The Hausdorff measure version of Gallagher's theorem - Closing the gap and beyond

In this paper we prove an upper bound on the “size” of the set of multiplicatively ψ-approximable points in Rd for d>1 in terms of f-dimensional Hausdorff measure. This upper bound exactly complements the known lower bound, providing a “zero-full” law which relates the Hausdorff measure to the convergence/divergence of a certain series in both the homogeneous and inhomogeneous settings. This zero-full law resolves a question posed by Beresnevich and Velani (2015) [6] regarding the “log factor” discrepancy in the convergent/divergent sum conditions of their theorem. We further prove the analogous result for the multiplicative doubly metric setup.