A sharp Trudinger-Moser type inequality involving $L^{n}$ norm in the entire space $\mathbb{R}^{n}$

Abstract

Let $W^{1,n} ( \mathbb{R}^{n} $ be the standard Sobolev space and $\left\Vert \cdot\right\Vert _{n}$ be the $L^{n}$ norm on $\mathbb{R}^n$. We establish a sharp form of the following Trudinger-Moser inequality involving the $L^{n}$ norm \[ \underset{\left\Vert u\right\Vert _{W^{1,n}\left(\mathbb{R} ^{n}\right) }=1}{\sup}\int_{ \mathbb{R}^{n}}\Phi\left( \alpha_{n}\left\vert u\right\vert ^{\frac{n}{n-1}}\left( 1+\alpha\left\Vert u\right\Vert _{n}^{n}\right) ^{\frac{1}{n-1}}\right) dx<+\infty \]in the entire space $\mathbb{R}^n$ for any $0\leq\alpha<1$, where $\Phi\left( t\right) =e^{t}-\underset{j=0}{\overset{n-2}{\sum}}% \frac{t^{j}}{j!}$, $\alpha_{n}=n\omega_{n-1}^{\frac{1}{n-1}}$ and $\omega_{n-1}$ is the $n-1$ dimensional surface measure of the unit ball in $\mathbb{R}^n$. We also show that the above supremum is infinity for all $\alpha\geq1$. Moreover, we prove the supremum is attained, namely, there exists a maximizer for the above supremum when $\alpha>0$ is sufficiently small. The proof is based on the method of blow-up analysis of the nonlinear Euler-Lagrange equations of the Trudinger-Moser functionals. Our result sharpens the recent work \cite{J. M. do1} in which they show that the above inequality holds in a weaker form when $\Phi(t)$ is replaced by a strictly smaller $\Phi^*(t)=e^{t}-\underset{j=0}{\overset{n-1}{\sum}}% \frac{t^{j}}{j!}$. (Note that $\Phi(t)=\Phi^*(t)+\frac{t^{n-1}}{(n-1)!}$).