K. Symanzik 
Euclidean Quantum Field Theory I [PDF ebook] 
Equations for a Scalar Model

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. Tion theory systems is our inaptitude to formulate properly the boundary conditions on such systems to make them mathematically meaningful. Prescriptions on how to break such systems off have been given at times but seem so far lacking in convincing justification as well as success.