Analytical approximations as close as desired to special functions

Abstract

We present a method for constructing global analytical expressions that approximate a function over its entire range. These approximations not only mirror the original function as accurately as desired, but are purposefully created to possess features that the original function lacks. This is particularly useful for functions that lack closed form and are defined by integrals or infinite series. Replacing these definitions with simple analytical expressions enables in-depth qualitative analysis and replaces the current methods of evaluation. We demonstrate this procedure by providing replacements for a variety of pivotal functions in physics and cosmology including the pressure and density of quantum gas, the one-loop correction in thermal field theory, common polylog functions, and the error function.