Friedrichs' early contributions are discussed in <A HREF="https://link.springer.com/chapter/10.1007/978-94-017-2012-0_9">On the Stone-von Neumann Uniqueness Theorem and Its Ramifications</A> by S.J. Summers: > In the early 1950's, K.O. Friedrichs undertook an influential attempt > to reduce the hand-waving typical of quantum field theory up to that > time [1,2]. The result of greatest interest was his construction of > some representations of the canonical commutation relations for > infinitely many degrees of freedom which were *not* unitarily > equivalent to the Fock representation. [1] K.O. Friedrichs, Mathematical Aspects of the Quantum Theory of Fields, 1953. [2] Commun. Pure Appl. Math., **4**, 161–224 (1951); **5**, 1–56 (1952); **6**, 1–72 (1953).