[Everyday Mathematics]20150206

$$\bex \sen{fg}_{L^1}\leq C\sen{f}_{L^{r,\al}}\sen{g}_{L^{r',\al'}}, \eex$$ 其中 $$\bex f\in L^{r,\al},\quad g\in L^{r',\al'},\quad \frac{1}{r}+\frac{1}{r'}=1=\frac{1}{\al}+\frac{1}{\al'},\quad 1<r<\infty,\quad 1\leq \al\leq \infty. \eex$$