Convergence Behavior of Variational Perturbation Expansions
  Variational weak-coupling perturbation theory yields converging approximations, uniformly in the coupling strength. This allows us to calculate directly the coefficients of `strong-coupling' expansions. For the anharmonic oscillator we explain the physical origin of the empirically observed convergence behavior which is exponentially fast with superimposed oscillations. 
Wolfhard Janke, Hagen Kleinert (JGU Mainz, FU Berlin)