ABSTRACT. A proof is given for the fact that the product of two self-adjoint operators, one of which is also positive, is again self-adjoint if and only if the product is normal. This theorem applies, in particular, if one operator is an orthogonal projection. In general, the posltlvity requirement cannot be dropped.KEY WORDS AND PHPSES. Self-adjoint given by the so-called "statistical operator". W, which is positive with trace (W) 1 and also named the "density operator" of the system. This probabilistic parlance stems from the intrinsic stochastic nature of quantum mechanics: property a, say, with representing operator A, will be found in the system not with certainty, but with a probability given by PW (a) trace (WA), and by measuring a, the original system changes into a new one whose density or state is given by AWA trace (WA)
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