Person: Cobos Díaz, Fernando
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First Name
Fernando
Last Name
Cobos Díaz
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Matemáticas
Department
Análisis Matemático Matemática Aplicada
Area
Análisis Matemático
Identifiers
65 results
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Now showing 1 - 10 of 65
Item Measure of non-compactness and limiting interpolation with slowly varying functions(Banach Journal of Mathematical Analysis, 2024) Cobos Díaz, Fernando; Fernández-Cabrera Marín, Luz María; Grover, ManviWe give estimates for the measure of non-compactness of an operator interpolated by the limiting methods involving slowly varying functions. As applications we establish estimates for the measure of non-compactness of operators acting between Lorentz–Karamata spaces.Item Diversity of Lorentz-Zygmund spaces of operators defined by approximation numbers(Analysis Mathematica, 2023) Cobos Díaz, Fernando; Kühn, ThomasWe prove the following dichotomy for the spaces ℒ (a) p,q,α (X, Y) of all operators T ∈ ℒ(X, Y) whose approximation numbers belong to the Lorentz-Zygmund sequence spaces ℓp,q(log ℓ)α: If X and Y are infinite-dimensional Banach spaces, then the spaces ℒ (a) p,q,α (X, Y) with 0 < p < ∞, 0 < q ≤ ∞ and α ∈ ℝ are all different from each other, but otherwise, if X or Y are finite-dimensional, they are all equal (to ℒ(X, Y)). Moreover we show that the scale is strictly increasing in q, where ℒ (a) ∈,q (X, Y) is the space of all operators in ℒ(X, Y) whose approximation numbers are in the limiting Lorentz sequence space ∓∈,q.Item Logarithmic interpolation methods and measure of non-compactness(Quarterly Journal of Mathematics, 2019) Cobos Díaz, Fernando; Fernández Besoy, BlancaWe derive interpolation formulae for the measure of non-compactness of operators interpolated by logarithmic methods with θ = 0, 1 between quasi-Banach spaces. Applications are given to operators between Lorentz-Zygmund spaces.Item Approximation and entropy numbers of embeddings between approximation spaces(Constructive Approximation, 2018) Cobos Díaz, Fernando; Domínguez Bonilla, Óscar; Kühn, ThomasItem Nuclear embeddings of Besov spaces into Zygmund spaces(Journal of Fourier analysis and applications, 2019) Cobos Díaz, Fernando; Edmunds, David E.; Kühn, TomasLet d ∈ N and let Ω be a bounded Lipschitz domain in Rd. We prove that the embedding Id : Bd (Ω) −→ L (log L) (Ω) is nuclear if a < −1 and 1 ≤ p, q ≤ ∞,p,q ≤∞, while if −1 < a < 0, 2 < p < ∞ and p ≤ q ≤ ∞ while if −1 < a < 0, 2 < p < ∞ and p ≤ q ≤ ∞ the embedding Id fails to be nuclear. Furthermore, if a = −1, the embedding Id : Bd∞,∞(Ω) −→ L∞ (log L)−1 (Ω) is not nuclear.Item On compactness results of Lions-Peetre type for bilinear operators(Nonlinear Analysis, 2019) Cobos Díaz, Fernando; Fernández-Cabrera Marín, Luz María; Martínez, AntónLet Ā = (A₀ , A₁) , B̄ = (B₀ , B₁) be Banach couples, let E be a Banach space and let T be a bilinear operator such that ||T(a, b)||ᴇ ≤ M[sub]j ||a||ᴀ[sub]j ||b||ʙ[sub]j for a ∈ A₀ ∩ A₁, b ∈ B₀ ∩ B₁, j = 0, 1. If T : A°[sub]j × B°[sub]j −→ E compactly for j = 0 or 1, we show that T may be uniquely extended to a compact bilinear operator from the complex interpolation spaces generated by Ā and B̄ to E. Furthermore, the corresponding result for the real method is given and we also study the case when E is replaced by a couple (E₀, E₁) of Banach function spaces on the same measure space.Item On Besov spaces of logarithmic smoothness and Lipschitz spaces(Journal of Mathematical Analysis and Applications, 2015) Cobos Díaz, Fernando; Domínguez Bonilla, ÓscarWe compare Besov spaces B-p,q(0,b) with zero classical smoothness and logarithmic smoothness b defined by using the Fourier transform with the corresponding spaces:B-p,q(0,b) defined by means of the modulus of smoothness. In particular, we show that B-p,q(0,b+1/2) = B-2,2(0,b) for b > -1/2. We also determine the dual of In:B-p,q(0,b) with the help of logarithmic Lipschitz spaces Lip(p,q)((1,-alpha)) Finally we show embeddings between spaces Lip(p,q)((1,-alpha)) and B-p,q(1,b) which complement and improve embeddings established by Haroske (2000).Item On function spaces of Lorentz–Sobolev type(Mathematische Annalen, 2021) Fernández Besoy, Blanca; Cobos Díaz, Fernando; Triebel, HansWe work with Triebel–Lizorkin spaces FsqLp,r(Rn) and Besov spaces BsqLp,r(Rn) with Lorentz smoothness. Using their characterizations by real interpolation we show how to transfer a number of properties of the usual Triebel–Lizorkin and Besov spaces to the spaces with Lorentz smoothness. In particular, we give results on diffeomorphisms, extension operators, multipliers and we also show sufficient conditions on parameters for FsqLp,r(Rn) and BsqLp,r(Rn) to be multiplication algebras.Item On a problem of Lions concerning real interpolation spaces. The quasi-Banach case(Journal of Mathematical Analysis and Applications, 2022) Cobos Díaz, Fernando; Cwikel, M.; Kühn, ThomasWe prove that, under a mild condition on a couple (A0;A1) of quasi-Banach spaces, all real interpolation spaces (A0;A1)θ,p with 0 < θ < 1 and 0 < p ≤ ∞ are different from each other. In the Banach case and for 1 ≤ p ≤ ∞ this was shown by Janson, Nilsson, Peetre and Zafran, thus solving an old problem posed by J.-L. Lions. Moreover, we give an application to certain spaces which are important objects in Operator Theory and which consist of bounded linear operators whose approximation numbers belong to Lorentz sequence spaces.Item On a result of Peetre about interpolation of operator spaces(Publicacions Matemàtiques, 2000) Cobos Díaz, Fernando; Signes, TeresaWe establish interpolation formulae for operator spaces that are components of a given quasi-normed operator ideal.