Repeated-root constacyclic codes over the finite chain ring $\mathbf{ \mathbb{F}_{p^m}[u]/\langle u^3 \rangle }$

Let $\mathcal{R}=\mathbb{F}_{p^m}[u]/\langle u^3 \rangle$ be the finite commutative chain ring with unity, where $p$ is a prime, $m$ is a positive integer and $\mathbb{F}_{p^m}$ is the finite field with $p^m$ elements. In this paper, we determine all repeated-root constacyclic codes of arbitrary lengths over $\mathcal{R},$ their sizes and their dual codes. As an application, we list some isodual constacyclic codes over $\mathcal{R}.$ We also determine Hamming distances, RT distances, and RT weight distributions of some repeated-root constacyclic codes over $\mathcal{R}.$