Nondeterminism of Linear Operators and Lower Entropy Estimates

Let u be a (bounded, linear) operator from a Hilbert space ℋ into the Banach space C(T), the space of continuous functions on the compact metric space T. We introduce and investigate numbers τ _n (u), n≥1, measuring the degree of determinism of the operator u. The slower τ _n (u) decreases, the less determined are functions in the range of u by their values on a certain set of points. It is shown that n ^−1/2 τ _n (u)≤2e _n (u), where e _n (u) are the (dyadic) entropy numbers of u. Furthermore, we transform the notion of strong local nondeterminism from the language of stochastic processes into that of linear operators. This property, together with a lower entropy estimate for the compact space T, leads to a lower estimate for τ _n (u), hence also for e _n (u). These results are used to prove sharp lower entropy estimates for some integral operators, among them, Riemann–Liouville operators with values in C(T) for some fractal set T. Some multi-dimensional extensions are treated as well.