Boundary quantum field theory is investigated in the Lagrangian framework.
Models are defined perturbatively around the Neumann boundary condition. The
analyticity properties of the Green functions are analyzed: Landau equations,
Cutkosky rules together with the Coleman-Norton interpretation are derived.
Illustrative examples as well as argument for the equivalence with other
perturbative expansions are presented.Comment: 29 pages, 19 eps and 11 pstex figures v2: some references and
comments added, misprints correcte