Continuous Unitary Transformations

In the present time we observe a growing number of publications where the, so-called, flow equations are successfully used to diagonalize Hamiltonians by means of an appropriate unitary transformation. Here we discuss and compare the flow equations (FE) method (proposed in 1994) with the method of one step continuous unitary transformations (OS CUT) (proposed in 1982). It is shown that the FE method can be considered as a generalization of the OS CUT approach to the case of parameter dependent generator. The OS CUT method gives linear differential equations for the diagonalization procedure. In the FE method the system of differential equations is nonlinear. Finally we discuss the generalization of idea of continuous unitary transformations for the case of quantum equations of motion (Heisenberg picture and density matrix).