Recently I read about the Gagliardo-Nirenberg inequality. And I would like to ask about the attainability and the maximizers of the GN inequality: $GN(N,p,q,r)=\sup\frac{∫|u|^{r}dx}{(∫|∇u|^{p}dx)^{a}(∫|u|^{q}dx)^{1-a}}$$(∫|u|^{r}dx)^{\frac{1}{r}} \leq GN(N,p,q,r)(∫|∇u|^{p}dx)^{\frac{a}{p}}(∫|u|^{q}dx)^{\frac{1-a}{q}}$. Can the supremumbest constant GN(N,p,q,r) be achieved in some cases? Does anyone know good references about this subject?