Simple Generating Functions for Certain Young Tableaux with Periodic Walls

Abstract

Recently, Banderier et. al. considered Young tableaux with walls, which are similar to standard Young tableaux, except that local decreases are allowed at some walls. We count the numbers $\overline{f}_m(n)$ of Young tableaux of shape $2\times mn$ with walls, that allow local decreases at the $(jm+i)$-th columns for all $j=0,\dots, n-1$ and $i=2,\dots, m$. We find that they have nice generating functions (thanks to the OEIS) as follows. $$\overline{F}_m(x)=\sum_{n\geq 0}\overline{f}_m(n)x^n=\prod_{k=1}^{m}C(e^{k\frac{2\pi i}{m}} x^\frac{1}{m})=\exp \left(\sum_{n\geq 1}\binom{2mn-1}{mn-1}\frac{x^n}{n}\right),$$ where $C(x)=\frac{1-\sqrt{1-4x}}{2x}$ is the well-known Catalan generating function. We prove generalizations of this result. Firstly, we use the Yamanouchi word to transform Young tableaux with horizontal walls into lattice paths. This results in a determinant formula. Then by lattice path counting theory, we obtain the generating functions $F_r(x)$ for the number of lattice paths from $(0,0)$ to $(\ell n-r,kn)$ that never go above the path $(N^kE^{\ell})^{n-1}N^kE^{\ell-r}$, where $N,E$ stand for north and east steps, respectively. We also obtain exponential formulas for $F_1(x)$ and $F_\ell(x)$. The formula for $\overline{F}_m(x)$ is thus proved since it is just $F_1(x)$ specializes at $k=\ell=m$.