Maximum likelihood estimation of regularization parameters in high-dimensional inverse problems: An empirical bayesian approach. part ii: Theoretical analysis

This paper presents a detailed theoretical analysis of the three stochastic approximation proximal gradient algorithms proposed in our companion paper [A. F. Vidal et al., SIAM J. Imaging Sci., 13 (2020), pp. 1945–1989] to set regularization parameters by marginal maximum likelihood estimation. We prove the convergence of a more general stochastic approximation scheme that includes the three algorithms of [A. F. Vidal et al., SIAM J. Imaging Sci., 13 (2020), pp. 1945–1989] as special cases. This includes asymptotic and nonasymptotic convergence results with natural and easily verifiable conditions, as well as explicit bounds on the convergence rates. Importantly, the theory is also general in that it can be applied to other intractable optimization problems. A main novelty of the work is that the stochastic gradient estimates of our scheme are constructed from inexact proximal Markov chain Monte Carlo samplers. This allows the use of samplers that scale efficiently to large problems and for which we have precise theoretical guarantees.