OFFSET
1,2
LINKS
Jean-Luc Baril, Pamela E. Harris, and José L. Ramírez, Flattened Catalan Words, arXiv:2405.05357 [math.CO], 2024. See pp. 21-22.
FORMULA
G.f.: x*(1 - x)*(1 - 2*x)/(1 - 5*x + 8*x^2 - 5*x^3 - x^2*y + 2*x^3*y).
Sum_{k>=0} T(n,k) = A007051(n-1).
EXAMPLE
The irregular triangle begins:
1;
2;
4, 1;
9, 5;
23, 17, 1;
63, 51, 8;
176, 149, 39, 1;
491, 439, 153, 11;
1362, 1308, 540, 70, 1;
3762, 3912, 1812, 342, 14;
...
T(4,1) = 5 since there are 5 flattened Catalan words of length 4 with 1 symmetric peak: 0100, 0101, 0010, 0110, and 0121.
MATHEMATICA
T[n_, k_]:=SeriesCoefficient[x(1-x)(1-2x)/(1-5x+8x^2-5x^3-x^2y+2x^3y), {x, 0, n}, {y, 0, k}]; Table[T[n, k], {n, 14}, {k, 0, Floor[(n-1)/2]}]//Flatten
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Stefano Spezia, May 15 2024
STATUS
approved