Abstract. 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 analyse 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.

PACS numbers: 64.60.−i, 05.30, 71.10

Print publication: Issue 25 (23 June 2006)
Received 30 November 2005, in final form 8 February 2006
Published 7 June 2006