We use a novel parameterization of the flowing Hamiltonian to show that the
flow equations based on continuous unitary transformations, as proposed by
Wegner, can be implemented through a nonlinear partial differential equation
involving one flow parameter and two system specific auxiliary variables. The
implementation is non-perturbative as the partial differential equation
involves a systematic expansion in fluctuations, controlled by the size of the
system, rather than the coupling constant. The method is applied to the Lipkin
model to construct a mapping which maps the non-interacting spectrum onto the
interacting spectrum to a very high accuracy. This function is universal in the
sense that the full spectrum for any (large) number of particles can be
obtained from it. In a similar way expectation values for a large class of
operators can be obtained, which also makes it possible to probe the stucture
of the eigenstates.Comment: 24 pages, 13 figure
