Asymptotic Consistency of $α-$Rényi-Approximate Posteriors

In this work, we study consistency properties of $\alpha$-R\'enyi approximate posteriors, a class of variational Bayesian methods that approximate an intractable Bayesian posterior with a member of a tractable family of distributions, the latter chosen to minimize the $\alpha$-R\'enyi divergence from the true posterior. Unique to our work is that we consider settings with $\alpha > 1$, resulting in approximations that upperbound the log-likelihood, and result in approximations with a wider spread than traditional variational approaches that minimize the Kullback-Liebler divergence from the posterior. We provide sufficient conditions under which consistency holds, centering around the existence of a 'good' sequence of distributions in the approximating family. We discuss examples where this holds and show how the existence of such a good sequence implies posterior consistency in the limit of an infinite number of observations.