The Quartic Oscillator: a Non-Perturbative Study by Continuous Unitary   Transformations
  The quantum quartic oscillator is investigated in order to test the many-body technique of the continuous unitary transformations. The quartic oscillator is sufficiently simple to allow a detailed study and comparison of various approximation schemes. Due to its simplicity, it can be used as pedagogical introduction in the unitary transformations. Both the spectrum and the spectral weights are discussed. 
S. Dusuel, Goetz S. Uhrig