Heisenberg writes on clarification of the question of the existence of a solution for the integral equation containing a simple pole. He gives his opinion on Källén's calculations.

Pauli regards Heisenberg's letter of 31 January as showing new progress. He talks about the mathematical difficulties of the Lee model and a general charaterization of the class of models with indefinite metric.

Mathematical appendix to the last letter.

Pauli recommends the establishing of a personal professorship for Källén who has done outstanding work in quantized field theory.

Pauli writes about Landau's paper on My-meson decay and about a note by Salam.