Simplicity of UHF and Cuntz algebras on $L^p$~spaces

We prove that, for $p\in [1,\mathrm{\infty }),$ and integers $d$ at least 2, the $L^p$ analog ${\mathcal{O}}_d^p$ of the Cuntz algebra ${\mathcal{O}}_d$ is a purely infinite simple amenable Banach algebra. The proof requires what we call the spatial $L^p$ UHF algebras, which are analogs of UHF algebras acting on $L^p$ spaces. As for the usual UHF C*-algebras, they have associated supernatural numbers. For fixed $p \in [1, \infty),$weprovethatanyspatial$L^p$ UHF algebra is simple and amenable, and that two such algebras are isomorphic if and only if they have the same supernatural number (equivalently, the same scaled ordered $K_0$-group). For distinct $p_1,p_2\in [1,\mathrm{\infty }),$ we prove that no spatial $L^{p_1}$ UHF algebra is isomorphic to any spatial $L^{p_2}$ UHF algebra.