High-dimensional integration on RdRd, weighted Hermite spaces, and orthogonal transforms

It has been found empirically that quasi-Monte Carlo methods are often efficient for very high-dimensional problems, that is, with dimension in the hundreds or even thousands. The common explanation for this surprising fact is that those functions for which this holds true behave rather like low-dimensional functions in that only few of the coordinates have a sizeable influence on its value. However, this statement may be true only after applying a suitable orthogonal transform to the input data, like utilizing the Brownian bridge construction or principal component analysis construction.We study the effect of general orthogonal transforms on functions on RdRd which are elements of certain weighted reproducing kernel Hilbert spaces. The notion of Hermite spaces is defined and it is shown that there are examples which admit tractability of integration. We translate the action of the orthogonal transform of RdRd into an action on the Hermite coefficients and we give examples where orthogonal transforms have a dramatic effect on the weighted norm, thus providing an explanation for the efficiency of using suitable orthogonal transforms.