Simple characterizations of exponential tractability for linear multivariate problems

We study linear multivariate problems defined as the approximation of compact linear multivariate operators over Hilbert spaces. We provide necessary and sufficient conditions on various notions of tractability. These conditions are mainly given in terms of sums of certain functions depending on the singular values of the multivariate problem. In particular, most of these conditions do not require the ordering of these singular values, which in many cases is difficult to achieve.