Classical Three-Box "paradox"

A simple classical probabilistic system (a simple card game) classically exemplifies Aharonov and Vaidman's "Three-Box 'paradox'" [J. Phys. A 24, 2315 (1991)], implying that the Three-Box example is neither quantal nor a paradox and leaving one less difficulty to busy the interpreters of quantum mechanics. An ambiguity in the usual expression of the retrodiction formula is shown to have misled Albert, Aharonov, and D'Amato [Phys. Rev. Lett. 54, 5 (1985)] to a result not, in fact, "curious"; the discussion illustrates how to avoid this ambiguity.Comment: 10 pages. v4: As published, with corrections and updated reference

Uniqueness of a convex sum of products of projectors

Relative to a given factoring of the Hilbert space, the decomposition of an operator into a convex sum of products over sets of distinct 1-projectors, one set linearly independent, is unique.Comment: 4 pages. v2: Minor clarifications in Section III; as accepted for publication in J Math Phy