# Ngô Quốc Anh

## October 29, 2013

### The mean curvature under conformal changes of Riemannian metrics: A global approach

Filed under: Riemannian geometry — Tags: — Ngô Quốc Anh @ 22:46

In a previous post, I showed how the mean curvature changes under a conformal change using slightly local coordinates approach. Today, I want to reconsider that topic using global approach.

As usual, suppose $(M,g)$ is an $n$-dimensional Riemannian manifold with boundary $\partial M$. We also assume that $N$ is an outward unit normal vector field along the boundary $\partial M$. By an unit normal vector field we mean $g(N,N)=1$ and $g(N,X)=0$ for any tangent vectors $X$ of $\partial M$. Note that $\partial M$ is just a hypersurface of $M$ and we also use $g$ to denote the induced metric of $g$ onto $\partial M$. Then we have the so-called second fundamental form $\mathrm{I\!I}$ associated to $\partial M$ defined to be

$\displaystyle \mathrm{I\!I} (X,Y)=g(X,\nabla_Y N)$

for any tangent vectors $X,Y$ of $\partial M$.

Regarding to the mean curvature $H$, we shall use the following definition

$\displaystyle H=\text{trace}_g (\mathrm{I\!I})=g^{ij}\mathrm{I\!I}(\partial_i, \partial_j)=g^{ij} g(\partial_i,\nabla_{\partial_j} N).$

Our aim is to calculate the mean curvature $H$ under the following conformal change $\widehat g=\phi^\kappa g$ for some smooth positive function $\phi$ and a real number $\kappa$. It is important to note that by $\widehat g=\phi^\kappa g$ we mean

$\displaystyle \widehat g_{ij} = \phi^\kappa g_{ij}.$

## October 20, 2013

### Rayleigh-Type Quotient For The Conformal Killing Operator on manifolds with boundary

Filed under: Uncategorized — Tags: — Ngô Quốc Anh @ 22:50

Following the previous note, today we discuss a similar Rayleigh-type quotient for the conformal Killing operator $\mathbb L$ on manifolds $(M,g)$ with boundary. We also prove that

whenever $M$ admits no non-zero conformal Killing vector fields, the following holds

$\displaystyle\inf \frac{{{{\left( {\int_M {|\mathbb LX|^2 d{v_g}} } \right)}^{1/2}}}}{{{{\left( {\int_M {|X|^{2n/(n - 2)}d{v_g}} } \right)}^{(n - 2)/(2n)}}}} > 0$

where the infimum is taken over all smooth vector fields $X$ on $M$ with $X \not\equiv 0$.

$\displaystyle \frac{1}{2}\int_M |\mathbb L X|^2 dv_g= \int_M |\nabla X|^2 dv_g + \left( 1-\frac{2}{n}\right)\int_M |{\rm div}X|^2 dv_g - \int_M {\rm Ric}(X,X)dv_g,$

is no longer available, we use a new approach in order to estimate $\int_M |\mathbb L X|^2 dv_g$ from below. To this purpose, we make use of a Riemannian version for the Korn inequality recently proved by S. Dain [here].

First, in view of Corollary 1.2 in Dain’s paper, the following inequality holds

$\displaystyle \int_M |\nabla X|^2 dv_g \leqslant C \left( \int_M |X|^2 dv_g + \int_M |\mathbb LX|^2 dv_g \right)$

for some positive constant $C$ independent of $X$. This helps us to conclude that

$\displaystyle C\int_M |\mathbb LX|^2 dv_g \geqslant \|X\|_{H^1}^2 - (C+1)\|X\|_{L^2}^2$

as in Dahl et al’ paper. Therefore, we can argue by contradiction by assuming that there exists a sequence of vector fields $\{X_k\}_k \in H^1(M)$ such that

## October 13, 2013

### Rayleigh-type quotient for the conformal Killing operator

Filed under: Riemannian geometry — Tags: — Ngô Quốc Anh @ 10:05

Suppose $(M,g)$ is a compact Riemannian manifold without boundary of dimension $n \geqslant 3$. We further assume that $M$ admits no conformal Killing vector fields.

In this entry, we discuss a beautiful result due to Dahl-Gicquaud-Humbert recently published in Duke Math. J. They proved that

$\displaystyle\inf \frac{{{{\left( {\int_M {|\mathbb LX|^2 d{v_g}} } \right)}^{1/2}}}}{{{{\left( {\int_M {|X|^{2n/(n - 2)}d{v_g}} } \right)}^{(n - 2)/(2n)}}}} > 0$

where the infimum is taken over all smooth vector fields $X$ on $M$ with $X \not\equiv 0$.

Their proof goes as follows: First by the compactness of $M$, ${\rm Ric} \leqslant \lambda g$ for some constant $\lambda$. We now use the the Bochner-type formula for the conformal Killing operator on manifolds without boundary, i.e.,

$\displaystyle \frac{1}{2}\int_M |\mathbb L X|^2 dv_g= \int_M |\nabla X|^2 dv_g + \left( 1-\frac{2}{n}\right)\int_M |{\rm div}X|^2 dv_g - \int_M {\rm Ric}(X,X)dv_g ,$

which now yields

$\displaystyle \frac{1}{2}\int_M |\mathbb L X|^2 dv_g \geqslant \int_M |\nabla X|^2 dv_g - \lambda\int_M |X|^2 dv_g,$

thanks to $1-\frac{2}{n}>0$. Using the standard norm for $H^1(M)$, we rewrite the preceeding inequality as follows

