0,4

A Schroeder path is a lattice path starting from (0,0), ending at a point on the x-axis, consisting only of steps U=(1,1), D=(1,-1) and H=(2,0) and never going below the x-axis. Schroeder paths are counted by the large Schroeder numbers (A006318).

Triangular array in A011117 transposed . - Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Mar 16 2005

This is an example of a Riordan (lower triangular) matrix. See the Shapiro et al. reference quoted under A053121. More precisely, this ordinary convolution triangle belongs to the Bell subgroup of the Riordan group. In the Shapiro et al. notation this is a Bell matrix (g(x),x*g(x)) with g(x)=(1+x-sqrt(1-6*x+x^2))/(4*x), the o.g.f. of A001003(n),n>=0. The g.f. for the row polynomials p(n,x):=sum(a(n,k)*x^k,k=0..n) is g(y)/(1-x*y*g(y))= (1-2*x*y+y-sqrt(1-6*y+y^2))/(2*y*(2-x-x*y+x^2*y)).

G.f.=2/[1+z+sqrt(1-6z+z^2)-2zt]

Another version of the triangle T(n, k), 0<=k<=n, read by = rows; given by [0, 1, 2, 1, 2, 1, 2, 1, 2, 1, ...] DELTA [1, 0, 0, 0, = 0, 0, 0, 0, 0, 0, ...] = 1; 0, 1; 0, 1, 1; 0, 3, 2, 1; 0, 11, 7, = 3, 1; 0, 45, 28, 12, 4, 1; ... where DELTA is the operator defined in = A084938 . - Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Mar 16 2005

a(n, k)= (k+1)*hypergeom([1-n+k, n+2], [2], -1) if n>k; a(n, n)=1; a(n, k)=0 if n<k. W. Lang, Sep 12 2005.

a(n, k)=((k+1)/(n-k))*sum(binomial(n-k, p)*binomial(n+p, p-1), p=1..(n-k)) if n>k; a(n, n)=1; a(n, k)=0 if n<k. Proof with Lagrange's inversion theorem based on eq. y = 1+x*(1-2/y) where y=1/g(x), with g(x) the o.g.f. of A001003(n), n>=0. Use G(k;y):=1/y^(k+1), k>=0 to find the coefficients a(n, k) of x^n of G(k;1/g(x)). For this method see also the Larcombe and French paper on Catalan convolutions quoted under A033184. W. Lang (wolfdieter.lang_AT_physik_DOT_uni-karlsruhe_DOT_de), Sep 12 2005.

G.f.: 1/(1-xy-x/(1-x-x/(1-x-x/(1-x-x/(1-x-x/(1-..... (continued fraction). [From Paul Barry (pbarry(AT)wit.ie), Feb 01 2009]

Triangle starts:

1;

1,1;

3,2,1;

11,7,3,1;

45,28,12,4,1;

T(3,1)=7 because we have HH(UD),H(UD)H,(UD)HH,UUDD(UD),(UD)UUDD,(UD)UHD, and

UHD(UD) (the peaks UD at height 1 are shown between parentheses).

G:=2/(1+z+sqrt(1-6*z+z^2)-2*z*t): Gser:=simplify(series(G, z=0, 13)): P[0]:=1: for n from 1 to 13 do P[n]:=coeff(Gser, z^n) od: for n from 0 to 11 do seq(coeff(t*P[n], t^k), k=1..n+1) od; # yields sequence in triangular form

(PARI) {T(n, k)=local(X=x+x*O(x^n), Y=y+y*O(y^k)); polcoeff(polcoeff(2/(1+X+sqrt(1-6*X+X^2)-2*X*Y), n, x), k, y)} (from Paul D. Hanna (pauldhanna(AT)juno.com), Mar 30 2005)

Row sums are the large Schroeder numbers (A006318). Column 0 yields the little Schroeder numbers (A001003).

Cf. A104967 (matrix inverse).

Sequence in context: A116071 A077756 A115080 this_sequence A123513 A117442 A118435

Adjacent sequences: A104216 A104217 A104218 this_sequence A104220 A104221 A104222

nonn,tabl

Emeric Deutsch (deutsch(AT)duke.poly.edu), Mar 14 2005