Källén proves a mathematical identity and he dicusses the properties of a function.

Källén talks about calculations as regards the non-unitarity of the S-matrix in the Lee model.

Källén writes on certain mathematical problems of the Lee model.

Källén attempts to clarify for himself the contents of the papers by Landau and his collaborators.

Källén talks about Thirring's work on quantum electrodynamics.

Källén enumerates various points of Heisenberg's theory, with which he does not agree.

Källén talks about calculation on an integral-representation of the S$_{c}$-function, avoiding the complex method of integration.

Källén encloses a manuscript dealing with the contents of his last letter of the 12 December 1949, in addition to a calculation on bosons in an external field.

Källén talks about some general observations on the mathematical formalism of the Lee model.

Källén writes about his work on quantum electrodynamics and especially about renormalization.