## Real and Complex Analysis, by C. Apelian and S. Surace, now publishedDecember 19, 2009

Posted by Akhil Mathew in analysis, General, math education.
Tags: , ,

(This material is reposted from here.)

The book Real and Complex Analysis, by Christopher Apelian and Steve Surace, was recently released.

It’s mainly for an introductory upper-level undergraduate course in real and complex analysis, especially at small liberal-arts colleges.  In this post, I’ll describe this book and how I was involved in its production.

At the start of my freshman year, my analysis teacher, Professor Surace, asked me to check over the drafts of a book he and his colleague (and my former teacher) Prof. Apelian were working on. It was the textbook for the course. At the time, if I remember correctly, there were six chapters: on the real and complex spaces, basic topology, limits, continuity, convergence of functions, and derivatives. The complex analysis part of the book was in its infancy (e.g., there was only a rudimentary outline of one chapter, which had been written some time back and was typeset in Word—they wrote it well before before they had switched to LaTeX). (more…)

## The Hahn-Banach theorem and two applicationsNovember 28, 2009

Posted by Akhil Mathew in analysis, functional analysis, MaBloWriMo.
Tags: , , , ,

I have been finishing my MaBloWriMo series on differential geometry with a proof of the Myers comparison theorem, which right now has only an outline, but will rely on the second variation formula for the energy integral.  After that, it looks like I’ll be posting somewhat more randomly.   Here I will try something different.

The Hahn-Banach theorem is a basic result in functional analysis, which simply states that one can extend a linear function from a subspace while preserving certain bounds, but whose applications are quite manifold.

Edit (12/5): This material doesn’t look so great on WordPress.  So, here’s the PDF version.  Note that the figure is omitted in the file.

The Hahn-Banach theorem

Theorem 1 (Hahn-Banach) Let ${X}$ be a vector space, ${g: X \rightarrow \mathbb{R}_{\geq 0}}$ a positive homogeneous (i.e. ${g(tx) = tg(x), t >0}$) and sublinear (i.e. ${g(x+y) \leq g(x) + g(y)}$) function.

Suppose ${Y}$ is a subspace and ${\lambda: Y \rightarrow \mathbb{R}}$ is a linear function with ${\lambda(y) \leq g(y)}$ for all ${y \in Y}$.

Then there is an extension of ${\lambda}$ to a functional ${\tilde{\lambda}: X \rightarrow \mathbb{R}}$ with

$\displaystyle \tilde{\lambda}(x) \leq g(x), \ x \in X.$

I’ll omit the proof; I want to discuss why it is so interesting.  One of its applications lies in questions of the form “are elements of this form dense in the space”? The reason is that if ${X}$ is a normed linear space and ${Y}$ a closed subspace, the quotient vector space ${X/Y}$ is a norm with the norm ${|x+Y| := \inf_{y\in Y} |x-y|.}$ (The closedness condition is necessary because otherwise there might be nonzero elements of ${X/Y}$ with zero norm.) (more…)

## A theorem of Mazur-Ulam on isometric maps of vector spacesNovember 22, 2009

Posted by Akhil Mathew in analysis, functional analysis, MaBloWriMo.
Tags: ,

I first posted this entry at Climbing Mount Bourbaki, where I have continued the MaBloWriMo series into topics in Riemannian geometry such as the Cartan-Hadamard theorem.  This particular material came up as part of the proof that distance-preserving maps between Riemannian manifolds are actually isometries.  However, the style of the entry seemed appropriate for this blog, so I’m placing it here as well.

The result in question is:

Theorem 1 (Mazur-Ulam) An isometry ${M: X \rightarrow X'}$ of a normed linear space ${X}$ onto another normed linear space ${X'}$ with ${M(0)=0}$ is linear. (more…)

## My new math blog: Climbing Mount BourbakiNovember 16, 2009

Posted by Akhil Mathew in General.
Tags:

I started a new mathematics blog at Climbing Mount Bourbaki.

Ultimately, as Steven observed in the comments yesterday, textbook-flavored posts–that is to say, those belonging to a long series on a given topic, aren’t quite what this blog is about.  In fact, I don’t think there has been much about “Mathematical research and problem-solving” as of late.  That said, I like doing such posts, and it helps me learn mathematics.  That’s why I started this new blath.

So, what do I plan to do here?  Of course, I’m still a contributor, though I probably will be less active than once-a-day.  I’m much less familiar with contest math-style problem-solving as some of the other contributors here.  At some point I will talk about my RSI project, but I’m still busy working on it.  Instead, I’ll probably aim to write more crisp, article-like posts that tell an interesting story without needing a whole series.  Those will appear here and on Climbing Mount Bourbaki.  The Bourbakist ones will be relegated to there.

## Hopf-Rinow II and an applicationNovember 15, 2009

Posted by Akhil Mathew in differential geometry, MaBloWriMo.
Tags: , , , ,

Now, let’s finish the proof of the Hopf-Rinow theorem (the first one) started yesterday. We need to show that given a Riemannian manifold ${(M,g)}$ which is a metric space ${d}$, the existence of arbitrary geodesics from ${p}$ implies that ${M}$ is complete with respect to ${d}$. Actually, this is slightly stronger than what H-R states: geodesic completeness at one point ${p}$ implies completeness.

