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设
[rho(x)=left{begin{array}{ll}e^{frac{1}{|x|^2-1}},&|x|<1,\0,&|x|geq 1.end{array}right.,quad xin R^N.]
则称 $\rho_n(\cdot)=n^N \rho(nx)$ 为磨光函数. 而有性质
若 $f\in L^p (1\leq p<\infty$, 则 $f*\rho_n\to f$ strongly in $L^p$.
若 $f\in L^\infty$, 则 $f*\rho_n\to f$ weakly $*$ in $L^\infty$.
如此, $C_c^\infty$ is dense in $L^p (1<p<\infty)$, and sequentially weak $*$ dense in $L^\infty$.
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