Three-loop critical exponents, amplitude functions, and amplitude ratios from variational perturbation theory

We use variational perturbation theory to calculate various universal amplitude ratios above and below Tc in minimally subtracted straight phi4 theory with N components in three dimensions. In order to best exhibit the method as a powerful alternative to Borel resummation techniques, we consider only two- and three-loops expressions where our results are analytic expressions. For the critical exponents, we also extend existing analytic expressions for two loops to three loops.