$\ell$-distance-balanced graphs

Let $\ell$ denote a positive integer. A connected graph $\G$ of diameter at least $\ell$ is said to be $\ell${\it -distance-balanced} whenever for any pair of vertices $u,v$ of $\G$ such that $d(u,v)=\ell$, the number of vertices closer to $u$ than to $v$ is equal to the number of vertices closer to $v$ than to $u$. In this paper we present some basic properties of $\ell$-distance-balanced graphs and study in more detail $\ell$-distance-balanced graphs of diameter at most $3$. We also investigate the $\ell$-distance-balanced property of some well known families of graphs such as the generalized Petersen graphs.