Bloch-Wilson Hamiltonian and a Generalization of the Gell-Mann-Low Theorem

The effective Hamiltonian introduced many years ago by Bloch and generalized later by Wilson, appears to be the ideal starting point for Hamiltonian perturbation theory in quantum field theory. The present contribution derives the Bloch-Wilson Hamiltonian from a generalization of the Gell-Mann-Low theorem, thereby enabling a diagrammatic analysis of Hamiltonian perturbation theory in this approach.