Parking functions on toppling matrices
  Let $\Delta$ be an integer $n \times n$-matrix which satisfies the   conditions: $\det \Delta\neq 0$, $\Delta_{ij}\leq 0\text{ for }i\neq j,$ and   there exists a vector ${\bf r}=(r_1,\ldots,r_n)>0$ such that ${\bf r}\Delta \geq 0$. Here the notation ${\bf r}> 0$ means that $r_i>0$ for all $i$, and ${\bf r}\geq {\bf r}'$ means that $r_i\geq r'_i$ for every $i$. Let $\mathscr{R}(\Delta)$ be the set of vectors ${\bf r}$ such that ${\bf r}>0$ and ${\bf r}\Delta\geq 0$. In this paper, $(\Delta,{\bf r})$-parking functions are defined for any ${\bf r}\in\mathscr{R}(\Delta)$. It is proved that the set of $(\Delta,{\bf r})$-parking functions is independent of ${\bf r}$ for any ${\bf r}\in\mathscr{R}(\Delta)$. For this reason, $(\Delta,{\bf r})$-parking functions are simply called $\Delta$-parking functions. It is shown that the number of $\Delta$-parking functions is less than or equal to the determinant of $\Delta$. Moreover, the definition of $(\Delta,{\bf r})$-recurrent configurations are given for any ${\bf r}\in\mathscr{R}(\Delta)$. It is proved that the set of $(\Delta,{\bf r})$-recurrent configurations is independent of ${\bf r}$ for any ${\bf r}\in\mathscr{R}(\Delta)$. Hence, $(\Delta,{\bf r})$-recurrent configurations are simply called $\Delta$-recurrent configurations. It is obtained that the number of $\Delta$-recurrent configurations is larger than or equal to the determinant of $\Delta$. A simple bijection from $\Delta$-parking functions to $\Delta$-recurrent configurations is established. It follows from this bijection that the number of $\Delta$-parking functions and the number of $\Delta$-recurrent configurations are both equal to the determinant of $\Delta$. 
Jun Ma, Yeong-Nan Yeh
math.CO