$\displaystyle \frac{1}{2}\int_M |\mathbb L X|^2 dv_g \geqslant \|X\|_{H^1}^2 - (\lambda+1)\|X\|_{L^2}^2.$

## October 12, 2013

### Bochner-type formula for the conformal Killing operator on manifolds with boundary

Filed under: Uncategorized — Tags: , — Ngô Quốc Anh @ 8:39

Given a Riemannian manifold $(M,g)$ without boundary, in the previous note, we derived a Bochner-type formula for the conformal Killing operator $\mathbb L$. Precisely, we obtained

$\displaystyle \frac{1}{2}\int_M |\mathbb L X|^2 dv_g= \int_M |\nabla X|^2 dv_g + \left( 1-\frac{2}{n}\right)\int_M |{\rm div}X|^2 dv_g - \int_M {\rm Ric}(X,X)dv_g$

or equivalenlty,

$\displaystyle \frac{1}{2}\int_M |\mathbb LX |^2 dv_g=- \int_M {({g^{ij}}{\nabla _i}{\nabla _j}{X^h}){X_h} - {\rm Ric}(X,X)d{v_g}}+\left( 1-\frac{2}{n}\right)\int_M |{\rm div}X|^2 dv_g.$

Today, we try to derive a similar formula for the operator $\mathbb L$ assuming the manifold has boundary $\partial M$.  Our starting point again is the Bochner formula for vector fields mentioned here, i.e.

$\displaystyle\frac{1}{2}\Delta (|X|^2) = |\nabla X|^2 + {\rm div}({\mathbb L_X}g)(X) - {\nabla _X}{\rm div}X - {\rm Ric}(X,X).$

Using this and the formula for $\Delta (|X|^2)$ that we derived here, we arrive at

$\displaystyle\frac{1}{2}{\Delta _g}(|X{|^2}) = ({g^{ij}}{\nabla _i}{\nabla _j}{X^h}){X_h} + |\nabla X{|^2}$

which now yields

$\displaystyle -{\rm div}({\mathbb L_X}g)(X)=-({g^{ij}}{\nabla _i}{\nabla _j}{X^h}){X_h}- {\nabla _X}{\rm div}X - {\rm Ric}(X,X).$

## October 11, 2013

### Bochner-type formula for the conformal Killing operator on manifolds without boundary

Filed under: Uncategorized — Tags: , — Ngô Quốc Anh @ 9:59

Assuming the Riemannian manifold $(M,g)$ is compact without boundary. In the previous post, we showed that

$\displaystyle \frac{1}{2}\int_M |\mathbb L_X g|^2 dv_g= \int_M |\nabla X|^2 dv_g +\int_M |{\rm div}X|^2 dv_g - \int_M {\rm Ric}(X,X)dv_g .$

Today, we use the above formula to derive a Bochner-type formula for the conformal Killing operator $\mathbb L$ given by

$\displaystyle \mathbb L X = \mathbb L_Xg - \frac{2}{n}\text{div}(X)g.$

Clearly, for any vector field $X$,

$\begin{array}{lcl} |\mathbb{L}X{|^2} &=& \displaystyle \Big|{\mathbb{L}_X}g - \frac{2}{n}( \text{div}X)g \Big|^2 \hfill \\ &=&\displaystyle {\left( {{\mathbb{L}_X}g - \frac{2}{n}( \text{div}X)g} \right)_{im}}{\left( {{\mathbb{L}_X}g - \frac{2}{n}( \text{div}X)g} \right)_{jn}}{g^{ij}}{g^{mn}} \hfill \\ &=& \displaystyle |{\mathbb{L}_X}g{|^2} + \frac{4}{{{n^2}}}{( \text{div}X)^2}\underbrace {{g_{im}}{g_{jn}}{g^{ij}}{g^{mn}}}_{\delta _n^i\delta _i^n = n} - \frac{2}{n}( \text{div}X)\left[ {{g_{im}}{\mathbb{L}_X}{g_{jn}} + {g_{jn}}{\mathbb{L}_X}{g_{im}}} \right]{g^{ij}}{g^{mn}} \hfill \\ &=& \displaystyle |{\mathbb{L}_X}g{|^2} + \frac{4}{n}{( \text{div}X)^2} - \frac{2}{n}( \text{div}X)[({\mathbb{L}_X}{g_{jn}})\underbrace {{g_{im}}{g^{ij}}{g^{mn}}}_{\delta _m^j{g^{mn}} = {g^{jn}}} + ({\mathbb{L}_X}{g_{im}})\underbrace {{g_{jn}}{g^{ij}}{g^{mn}}}_{\delta _n^i{g^{mn}} = {g^{im}}}] \hfill \\ &=&\displaystyle |{\mathbb{L}_X}g{|^2} + \frac{4}{n}{( \text{div}X)^2} - \frac{2}{n}( \text{div}X)[({\nabla _j}{X_n}+\nabla_n X_j){g^{jn}} + ({\nabla _i}{X_m}+\nabla_m X_i){g^{im}}] \hfill \\ &=& \displaystyle |{\mathbb{L}_X}g{|^2} + \frac{4}{n}{( \text{div}X)^2} - \frac{2}{n}( \text{div}X)[\underbrace {2{\nabla ^n}{X_n} + 2{\nabla ^m}{X_m}}_{4 \text{div}X}] \hfill \\ &=& \displaystyle |{\mathbb{L}_X}g{|^2} - \frac{4}{n}{( \text{div}X)^2}. \end{array}$