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When do Projections Commute?

Rehder, W.

Necessary and sufficient conditions for commutativity of two projections in Hilbert space are given through properties of so-called conditional connectives which are derived from the conditional probability operator PQP. This approach unifies most of the known proofs, provides a few new criteria, and permits certain suggestive interpretations for compound properties of quantum-mechanical systems.
Published in: Zeitschrift fĂĽr Naturforschung A, 10.1515/zna-1980-0415, De Gruyter