Ngô Quốc Anh

July 8, 2010

The Moser-Trudinger inequality

Followed by an entry where the Trudinger inequality had been discussed we now consider an important variant of it known as the Moser-Trudinger inequality.

Let us remind the Trudinger inequality

Theorem (Trudinger). Let \Omega \subset \mathbb R^n be a bounded domain and u \in W_0^{1, n}(\Omega) with

\displaystyle\int_\Omega {{{\left| {\nabla u} \right|}^n}dx} \leqslant 1.

Then there exist universal constants \beta>0, C_1>0 such that

\displaystyle\int_\Omega {\exp (\beta {|u|^\frac{n}{n-1}})dx} \leqslant {C_1}|\Omega |.

The Trudinger inequality has lots of application. For application to the prescribed Gauss curvature equation, one requires a particular value for the best constant \beta_0. In connection with his work on the Gauss curvature equation, J. Moser [here] sharpended the above result of Trungdier as follows

Theorem (Moser). Let \Omega \subset \mathbb R^n be a bounded domain and u \in W_0^{1, n}(\Omega) with

\displaystyle\int_\Omega {{{\left| {\nabla u} \right|}^n}dx} \leqslant 1.

Then there exist sharp constants \beta_0=\beta(n)>0, C_1=C_1(n)>0 given by

\displaystyle \beta_0=n\omega_{n-1}^{\frac{1}{n}-1}

such that

\displaystyle\int_\Omega {\exp (\beta {|u|^\frac{n}{n-1}})dx} \leqslant {C_1}|\Omega|, \quad \forall \beta \leqslant \beta_0.

The constant \beta_0 is sharp in the sense that for all \beta>\beta_0 there is a sequence of functions u_k \in W_0^{1,n}(\Omega) satisfying

\displaystyle\int_\Omega {{{\left| {\nabla u_k} \right|}^n}dx} \leqslant 1

but the integral

\displaystyle\int_\Omega {\exp (\beta {|u_k|^\frac{n}{n-1}})dx}

grow without bound.

For general compact closed manifold (M,g) the constant on the right hand side of the Moser-Trudinger inequality depends on the metric g. Working on a sphere (\mathbb S^2,g_c) with a canonical metric allows us to control the constants.

Theorem (Moser). There is a universal constant C_1>0 such that for all u \in W^{1,2}(\mathbb S^2) with

\displaystyle\int_{\mathbb S^2} {{{\left| {\nabla u} \right|}^n}dv_{g_c}}  \leqslant 1

and

\displaystyle\int_{\mathbb S^2} u dv_{g_c}=0

we have

\displaystyle\int_{\mathbb S^2} \exp(4\pi u^2)  \leqslant {C_1}.

Observe that

\displaystyle 4\pi = \int_{{\mathbb{S}^2}} {d{v_{{g_c}}}} <\int_{{\mathbb{S}^2}} {{e^{4\pi {u^2}}}d{v_{{g_c}}}} \leqslant {C_1}.

In the same way as we introduce in the entry concerning the Trudinger inequality one can show

Corollary. For

\displaystyle C_2 := \log C_1 +\log\frac{1}{4\pi}

one has

\displaystyle \log \overline\int_{{\mathbb{S}^2}} {{e^{2u}}d{v_{{g_c}}}} \leqslant \left[ {\frac{1}{{4\pi }}\int_{{\mathbb{S}^2}} {{{\left| {\nabla u} \right|}^2}d{v_{{g_c}}}} + 2\overline u } \right] + {C_2}

for all u \in W^{1,2}(\mathbb S^2).

Obviously, C_2 >0 since C_1 >4\pi. It turns out to determine the best constant C_2. This had been done by Onofri known as the Onofri inequality [here].

Theorem (Onofri).Let u \in W^{1,2}(\mathbb S^2) then we have

\displaystyle\log \overline\int_{{\mathbb{S}^2}} {{e^{2u}}d{v_{{g_c}}}} \leqslant \frac{1}{{4\pi }}\int_{{\mathbb{S}^2}} {{{\left| {\nabla u} \right|}^2}d{v_{{g_c}}}} + 2\overline u

with the equality iff

\Delta u +e^{2u}=1.

The proof of the Onofri inequality relies on a result due to Aubin

Theorem (Aubin). For all \varepsilon>0 there exists a constant C_\varepsilon such that

\displaystyle\log \overline\int_{{\mathbb{S}^2}} {{e^{2u}}d{v_{{g_c}}}} \leqslant \left[ {\left( {\frac{1}{2} + \varepsilon } \right)\frac{1}{{4\pi }}\int_{{\mathbb{S}^2}} {{{\left| {\nabla u} \right|}^2}d{v_{{g_c}}}} + 2\overline u } \right] + {C_\varepsilon }

for any u belonging to the following class

\displaystyle S = \left\{ {u \in {W^{1,2}}({\mathbb{S}^2}):\int_{{\mathbb{S}^2}} {{e^{2u}}{x_j}d{v_{{g_c}}}} = 0,j = \overline {1,3} } \right\}.

Source: S-Y.A. Chang, Non-linear elliptic equations in conformal geometry, EMS, 2004.

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