Pauli asks questions about the algebraic properties of certain relations and makes some remarks about spin operator.

The letter contains detailed discussions of systems of hyper-complex numbers

Addition to the former letter.

Pauli wants to continue his work with Kemmer on systems of hyper-complex numbers. He asks questions about application of quantum theory (perturbation) and on scattering of neutral mesons.

Pauli continues with his calculations with hyper-complex numbers. He adds some remarks on "neutretto"-scattering and the theory of mesons.

A short addendum to the last letter.

Pauli thanks him for his letter of 26 Dec. He would like to hear more from him about Heisenberg's S-function. He writes about the translation of Pauli's handbook article into Japanese.

Pauli talks about reasons for writing to Klein and not to Bohr, involvement with Heisenberg in field theory, Heisenberg's "artificial" solution of the commutation problem, difficulty with continuous beta-decay energy, and shortcomings of the field theory.

Pauli congratulates Klein on his work, his new position and remarks on physics in Sweden and Klein's work. Pauli describes his attempts to treat beta decay using "neutron" hypothesis. Pauli gives arguments against failure of energy

Pauli talks about the question of Fermi-statistics for zero-spin particles.