Optimized evaluation of a large sum of functions using a three-grid approach

The efficient evaluation of numerous values of functions that vary rapidly only in a small part of the region of interest is presented. An optimized algorithm using a sequence of grids with increasing resolution is developed. The algorithm does not make any assumptions about special properties of the function to be evaluated, e.g., symmetry. An additional speed-up is obtained by exploiting the asymptotic behaviour of the functions to be summed. Two applications from high resolution atmospheric radiative transfer modelling in the infrared and microwave are presented. In a third example asymmetric Rautian line shapes important for high resolution molecular spectroscopy are considered. Computational gains by more than two orders of magnitude with relative errors less than 10⁻³ have been achieved.