Variational Interpolation Algorithm between Weak- and Strong-Coupling Expansions

For many physical quantities, theory supplies weak- and strong-coupling expansions of the types $\sum a_n \alpha ^n$ and \alpha ^p\sum b_n (\alpha^{-2/q) ^n, respectively. Either or both of these may have a zero radius of convergence. We present a simple interpolation algorithm which rapidly converges for an increasing number of known expansion coefficients. The accuracy is illustrated by calculating the ground state energies of the anharmonic oscillator using only the leading large-order coefficient $b_0$ (apart from the trivial expansion coefficent $a_0=1/2$). The errors are less than 0.5 for all g. The algorithm is applied to find energy and mass of the Fr\"ohlich-Feynman polaron. Our mass is quite different from Feynman's variational approach.