(Multiplicative Sobolev inequality). Let $\mu,\lambda$ and $\gamma$ be three parameters that satisfy $$\bex 1\leq \mu,\lm<\infty,\quad \frac{2}{\lm}+\frac{1}{\mu}>1\quad\mbox{and}\quad 1+\frac{3}{\gamma}=\frac{2}{\lm}+\frac{1}{\mu}. \eex$$ Assume $\…