# Ngô Quốc Anh

## March 31, 2021

### Monotonicity of (1+1/x)^(x+α)

Filed under: Uncategorized — Ngô Quốc Anh @ 1:11

This post concerns the monotonicity of

$\displaystyle f: x \mapsto \Big(1+\frac 1x\Big)^{x+\alpha}$

on $(0,+\infty)$ with $\alpha \geq 0$. The two cases $\alpha=0$ and $\alpha=1$ are of special because these are always mentioned in many textbooks as

$\displaystyle \lim_{n \to \infty} \Big(1+\frac 1n\Big)^n= \lim_{n \to \infty} \Big(1+\frac 1n\Big)^{n+1} =e.$

Clearly, $f$ is monotone increasing with respect to $\alpha$. Hence we are left with the monotonicity of $f$ with respect to $x$. To study this problem, we examine $f'$ with respect to $x$.

Derivative of $f$. It is easy to get

$\displaystyle f'(x) = \Big(1+\frac 1x\Big)^{x+\alpha} \Big[ \log \Big(1+\frac 1x\Big)- \frac{x+a}{x(x+1)}\Big].$

Hence the sign of $f'$ is determined by the sign of

$\displaystyle g(x) = \log \Big(1+\frac 1x\Big)- \frac{x+a}{x(x+1)} .$

on $(0, \infty)$.

(more…)

## April 16, 2020

### Restriction of gradient, Laplacian, etc on level sets and applications

Filed under: Uncategorized — Tags: — Ngô Quốc Anh @ 18:10

This topic is devoted to proofs of several interesting identities involving derivatives on level sets. First, we start with the case of gradient. We shall prove

The first identity

$\displaystyle \partial_\nu f = \pm |\nabla f|$

on the level set

$\displaystyle \big\{ x \in \mathbf R^n : f(x) =0 \big\}.$

The above identity shows that while the right hand side involves the value of $f$ in a neighborhood, however, the left hand side indicates that only the normal direction is affected. Heuristically, any change of $f$ along the level set does not contribute to any derivative of $f$, namely, on the boundary of the level set, the norm of $\nabla f$ is actually the normal derivative $\partial_\nu f$. Therefore, the only direction taking into derivatives of $f$ is in the normal direction and this should be true for higher-order derivatives of $f$.

Next we prove the following

The second identity

$\displaystyle \partial_\nu \big(x \cdot \nabla f \big)=(\partial_\nu^2 f) (x \cdot \nu)$

on the level set

$\displaystyle \big\{ x \in \mathbf R^n : \partial_1 f(x) = \cdots = \partial_n f(x) = 0 \big\}.$

Combining the above two identities, we can prove

The third identity

$\displaystyle \partial_\nu^2 f=\Delta f$

on the level set

$\displaystyle \big\{ x \in \mathbf R^n : \partial_1 f(x) = \cdots = \partial_n f(x) = 0 \big\}$

which basically tells us how to compute the restriction of Laplacian on level sets. This note is devoted to a rigorous proof of the above facts together with a simple application of all these identities.

## April 14, 2019

### Extending functions between metric spaces: Continuity, uniform continuity, and uniform equicontinuity

Filed under: Giải Tích 3, Giải tích 8 (MA5206) — Tags: — Ngô Quốc Anh @ 15:02

This topic concerns a very classical question: extend of a function $f : X \to Y$ between two metric spaces to obtain a new function $\widetilde f : \overline X \to Y$ enjoying certain properties. I am interested in the following three properties:

• Continuity,
• Uniformly continuity,
• Pointwise equi-continuity, and
• Uniformly equi-continuity.

Throughout this topic, by $X$ and $Y$ we mean metric spaces with metrics $d_X$ and $d_Y$ respectively.

CONTINUITY IS NOT ENOUGH. Let us consider the first situation where the given function $f : X \to Y$ is only assumed to be continuous. In this scenario, there is no hope that we can extend such a continuous function $f$ to obtain a new continuous function $\widetilde f : \overline X \to Y$. The following counter-example demonstrates this:

Let $X = [0,\frac 12 ) \cup (\frac 12, 1]$ and let $f$ be any continuous function on $X$ such that there is a positive gap between $f(\frac 12+)$ and $f(\frac12-)$. For example, we can choose

$\displaystyle f(x)=\begin{cases}x^2&\text{ if } x<\frac 12,\\x^3 & \text{ if } x>\frac 12.\end{cases}$

Since $f$ is monotone increasing, we clearly have

$\displaystyle f(\frac12-)-f(\frac 12+)=\frac18.$

Hence any extension $\widetilde f$ of $f$ cannot be continuous because $\widetilde f$ will be discontinuous at $x =\frac 12$. Thus, we have just shown that continuity is not enough. For this reason, we require $f$ to be uniformly continuous.

