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Sharp constant in Riemannian L^p-Gagliardo-Nirenberg inequalities

Let (M,g) be a smooth compact Riemannian manifold of dimension n \geq 2, 1 < p < n and 1 \leq q < r < p^\ast = \frac{np}{n-p} be real parameters. This paper concerns to the validity of the optimal Gagliardo-Nirenberg inequality (\int_M |u|^r\; dv_g)^{\fracτ{r θ}} \leq (A_{opt} (\int_M |\nabla_g u|^p\; dv_g)^{\fracτ{p}} + B_{opt} (\int_M |u|^p\; dv_g)^{\fracτ{p}}) (int_M |u|^q\; dv_g)^{\frac{τ(1 - θ)}{θq}} \; . This kind of inequality is studied in Chen and Sun (Nonlinear Analysis 72 (2010), pp. 3159-3172) where the authors established its validity when 2 < p < r < p^\ast and (implicitly) τ= 2. Here we solve the case p \geq r and introduce one more parameter 1 \leq τ\leq \min\{p,2\}. Moreover, we prove the existence of extremal function for the optimal inequality above.