Exact Results in Quantum Theory

The Dyson formula for the scattering operator usually gives ambiguous results for diagrams with corrections to external lines. The well-known solution to this problem is based on the Lehmann–Symanzik–Zimmermann (LSZ) formalism. In this talk, after reviewing the problem and its solution, I will give a simple prescription for LSZ-reduction in the case of mixing between an arbitrary number of fields.