  For the Cos(2x)-Potential the coefficients of the weak- and strong coupling
perturbation series of the ground state energy are constructed recursively.
They match the well-known expansion coefficients of the Mathieu equation's
characteristic values. However presently there is no physically intuitive
method to extract the coefficients of the strong coupling series from those of
the weak one. The standard rule while giving exellent results for the
anharmonic oscillator fails completely in this case.
