Abstract
We consider the problem of the maximum concentration in a fixed measurable subset \(\Omega \subset {\mathbb {R}}^{2d}\) of the time-frequency space for functions \(f\in L^2({\mathbb {R}}^{d})\). The notion of concentration can be made mathematically precise by considering the \(L^p\)-norm on \(\Omega \) of some time–frequency distribution of f such as the ambiguity function A(f). We provide a positive answer to an open maximization problem, by showing that for every subset \(\Omega \subset {\mathbb {R}}^{2d}\) of finite measure and every \(1\le p<\infty \), there exists an optimizer for
The lack of weak upper semicontinuity and the invariance under time-frequency shifts make the problem challenging. The proof is based on concentration compactness with time–frequency shifts as dislocations, and certain integral bounds and asymptotic decoupling estimates for the ambiguity function. We also discuss the case \(p=\infty \) and related optimization problems for the time correlation function, the cross-ambiguity function with a fixed window, and for functions in the modulation spaces \(M^q({\mathbb {R}}^{d})\), \(0<q<2\), equipped with continuous or discrete-type (quasi-)norms.
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Acknowledgements
The authors are very grateful to Karlheinz Gröchenig for bringing to their attention the problem solved here, in connection to an unpublished manuscript of his and Markus Neuhauser. The present research has been partially supported by the MIUR grant Dipartimenti di Eccellenza 2018–2022, CUP: E11G18000350001, DISMA, Politecnico di Torino. J. L. R. gratefully acknowledges support from the Austrian Science Fund (FWF): Y 1199. S. I. T. is member of the Machine Learning Genoa (MaLGa) Center, Università di Genova. F. N. and S. I. T. are members of the Gruppo Nazionale per l’Analisi Matematica, la Probabilità e le loro Applicazioni (GNAMPA) of the Istituto Nazionale di Alta Matematica (INdAM).
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Nicola, F., Romero, J.L. & Trapasso, S.I. On the existence of optimizers for time–frequency concentration problems. Calc. Var. 62, 21 (2023). https://doi.org/10.1007/s00526-022-02358-6
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DOI: https://doi.org/10.1007/s00526-022-02358-6