The first thing to notice is that ${\exp: T_p(M) \rightarrow M}$ is smooth by the global smoothness theorem and the assumption that arbitrary geodesics from ${p}$ exist. Moreover, it is surjective by the second Hopf-Rinow theorem.

Now fix a ${d}$-Cauchy sequence ${q_n \in M}$. We will show that it converges. Draw minimal geodesics ${\gamma_n}$ travelling at unit speed with

$\displaystyle \gamma_n(0)=p, \quad \gamma_n( d(p,q_n)) = q_n.$  (more…)

## The Hopf-Rinow theorems and geodesic completenessNovember 14, 2009

Posted by Akhil Mathew in differential geometry, MaBloWriMo.
Tags: , , , ,

Ok, yesterday I covered the basic fact that given a Riemannian manifold ${(M,g)}$, the geodesics on ${M}$ (with respect to the Levi-Civita connection) locally minimize length. Today I will talk about the phenomenon of “geodesic completeness.”

Henceforth, all manifolds are assumed connected.

The first basic remark to make is the following. If ${c: I \rightarrow M}$ is a piecewise ${C^1}$-path between ${p,q}$ and has the smallest length among piecewise ${C^1}$ paths, then ${c}$ is, up to reparametrization, a geodesic (in particular smooth). The way to see this is to pick ${a,b \in I}$ very close to each other, so that ${c([a,b])}$ is contained in a neighborhood of ${c\left( \frac{a+b}{2}\right)}$ satisfying the conditions of yesterday’s theorem; then ${c|_{[a,b]}}$ must be length-minimizing, so it is a geodesic. We thus see that ${c}$ is locally a geodesic, hence globally.

Say that ${M}$ is geodesically complete if ${\exp}$ can be defined on all of ${TM}$; in other words, a geodesic ${\gamma}$ can be continued to ${(-\infty,\infty)}$. The name is justified by the following theorem:

Theorem 1 (Hopf-Rinow)

The following are equivalent:

• ${M}$ is geodesically complete.
• In the metric ${d}$ on ${M}$ induced by ${g}$ (see here), ${M}$ is a complete metric space (more…)

## Geodesics are locally length-minimizingNovember 13, 2009

Posted by Akhil Mathew in differential geometry, MaBloWriMo.
Tags: , ,

Fix a Riemannian manifold with metric ${g}$ and Levi-Civita connection ${\nabla}$. Then we can talk about geodesics on ${M}$ with respect to ${\nabla}$. We can also talk about the length of a piecewise smooth curve ${c: I \rightarrow M}$ as

$\displaystyle l(c) := \int g(c'(t),c'(t))^{1/2} dt .$

Our main goal today is:

Theorem 1 Given ${p \in M}$, there is a neighborhood ${U}$ containing ${p}$ such that geodesics from ${p}$ to every point of ${U}$ exist and also such that given a path ${c}$ inside ${U}$ from ${p}$ to ${q}$, we have

$\displaystyle l(\gamma_{pq}) \leq l(c)$

with equality holding if and only if ${c}$ is a reparametrization of ${\gamma_{pq}}$.

In other words, geodesics are locally path-minimizing.   Not necessarily globally–a great circle is a geodesic on a sphere with the Riemannian metric coming from the embedding in $\mathbb{R}^3$, but it need not be the shortest path between two points. (more…)

## The test case: flat Riemannian manifoldsNovember 12, 2009

Posted by Akhil Mathew in differential geometry, MaBloWriMo.
Tags: , , ,
1 comment so far

Recall that two Riemannian manifolds ${M,N}$ are isometric if there exists a diffeomorphism ${f: M \rightarrow N}$ that preserves the metric on the tangent spaces. The curvature tensor  (associated to the Levi-Civita connection) measures the deviation from flatness, where a manifold is flat if it is locally isometric to a neighborhood of ${\mathbb{R}^n}$.

Theorem 1 (The Test Case) The Riemannian manifold ${M}$ is locally isometric to ${\mathbb{R}^n}$ if and only if the curvature tensor vanishes. (more…)

## Identities for the curvature tensorNovember 11, 2009

Posted by Akhil Mathew in differential geometry, MaBloWriMo.
Tags: , , , ,

It turns out that the curvature tensor associated to the connection from a Riemannian pseudo-metric ${g}$ has to satisfy certain conditions.  (As usual, we denote by $\nabla$ the Levi-Civita connection associated to $g$, and we assume the ground manifold is smooth.)

First of all, we have skew-symmetry

$\displaystyle R(X,Y)Z = -R(Y,X)Z.$

This is immediate from the definition.

Next, we have another variant of skew-symmetry:

Proposition 1 $\displaystyle g( R(X,Y) Z, W) = -g( R(X,Y) W, Z)$  (more…)

## The fundamental theorem of Riemannian geometry and the Levi-Civita connectionNovember 10, 2009

Posted by Akhil Mathew in differential geometry, MaBloWriMo.
Tags: , ,
Ok, now onto the Levi-Civita connection. Fix a manifold ${M}$ with the pseudo-metric ${g}$. This means essentially a metric, except that ${g}$ as a bilinear form on the tangent spaces is still symmetric and nondegenerate but not necessarily positive definite. It is still possible to say that a pseudo-metric is compatible with a given connection.
Theorem 1 There is a unique symmetric connection ${\nabla}$ on ${M}$ compatible with ${g}$. (more…)