Given a subset E of R^n with empty interior, what is the maximum regularity exponent s for which there exist non-zero distributions in the Bessel potential Sobolev space H^s_p(R^n) that are supported entirely inside E? This question has arisen many times in my recent investigations into boundary integral equation formulations of linear wave scattering by fractal screens, and it is closely related to other fundamental questions concerning Sobolev spaces defined on ``rough'' (i.e. non-Lipschitz) domains. Roughly speaking, one expects that the ``fatter'' the set, the higher the maximum regularity that can be supported. For sets of zero Lebesgue measure one can show, using results on certain set capacities from classical potential theory, that the maximum regularity (if it exists) is negative, and is (almost) characterised by the fractal (Hausdorff) dimension of E. For sets with positive measure the maximum regularity (if it exists) is non-negative,but appears more difficult to characterise in terms of geometrical properties of E. I will present some partial results in this direction, which have recently been obtained by studying the asymptotic behaviour of the Fourier transform of the characteristic functions of certain fat Cantor sets.
