 1],

[51, 126, 135, 80, 30, 6, 1],

MAPLE

T := proc(n, k) if k<n then binomial(n-1, k)*sum(binomial(n-k, j)*binomial(n-k-j, j-1), j=0..ceil((n-k)/2))/(n-k) else 0 fi end: seq(seq(T(n, k), k=0..n-1), n=1..11);

# second Maple program:

b:= proc(x, y, t) option remember; expand(`if`(x=0, 1,

      `if`(y>0, b(x-1, y-1, 0)*z^irem(t*y+t, 2), 0)+

      `if`(y<x-1, b(x-1, y+1, 1), 0)))

    end:

T:= n-> (p-> seq(coeff(p, z, i), i=0..degree(p)))(b(2*n, 0$2)):

seq(T(n), n=1..16);  # Alois P. Heinz, May 12 2017

MATHEMATICA

(* m = MotzkinNumber *) m[0] = 1; m[n_] := m[n] = m[n - 1] + Sum[m[k]*m[n - 2 - k], {k, 0, n - 2}]; t[n_, n_] = 1; t[n_, k_] := m[n - k]*Binomial[n - 1, k - 1]; Table[t[n, k], {n, 1, 11}, {k, 1, n}] // Flatten (* Jean-François Alcover, Jul 10 2013 *)

CROSSREFS

Cf. A000108, A001006, A001791, A091187, A243752.

Sequence in context: A068957 A119468 A175136 * A112307 A228336 A111062

Adjacent sequences:  A091866 A091867 A091868 * A091870 A091871 A091872

KEYWORD

nonn,tabl,changed

AUTHOR

Emeric Deutsch, Mar 10 2004

STATUS

approved

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Last modified May 23 01:01 EDT 2017. Contains 286908 sequences.