$\boldsymbol{L}_{\infty}$-approximation in Korobov spaces with Exponential Weights

Abstract: We study multivariate $\boldsymbol{L}{\infty}$-approximation for a weighted Korobov space of periodic functions for which the Fourier coefficients decay exponentially fast. The weights are defined, in particular, in terms of two sequences $\boldsymbol{a}={a_j}$ and $\boldsymbol{b}={b_j}$ of positive real numbers bounded away from zero. We study the minimal worst-case error $e^{\boldsymbol{L}{\infty}\mathrm{-app},\Lambda}(n,s)$ of all algorithms that use $n$ information evaluations from a class $\Lambda$ in the $s$-variate case. We consider two classes $\Lambda$ in this paper: the class $\Lambda^{{\rm all}}$ of all linear functionals and the class $\Lambda^{{\rm std}}$ of only function evaluations. We study exponential convergence of the minimal worst-case error, which means that $e^{\boldsymbol{L}_{\infty}\mathrm{-app},\Lambda}(n,s)$ converges to zero exponentially fast with increasing $n$. Furthermore, we consider how the error depends on the dimension $s$. To this end, we define the notions of $\kappa$-EC-weak, EC-polynomial and EC-strong polynomial tractability, where EC stands for "exponential convergence". In particular, EC-polynomial tractability means that we need a polynomial number of information evaluations in $s$ and $1+\log\,\varepsilon^{-1}$ to compute an $\varepsilon$-approximation. We derive necessary and sufficient conditions on the sequences $\boldsymbol{a}$ and $\boldsymbol{b}$ for obtaining exponential error convergence, and also for obtaining the various notions of tractability. The results are the same for both classes $\Lambda$.