Suppose $C$ is a Borel set in $\mathbb{R}^n$. Consider the set$$Q=\{x|\lim_{r\to 0}\frac{\mathscr{L}^n(B(x,r)\cap C)}{r^n}=0\}$$How can I show that $Q$ is a Borel set? Thx.
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Suppose $C$ is a Borel set in $\mathbb{R}^n$. Consider the set$$Q=\{x|\lim_{r\to 0}\frac{\mathscr{L}^n(B(x,r)\cap C)}{r^n}=0\}$$How can I show that $Q$ is a Borel set? Thx.