Sections of functions and Sobolev type inequalities

Abstract

We study functions of two variables whose sections by the lines parallel to the coordinate axis satisfy Lipschitz condition of the order $0<\a\le 1.$ We prove that if for a function $f$ the $\operatorname{Lip} \a-$ norms of these sections belong to the Lorentz space $L^{p,1}(\R) \,(p=1/\a),$ then $f$ can be modified on a set of measure zero so as to become bounded and uniformly continuous on ${\mathbb{R}}^2.$ For $\a=1$ this gives an extension of Sobolev's theorem on continuity of functions of the space $W_1^{2,2}({\mathbb{R}}^2)$. We show that the exterior $L^{p,1}-$ norm cannot be replaced by a weaker Lorentz norm $L^{p,q}$ with $q>1$.

To appear in Proc. Steklov Inst. Math., v 284