A121462 - OEIS

A121462

Triangle read by rows: T(n,k) is the number of nondecreasing Dyck paths of semilength n, having pyramid weight k (1 <= k <= n).

1, 0, 2, 0, 1, 4, 0, 1, 4, 8, 0, 1, 5, 12, 16, 0, 1, 6, 18, 32, 32, 0, 1, 7, 25, 56, 80, 64, 0, 1, 8, 33, 88, 160, 192, 128, 0, 1, 9, 42, 129, 280, 432, 448, 256, 0, 1, 10, 52, 180, 450, 832, 1120, 1024, 512, 0, 1, 11, 63, 242, 681, 1452, 2352, 2816, 2304, 1024, 0, 1, 12, 75, 316

(list; table; graph; refs; listen; history; text; internal format)

OFFSET

1,3

COMMENTS

A pyramid in a Dyck word (path) is a factor of the form U^h D^h, where U=(1,1), D=(1,-1) and h is the height of the pyramid. A pyramid in a Dyck word w is maximal if, as a factor in w, it is not immediately preceded by a u and immediately followed by a d. The pyramid weight of a Dyck path (word) is the sum of the heights of its maximal pyramids.

Row sums are the odd-subscripted Fibonacci numbers (A001519). T(n,n)=2^(n-1). Sum_{k=1..n} k*T(n,k) = A030267(n).

Mirror image of triangle in A153342. - Philippe Deléham, Dec 31 2008

Essentially triangle given by (0,1/2,1/2,0,0,0,0,0,0,0,...) DELTA (2,0,0,0,0,0,0,0,0,0,...) where DELTA is the operator defined in A084938. - Philippe Deléham, Oct 30 2011

A121462 is jointly generated with A208341 as an array of coefficients of polynomials u(n,x): initially, u(1,x)=v(1,x)=1; for n > 1, u(n,x) = x*u(n-1,x) + x*v(n-1) and v(n,x) = x*u(n-1,x) + (x+1)*v(n-1,x). See the Mathematica section. - Clark Kimberling, Mar 11 2012

LINKS

Table of n, a(n) for n=1..71.

Elena Barcucci, Alberto del Lungo, S. Fezzi and Renzo Pinzani, Nondecreasing Dyck paths and q-Fibonacci numbers, Discrete Math., 170, 1997, 211-217.

Nicolas Bělohoubek and Antonín Slavík, L-Tetromino Tilings and Two-Color Integer Compositions, Univ. Karlova (Czechia, 2025). See p. 10.

Alain Denise and Rodica Simion, Two combinatorial statistics on Dyck paths, Discrete Math., 137, 1995, 155-176.

Emeric Deutsch and Helmut Prodinger, A bijection between directed column-convex polyominoes and ordered trees of height at most three, Theoretical Comp. Science, 307, 2003, 319-325.

FORMULA

T(n,k) = Sum_{j=0..k-1} binomial(k-1,j)*binomial(n-k-1+j,j-1) for 2 <= k <= n; T(1,1)=1; T(n,1)=0 for n >= 2.

G.f.: G = G(t,z) = tz(1-z)/(1-2tz-z+tz^2).

T(n+1,k+1) = A062110(n,k)*2^(2*k-n). - Philippe Deléham, Aug 01 2006

EXAMPLE

T(4,3)=4 because we have (UD)U(UD)(UD)D, U(UD)(UD)(UD)D, U(UD)(UUDD)D and U(UUDD)(UD)D, where U=(1,1) and D=(1,-1) (the maximal pyramids are shown between parentheses).

Triangle starts:

0, 2;

0, 1, 4;

0, 1, 4, 8;

0, 1, 5, 12, 16;

0, 1, 6, 18, 32, 32;

MAPLE

T:=proc(n, k) if n=1 and k=1 then 1 elif k=1 then 0 elif k<=n then sum(binomial(k-1, j)*binomial(n-k-1+j, j-1), j=0..k-1) else 0 fi end: for n from 1 to 13 do seq(T(n, k), k=1..n) od; # yields sequence in triangular form

MATHEMATICA

u[1, x_] := 1; v[1, x_] := 1; z = 16;

u[n_, x_] := x*u[n - 1, x] + x*v[n - 1, x];

v[n_, x_] := x*u[n - 1, x] + (x + 1) v[n - 1, x];

Table[Expand[u[n, x]], {n, 1, z/2}]

Table[Expand[v[n, x]], {n, 1, z/2}]

cu = Table[CoefficientList[u[n, x], x], {n, 1, z}];

TableForm[cu]

Flatten[%] (* A121462 *)

Table[Expand[v[n, x]], {n, 1, z}]

cv = Table[CoefficientList[v[n, x], x], {n, 1, z}];

TableForm[cv]