Continuous Gaussian multifractional processes with random pointwise Hölder regularity

Abstract: Let X be an arbitrary centered Gaussian process whose trajectories are, with probability one, continuous nowhere differentiable functions. It follows from a classical result, derived from zero-one law, that, with probability one, the trajectories of X have the same global H\"older regularity over any compact interval, that is the uniform H\"older exponent does not depend on the choice of a trajectory. A similar phenomenon happens with their local H\"older regularity measured through the local H\"older exponent. Therefore, it seems natural to ask the following question: does such a phenomenon also occur with their pointwise H\"older regularity measured through the pointwise H\"older exponent? In this article, using the framework of multifractional processes, we construct a family of counterexamples showing that the answer to this question is not always positive.