Approximation of Mixed Order Sobolev Functions on the d-Torus: Asymptotics, Preasymptotics, and d-Dependence
文摘
We investigate the approximation of d-variate periodic functions in Sobolev spaces of dominating mixed (fractional) smoothness \(s>0\) on the d-dimensional torus, where the approximation error is measured in the \(L_2\)-norm. In other words, we study the approximation numbers \(a_n\) of the Sobolev embeddings \(H^s_\mathrm{mix}(\mathbb {T}^d)\hookrightarrow L_2(\mathbb {T}^d)\), with particular emphasis on the dependence on the dimension d. For any fixed smoothness \(s>0\), we find two-sided estimates for the approximation numbers as a function in n and d. We observe super-exponential decay of the constants in d, if n, the number of linear samples of f, is large. In addition, motivated by numerical implementation issues, we also focus on the error decay that can be achieved by approximations using only a few linear samples (small n). We present some surprising results for the so-called “preasymptotic-decay and point out connections to the recently introduced notion of quasi-polynomial tractability of approximation problems. Keywords Approximation numbers Sobolev spaces of mixed smoothness Rate of convergence Preasymptotics d-Dependence Quasi-polynomial tractability