Improved Moser-Trudinger type inequalities in the hyperbolic space $\mathbb H^n$

Abstract

We establish an improved version of the Moser-Trudinger inequality in the hyperbolic space $\mathbb H^n$, $n\geq 2$. Namely, we prove the following result: for any $0 \leq \lambda < \left(\frac{n-1}n\right)^n$, then we have $$ \sup_{\substack{u\in C_0^\infty(\mathbb H^n) \int_{\mathbb H^n} |\nabla_g u|_g^n d\text{Vol}_g -\lambda \int_{\mathbb H^n} |u|^n d\text{ Vol}_g \leq 1}} \int_{\mathbb H^n} \Phi_n(\alpha_n |u|^{\frac{n}{n-1}}) d\text{ Vol}_g < \infty, $$ where $\alpha_n = n \omega_{n-1}^{\frac1{n-1}}$, $\omega_{n-1}$ denotes the surface area of the unit sphere in $\mathbb R^n$ and $\Phi_n(t) = e^t -\sum_{j=0}^{n-2}\frac{t^j}{j!}$. This improves the Moser-Trudinger inequality in hyperbolic spaces obtained recently by Mancini and Sandeep, by Mancini, Sandeep and Tintarev and by Adimurthi and Tintarev. In the limiting case $\lambda =(\frac{n-1}n)^n$, we prove a Moser-Trudinger inequality with exact growth in $\mathbb H^n$, $$ \sup_{\substack{u\in C_0^\infty(\mathbb H^n) \int_{\mathbb H^n} |\nabla_g u|_g^n d\text{ Vol}_g -(\frac{n-1}n)^n \int_{\mathbb H^n} |u|^n d\text{ Vol}_g \leq 1}} \frac{1}{\int_{\mathbb H^n} |u|^n d\text{ Vol}_g}\int_{\mathbb H^n} \frac{\Phi_n(\alpha_n |u|^{\frac{n}{n-1}})}{(1+ |u|)^{\frac n{n-1}}} d\text{ Vol}_g < \infty. $$ This improves the Moser-Trudinger inequality with exact growth in $\mathbb H^n$ established by Lu and Tang.