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Abstract

Under certain conditions on a function space X, it is proved that for every $L^∞$-function f with $∥f∥_{∞} ≤ 1$ one can find a function φ, 0 ≤ φ ≤ 1, such that φf ∈ X, $mes{φ ≠ 1} ≤ ɛ∥f∥_1$ and $∥φf∥_X ≤ const(1 + log ɛ^{-1})$. For X one can take, e.g., the space of functions with uniformly bounded Fourier sums, or the space of $L^∞$-functions on $ℝ^n$ whose convolutions with a fixed finite collection of Calderón-Zygmund kernels are also bounded.