Multipliers between model spaces
Résumé
In this paper we characterize the multipliers from one model space (of the disk) to another. Our characterization involves kernels of Toeplitz operators and Carleson measures. Using a sub-level set condition of Cohn [12], we show, under certain circumstances , that every multiplier between the two model spaces is a bounded function. However, this is not always the case. In the case of onto multipliers, this answers a question posed by Crofoot [13]. When considering model spaces of the upper-half plane, we will discuss in some detail when the associated inner function is a meromorphic inner function. This connects to de Branges spaces of entire functions which are closely related to different important problems in complex analysis (e.g., zero distribution, differential equations, and completeness problems). When the derivative of the associated inner function is bounded, we show that the multipliers can be described as the kernel of a certain Toeplitz operator.
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