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Complete Pick positivity and unitary invariance

Angshuman Bhattacharya, Tirthankar Bhattacharyya (2010)

Studia Mathematica

The characteristic function for a contraction is a classical complete unitary invariant devised by Sz.-Nagy and Foiaş. Just as a contraction is related to the Szegö kernel k S ( z , w ) = ( 1 - z w ̅ ) - 1 for |z|,|w| < 1, by means of ( 1 / k S ) ( T , T * ) 0 , we consider an arbitrary open connected domain Ω in ℂⁿ, a complete Pick kernel k on Ω and a tuple T = (T₁, ..., Tₙ) of commuting bounded operators on a complex separable Hilbert space ℋ such that (1/k)(T,T*) ≥ 0. For a complete Pick kernel the 1/k functional calculus makes sense in a beautiful...

Complex symmetry of Toeplitz operators on the weighted Bergman spaces

Xiao-He Hu (2022)

Czechoslovak Mathematical Journal

We give a concrete description of complex symmetric monomial Toeplitz operators T z p z ¯ q on the weighted Bergman space A 2 ( Ω ) , where Ω denotes the unit ball or the unit polydisk. We provide a necessary condition for T z p z ¯ q to be complex symmetric. When p , q 2 , we prove that T z p z ¯ q is complex symmetric on A 2 ( Ω ) if and only if p 1 = q 2 and p 2 = q 1 . Moreover, we completely characterize when monomial Toeplitz operators T z p z ¯ q on A 2 ( 𝔻 n ) are J U -symmetric with the n × n symmetric unitary matrix U .

Complex tangential characterizations of Hardy-Sobolev spaces of holomorphic functions.

Sandrine Grellier (1993)

Revista Matemática Iberoamericana

Let Ω be a C∞-domain in Cn. It is well known that a holomorphic function on Ω behaves twice as well in complex tangential directions (see [GS] and [Kr] for instance). It follows from well known results (see [H], [RS]) that some converse is true for any kind of regular functions when Ω satisfies(P)    The real tangent space is generated by the Lie brackets of real and imaginary parts of complex tangent vectorsIn this paper we are interested in the behavior of holomorphic Hardy-Sobolev functions in...

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