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 T_c in minimally subtracted φ⁴ 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.