## October 18, 2018

### Jacobian determinant of diffeomorphisms measures the quotient of area of small balls

Filed under: Uncategorized — Ngô Quốc Anh @ 23:50

This post concerns a widely mentioned feature of the Jacobian determinant of diffeomorphisms whose proof is not easy to find. The precise statement of the result is as follows:

Geometric meaning of the Jacobian determinant: Let $U \subset \mathbf R^n$ be open and $\phi : U \to \phi (U)$ be a diffeomorphism in $\mathbf R^n$. Fix a point $a \in U$. Then

$\displaystyle |\det J_\phi (a) | = \lim_{r \searrow 0} \frac{{\rm vol}(\phi(B(a,r)))}{{\rm vol}(B(a,r))},$

where $B(a, r)$ denotes the open ball in $\mathbf R^n$ centered at $a$ with radius $r$.

As a remark and to be more exact, we require $\phi$ to be a $C^1$-diffeomorphism. Before proving the above result, it is worth noting that it is true for linear maps, whose proof is not hard. One way to realize this is to make use of the change of variable formula for multiple integrals. The proof presented here is inspired by the proof of Lemma 5.1.12 in this book.

We now proceed with the proof whose proof is divide into a few steps.

Step 1. First we use $\| \cdot \|$ to denote a norm on $\mathbf R^n$. Clearly, because $\phi$ is a $C^1$-diffeomorphism we can write

$\displaystyle \phi (x) = \phi (a) + J_\phi (a) \cdot (x- a) + \vec \varepsilon (x) \| x- a\|,$

where the error vector-valued function $\vec \varepsilon$ enjoys the following properties

$\displaystyle \| \vec \varepsilon (x) \| \to 0$

## September 14, 2018

### Leibniz rule for proper integral with parameter whose limits also depends on the parameter

Filed under: Uncategorized — Ngô Quốc Anh @ 21:24

The following Leibniz integral rule is well-known

Theorem. Let $f(x, t)$ be a function such that both $f(x, t)$ and its partial derivative $f_x(x, t)$ are continuous in $t$ and $x$ in some region of the $(x, t)$-plane, including $a(x) \leqslant t \leqslant b(x)$, $x_0 \leqslant x \leqslant x_1$. Also suppose that the functions $a(x)$ and $b(x)$ are both continuous and both have continuous derivatives for $x_0 \leqslant x \leqslant x_1$. Then, for $x_0 \leqslant x \leqslant x_1$,

$\displaystyle \frac {d}{dx}\left(\int _{a(x)}^{b(x)}f(x,t)\,dt\right)=f\big (x,b(x)\big )b'(x)-f\big (x,a(x)\big) a'(x)+\int _{a(x)}^{b(x)}{\frac {\partial f }{\partial x}}(x,t)\,dt.$

The purpose of this note is to show that, in fact, it is not  is not necessary to assume the function $f$ to be continuous. We note that this is indeed the case in which the limits of the integral $\int_{a(x)}^{b(x)}f(x,t)\,dt$ do not depend on the parameter $x$. For convenience, it is routine to assume the continuity, which immediately implies that all integrals are well-defined.

As mentioned above, we want to show that this is also the case for integrals of the form above.

## October 9, 2017

### Jacobian of the stereographic projection not at the North pole

Filed under: Uncategorized — Ngô Quốc Anh @ 18:57

Denote $\pi_P : \mathbb S^n \to \mathbb R^n$ the stereographic projection performed with $P$ as the north pole to the equatorial plane of $\mathbb S^n$. Clearly when $P$ is the north pole $N$, i.e. $N = (0,...,0,1)$, then $\pi_N$ is the usual stereographic projection.

As we have already known that, for arbitrary $\xi \in \mathbb S^n$, the image of $\xi$ is

$\displaystyle \pi_P : \xi \mapsto x = P+\frac{\xi-P}{1-\xi \cdot P}.$

Here the point $x \in \mathbb R^n$ is being understood as a point in $\mathbb R^{n+1}$ by adding zero in the last coordinate. For the inverse map, it is not hard to see that

$\displaystyle \pi_P^{-1} : x \mapsto \xi =\frac{|x|^2-1}{|x|^2+1}P+\frac 2{|x|^2+1}x.$

The purpose of this entry is to compute the Jacobian of the, for example, $\pi_P^{-1}$ by comparing the ratio of volumes.

First pick two arbitrary points $x, y \in \mathbb R^n$ and denote $\xi = \pi_P^{-1}(x)$ and $\eta = \pi_P^{-1}(y)$. The Euclidean distance between $\xi$ and $\eta$ is

$\displaystyle |\xi -\eta|^2 = \sum_{i=1}^{n+1} |\xi_i - \eta_i|^2 =\sum_{i=1}^n |\xi_i - \eta_i|^2+|\xi_{n+1} - \eta_{n+1}|^2.$

## October 3, 2017

### Stereographic projection not at the North pole and an example of conformal transformation on S^n

Filed under: Riemannian geometry — Tags: , — Ngô Quốc Anh @ 12:09

Denote $\pi_P : \mathbb S^n \to \mathbb R^n$ the stereographic projection performed with $P$ as the north pole to the equatorial plane of $\mathbb S^n$. Clearly when $P$ is the north pole $N$, i.e. $N = (0,...,0,1)$, then $\pi_N$ is the usual stereographic projection.

