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CONSTRAINED UNITARY DILATIONS AND NUMERICAL RANGES

It is shown that each contraction A on a Hilbert space H, with A + A 6 µI for some µ 2 R, has a unitary dilation U on H H satisfying U +U 6 µI. This is used to settle a conjecture of Halmos in the armative: The closure of the numerical range of each contraction A is the intersection of the closures of the numerical ranges of all unitary dilations of A. By means of the duality theory of completely positive linear maps, some further results concerning numerical ranges inclusions and dilations are deduced.