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Källén writes on his recent attempts to simplify the calculations on quantum electrodynamics.

Källén make some mathematical comments on Thirring's calculations.

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

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.