The $p$-Hardy-Rellich-Birman inequalities on the half-line

Abstract

The classical discrete $p$-Hardy inequality establishes a sharp relationship between the ${\ell }^p$-norms of a sequence and its discrete derivative. In this paper, we generalize this inequality to discrete derivatives of arbitrary integer order $\ell \ge 1$, yielding discrete $p$-Rellich ($\ell =2$) and general $p$-Birman ($\ell \ge 3$) inequalities. As a key step in the proof, we deduce a variant of the Copson inequality with a negative exponent, which may be of independent interest. Furthermore, we demonstrate how the continuous $p$-Birman inequality can be recovered from our discrete version, providing an alternative proof of this classical result. All constants in the obtained inequalities are shown to be optimal.