The mod-$p$ representation theory of the metaplectic cover of ${\mathrm{GL}}_2({\mathbf{Q}}_p)$

Half-integral weight modular forms are naturally viewed as automorphic forms on the so-called metaplectic covering of ${\mathrm{GL}}_2({\mathbf{A}}_{\mathbf{Q}})$ – a central extension by the roots of unity ${\mu }_2$ in $\mathbf{Q}$. For an odd prime number $p$, we give a complete classification of the smooth irreducible genuine mod-$p$ representations of the corresponding covering of ${\mathrm{GL}}_2({\mathbf{Q}}_p)$ by showing that the functor of taking pro-$p$-Iwahori-invariants and its left adjoint define a bijection onto the set of simple right modules of the pro-$p$ Iwahori Hecke algebra. As an application of our investigation of the irreducible subquotients of the universal module over the spherical Hecke algebra depending on some weight, we prove that being of finite length is equivalent to being finitely generated and admissible. Finally, we explain a relation to locally algebraic irreducible unramified genuine principal series representations.