Impossibility of almost extension
نویسندگان
چکیده
Let (X,???X),(Y,???Y) be normed spaces with dim?(X)=n. Bourgain's almost extension theorem asserts that for any ?>0, if N is an ?-net of the unit sphere X and f:N?Y 1-Lipschitz, then there exists O(1)-Lipschitz F:X?Y such ?F(a)?f(a)?Y?n? all a?N. We prove this optimal up to lower order factors, i.e., sometimes maxa?N??F(a)?f(a)?Y?n1?o(1)? every F:X?Y. This improves bound maxa?N??F(a)?f(a)?Y?nc? some 0<c<12. If X=?2n, approximation in can improved maxa?N??F(a)?f(a)?Y?n?. sharp, maxa?N??F(a)?f(a)?Y?n? F:?2n?Y.
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ژورنال
عنوان ژورنال: Advances in Mathematics
سال: 2021
ISSN: ['1857-8365', '1857-8438']
DOI: https://doi.org/10.1016/j.aim.2021.107761