Coherently complete algebraic stacks in positive characteristic

Abstract: With the long-term goal of proving local structure theorems of algebraic stacks in positive characteristic near points with reductive (but possibly non-linearly reductive) stabilizer, we conjecture that quotient stacks of the form $[\mathrm{Spec}\, A/G]$, with $G$ reductive and $A^G$ complete local, are coherently complete along the unique closed point. We establish this conjecture in two interesting cases: (1) $A^G$ is artinian and (2) $G$ acts trivially on $\mathrm{Spec}\, A$. We also establish coherent completeness results for graded unipotent group actions. In order to establish these results, we prove a number of foundational statements concerning cohomological and completeness properties of algebraic stacks -- including on how these properties ascend and descend along morphisms.