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Trudinger type inequalities in and their best exponents
Authors:Shinji Adachi  Kazunaga Tanaka
Institution:Department of Mathematics, School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan ; Department of Mathematics, School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
Abstract:We study Trudinger type inequalities in ${\mathbf{R}}^{N}$ and their best exponents $\alpha _{N}$. We show for $\alpha \in (0,\alpha _{N})$, $\alpha _{N}=N\omega _{N-1}^{1/(N-1)}$ ($\omega _{N-1}$ is the surface area of the unit sphere in ${\mathbf{R}}^{N}$), there exists a constant $C_{\alpha }>0$ such that

\begin{equation*}\tag{$*$} \int _{\mathbf{R} ^{N}} \Phi _{N}\left (\alpha \left ( \frac{\left |u(x)\right | }{\|\nabla u\| _{L^{N}(\mathbf{R} ^{N})}} \right )^{\frac{N}{N-1}}\right )\, dx \leq C_{\alpha } \frac {\|u\|_{L^{N}(\mathbf{R} ^{N})} ^{N}}{\|\nabla u\|_{L^{N}(\mathbf{R} ^{N})}^{N}} \end{equation*}

for all $u \in W^{1,N} (\mathbf{R} ^{N})\setminus \{ 0\}$. Here $\Phi _{N}(\xi )$ is defined by

\begin{equation*}\Phi _{N}(\xi ) = \exp (\xi ) - \sum _{j=0}^{N-2} {\frac{1}{j!}}\xi ^{j}. \end{equation*}

It is also shown that $(*)$ with $\alpha \geq \alpha _{N}$ is false, which is different from the usual Trudinger's inequalities in bounded domains.

Keywords:
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