Abstract
We study the empirical process \({{\rm sup}_{f \in F}|N^{-1}\sum_{i=1}^{N}\,f^{2}(X_i)-\mathbb{E}f^{2}|}\), where F is a class of mean-zero functions on a probability space (Ω, μ), and \({(X_{i})_{i =1}^N}\) are selected independently according to μ.
We present a sharp bound on this supremum that depends on the Ψ 1 diameter of the class F (rather than on the Ψ 2 one) and on the complexity parameter γ 2(F,Ψ 2). In addition, we present optimal bounds on the random diameters \({{\rm sup}_{f \in F} {\rm max}_{|I|=m}\left(\sum_{i \in I} f^{2}(X_{i})\right)^{1/2}}\) using the same parameters. As applications, we extend several well-known results in Asymptotic Geometric Analysis to any isotropic, log-concave ensemble on \({\mathbb{R}^n}\).
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Mendelson, S. Empirical Processes with a Bounded Ψ 1 Diameter. Geom. Funct. Anal. 20, 988–1027 (2010). https://doi.org/10.1007/s00039-010-0084-5
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DOI: https://doi.org/10.1007/s00039-010-0084-5