For partitions $\lambda$ and $\mu$, the [tensor product](/page/Tensor%20Product) of type $A$ highest weight crystals decomposes as \begin{align*} B(\lambda)\otimes B(\mu)\cong \bigsqcup_\nu\bigsqcup_{a=1}^{c_{\lambda\mu}^{\nu}} B(\nu,a). \end{align*} Here each $B(\nu,a)$ is isomorphic to the irreducible highest weight crystal $B(\nu)$, and $c_{\lambda\mu}^{\nu}$ is the number of Littlewood--Richardson tableaux of skew shape $\nu/\lambda$ and content $\mu$.