  First we give an introduction to the method of diagonalizing or
block-diagonalizing continuously a Hamiltonian and explain how this procedure
can be used to analyze the two-dimensional Hubbard model. Then we give a short
survey on applications of this flow equation on other models. Finally we
outline, how symmetry breaking can be introduced by means of a symmetry
breaking of the normal ordering, not of the Hamiltonian.
