Interpolation of Hilbert spaces

Sammanfattning: (i) We prove that intermediate Banach spaces A, B with respect to arbitrary Hilbert couples H, K are exact interpolation iff they are exact K-monotonic, i.e. the condition f0?A and the inequality K(t,g0;K)?K(t,f0;H), t>0 imply g0?B and ||g0||B?||f0||A (K is Peetre's K-functional). It is well-known that this property is implied by the following: for each ?>1 there exists an operator T : H?K such that Tf0=g0, and K(t,Tf;K)??K(t,f;H), f?H0+H1, t>0.Verifying the latter property, it suffices to consider the "diagonal" case where H=K is finite-dimensional. In this case, we construct the relevant operators by a method which allows us to explicitly calculate them. In the strongest form of the theorem, it is shown that the statement remains valid when substituting ?=1.(ii) A new proof is given to a theorem of W. F. Donoghue which characterizes certain classes of functions whose domain of definition are finite sets, and which are subject to certain matrix inequalities. The result generalizes the classical Löwner theorem on monotone matrix functions, and also yields some information with respect to the finer study of monotone functions of finite order.(iii) It is shown that with respect to a positive concave function ? there exists a function h, positive and regular on ?+ and admitting of analytic continuation to the upper half-plane and having positive imaginary part there, such that h??? 2h. This fact is closely related to a theorem of Foia?, Ong and Rosenthal, which states that regardless of the choice of a concave function ?, and a weight ?, the weighted l2-space l2(?(?)) is c-interpolation with respect to the couple (l2,l2(?)), where we have c??2 for the best c. It turns out that c=?2 is best possible in this theorem; a fact which is implicit in the work of G. Sparr.(iv) We give a new proof and new interpretation (based on the work (ii) above) of Donoghue's interpolation theorem; for an intermediate Hilbert space H' to be exact interpolation with respect to a regular Hilbert couple H it is necessary and sufficient that the norm in H' be representable in the form ||f||'= (?[0,?] (1+t-1)K2(t,f;H)2d?(t))1/2 with some positive Radon measure ? on the compactified half-line [0,?].(v) The theorem of W. F. Donoghue (item (ii) above) is extended to interpolation of tensor products. Our result is related to A. Korányi's work on monotone matrix functions of several variables.