Douglas C. Howroyd's research while affiliated with University of St Andrews and other places

We study dimensions of sumsets and iterated sumsets and provide natural conditions which guarantee that a set $F \subseteq \mathbb{R}$ satisfies $\overline{\dim_{\text{B}}} F+F > \overline{\dim_{\text{B}}} F$ or even $\overline{\dim_{\text{B}}} n F \to 1$. Our results apply to, for example, all uniformly perfect sets, which include Ahlfors-David regular sets. Our proofs rely on the inverse theorems of Bourgain and Hochman and the Assouad and lower dimensions play a critical role. We give several applications of our results including an Erd\H{o}s-Volkmann type theorem for semigroups and new lower bounds for the dimensions of distance sets for sets with small dimension.

We study the upper and lower regularity dimensions in relation to the notions of doubling and uniformly perfect. These two regularity properties are closely related which is quantified thanks to the regularity dimensions. The regularity dimensions of pushforward measures onto graphs of Brownian motion are calculated, similarly for pushforwards with respect to quasisymmetric homeomorphisms. We finish by introducing an application to Diophantine approximation in the setting of Kleinian groups.

We investigate how the Hausdorff dimensions of microsets are related to the dimensions of the original set. It is known that the maximal dimension of a microset is the Assouad dimension of the set. We prove that the lower dimension can analogously be obtained as the minimal dimension of a microset. In particular, the maximum and minimum exist. We also show that for an arbitrary F σ \mathcal {F}_\sigma set Δ ⊆ [ 0 , d ] \Delta \subseteq [0,d] containing its infimum and supremum there is a compact set in [ 0 , 1 ] d [0,1]^d for which the set of Hausdorff dimensions attained by its microsets is exactly equal to the set Δ \Delta . Our work is motivated by the general programme of determining what geometric information about a set can be determined at the level of tangents.

We investigate how the Hausdorff dimensions of microsets are related to dimensions of the original set. It is known that the maximal dimension of a microset is the Assouad dimension of the set. We prove that the lower dimension can analogously be obtained as the minimal dimension of a microset. In particular, the maximum and minimum exist. We also show that for an arbitrary set $\Delta \subseteq [0,d]$ containing its infimum and supremum there is a compact set in $[0,1]^d$ for which the set of Hausdorff dimensions attained by its microsets is exactly equal to the set $\Delta$. Our work is motivated by the general programme of determining what geometric information about a set can be determined at the level of tangents.

In this paper we study the Assouad dimension of graphs of certain L\'evy processes and functions defined by stochastic integrals. We do this by introducing a convenient condition which guarantees a graph to have full Assouad dimension and then show that graphs of our studied processes satisfy this condition.

We study the upper regularity dimension which describes the extremal local scaling behaviour of a measure. A measure is doubling if and only if it has finite upper regularity dimension and so in some sense this dimension gives a finer, and quantifiable, description of doubling. We investigate the relationships between the upper regularity dimension and other notions of dimension such as the Assouad dimension (of the support), the upper local dimension, and the $L^q$-spectrum. Motivated by work on the Assouad dimension, we show that the upper regularity dimension of weak tangent measures cannot exceed that of the original measure, hence proving that all weak tangents of a doubling measure are themselves doubling. We also compute the upper regularity dimension explicitly in a number of important contexts including self-similar measures, self-affine measures, and measures on sequences.

Recently self-affine sponges have been shown to be interesting examples and counter-examples to several previously open problems. One class of recently discovered sponges are partially affine Bedford-McMullen sponges whose Assouad type dimensions cannot be calculated like the dimensions of regular Bedford-McMullen sponges are. We calculate the Assouad type dimensions for such partially affine sponges and discuss some of their more subtle details.

We study the Assouad and lower dimensions of self-affine sponges; the higher dimensional analogue of the planar self-affine carpets of Bedford and McMullen. Our techniques involve the weak tangents of Mackay and Tyson as well as regularity properties of doubling measures in the context studied by Bylund and Gudayol.