The Highest-Derivative Version of Variational Perturbation Theory

We systematically investigate different versions of variational perturbation theory by forcing not only the first or second but also higher derivatives of the approximant with respect to the variational parameter to vanish. The choice of the highest derivative version turns out to be the most successful one for approximating the ground-state energy of the anharmonic oscillator. It is therefore used to determine the critical exponent alpha of the specific heat in superfluid 4He in agreement with the value measured in recent space shuttle experiments.Comment: The article is a contribution to the book "Fluctuating Paths and Fields - Dedicated to Hagen Kleinert on the Occasion of His 60th Birthday", Eds. Wolfhard Janke, Axel Pelster, Hans-Juergen Schmidt, and Michael Bachmann (World Scientific, Singapore, 2001), p. 347-36