Clearly, for arbitrary $x \in \mathbb S^n$, the image of $x$ is

$\displaystyle \pi_P : x \mapsto y = P+\frac{x-P}{1-x \cdot P}.$

For the inverse map, it is not hard to see that

$\displaystyle \pi_P^{-1} : y \mapsto x =\frac{|y|^2-1}{|y|^2+1}P+\frac 2{|y|^2+1}y.$

Derivation of $\pi_P$ and $\pi_P^{-1}$ are easy, for interested reader, I refer to an answer in . Let us now define the usual conformal transformation $\varphi_{P,t} : \mathbb S^n \to \mathbb S^n$ given by

$\displaystyle \varphi_{P,t} : x \mapsto \pi_P^{-1} \big( t \pi_P ( x) \big)$

## February 23, 2017

### In a normed space, finite linearly independent systems are stable under small perturbations

Filed under: Giải tích 8 (MA5206) — Ngô Quốc Anh @ 23:21

In this topic, we show that in a normed space, any finite linearly independent system is stable under small perturbations. To be exact, here is the statement.

Suppose $(X, \|\cdot\|)$ is a normed space and $\{x_1,...,x_n\}$ is a set of linearly independent elements in $X$. Then $\{x_1,...,x_n\}$ is stable under a small perturbation in the sense that there exists some small number $\varepsilon>0$ such that for any $\|y_i\| < \varepsilon$ with $1 \leqslant i \leqslant n$, the all elements of $\{x_1+y_1,...,x_n+y_n\}$ are also linearly independent.

We prove this result by way of contradiction. Indeed, for any $\varepsilon>0$, there exist $n$ elements $y_i \in X$ with $\|y_i\| < \varepsilon$ such that all elements of $\{x_1+y_1,...,x_n+y_n\}$ are linearly dependent, that is, there exist real numbers $\alpha_i$ with $1 \leqslant i \leqslant n$ such that

$\displaystyle \alpha_1 (x_1+y_1) + \cdots + \alpha_n (x_n+y_n) =0$

with

$\displaystyle |\alpha_1| + \cdots + |\alpha_n| >0.$

## December 11, 2016

### Why the equation Δu+1/u^α=0 in R^n has no positive solution?

Filed under: Uncategorized — Ngô Quốc Anh @ 23:50

Following the idea due to Brezis in his note that we had already mentioned once, it seems that we can show that the following equation

$\displaystyle \Delta u + \frac 1{u^\alpha}=0$

in $\mathbb R^n$ with $n\geqslant 2$ has no positive $C^2$-solution. Here we require $\alpha>0$.

Indeed, to see this, we first denote

$f(t)=-t^{-\alpha}$

with $t>0$. Clearly the function $f$ is monotone increasing in its domain. Using this, we rewrite our equation as follows

$\Delta u =f(u).$

## September 9, 2016

### Benefits of “complete” and “compact” for analysis on Riemannian manifolds

Filed under: Riemannian geometry — Tags: — Ngô Quốc Anh @ 10:08

When working on Riemannian manifolds, it is commonly assumed that the manifold is complete and compact. (The case of non-compactness is also of interest too.) In this entry, let us review the role of completeness and compactness in this setting.

How important the completeness is? Let us recall that for given a Riemannian manifold $(M,g)$, what we have is a nice structure as well as an appropriate analysis on any tangent space $T_pM$. For a $C^1$-curve $\gamma : [a,b] \to M$ on $M$, the length of $\gamma$ is

$\displaystyle L(\gamma) = \int_a^b \sqrt{g(\gamma (t)) \langle \partial_t \gamma \big|_t, \partial_t \gamma \big|_t\rangle} dt$

where $\partial_t \gamma\big|_t \in T_{\gamma (t)}M$ a tangent vector. (Note that by using curves, the tangent vector $\partial_t \gamma\big|_t$ is being understood as follows

$\displaystyle \partial_t \gamma\big|_t (f) = (f \circ \gamma)'(t)$

for any differentiable function $f$ at $\gamma(t)$.) Length of piecewise $C^1$ curves can be defined as the sum of the lengths of its pieces. From this a distance on $M$ whose topology coincides with the old one on $M$ is given as follows

$\displaystyle d_g(x,y) = \inf_\gamma L(\gamma)$

where the infimum is taken on all over the set of all piecewise $C^1$ curves connecting $x$ and $y$.

Older Posts »