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The symmetric decreasing rearrangement $u^{\ast }$ of a measurable function is the radial, radially nonincreasing function whose superlevel sets are centered balls with the same measures as those of $|u|$. It preserves $L^p$ norms, and the Pólya-Szegő inequality gives $\Vert \nabla u^{\ast }{\Vert }_2\le \Vert \nabla u{\Vert }_2$.