# Sergei Yakovenko's blog: on Math and Teaching

## Saturday, September 21, 2019

### Alexandre M. Vinogradov 18.02.1938–20.09.2019

Filed under: Calculus on manifolds course,opinion — Sergei Yakovenko @ 3:40
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Yesterday, on his 82nd year, passed away a wonderful mathematician and pedagogue Alexandre Mikhailovich Vinogradov.

In 1977/8 in Moscow University he taught me a course (formally it was “Exercises”, not a full-fledged course) which contained in its DNA most of the ideas that I used much later for the course “Calculus on Manifolds”, which is featured on these pages.

At the same time A.M. was among the founding fathers of the “Jewish People’s University“, an unofficial attempt to give access to the modern mathematics to the Jewish candidates who were turned down at the entrance examinations to the Moscow University because of the state-sponsored antisemitism.

The last years of his life he spent in Salerno, investing his inexhaustible energy to organization of mathematical schools and workshops on modern theory of PDEs.

May his memory be blessed, יהיה זיכרו ברוך

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## Wednesday, September 11, 2019

### Solve or transform? Lecture on the Uplana de Shalit-2019

One of the  profound ideas of Poincare, broadly popularized by Arnold, was that equations that you need to solve, are better transformed to a simple form rather than solved explicitly. The reason is quite obvious: solutions of even simple equations often involve transcendental functions (exponents, logarithms, trigonometric functions), while transformation of equations to simple forms can very often be achieved by transformations which are (converging or diverging) Taylor series (polynomials of infinite degree).

I tried my best to convey the spirit of this theory in 60 min of available time.

Failed, predictably, but the lecture notes (very preliminary and raw version) are available for anybody to look.

Enjoy! And don’t hesitate to leave questions in the talkback section, I will be happy to clarify whatever is obscure.

## Sunday, September 2, 2018

### What is logarithm?

Filed under: lecture — Sergei Yakovenko @ 3:31
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The way logarithms, exponential and trigonometric functions are defined in the high school, is concealing more than explaining. In these notes, delivered in the framework of the annual Ulpana de Shalit, I try to explain how these functions (and their complex and matrix generalizations) naturally appear as solutions of ordinary differential equations.

## Friday, July 6, 2018

### Calculus on manifolds 2017/8 – Exam

Filed under: Calculus on manifolds course,problems & exercises — Sergei Yakovenko @ 5:12
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Finally the summer is here, and with it the exams.

The revised lecture notes are available here. The problems for the exam are at the end of the notes.

The rules of the game are extremely simple.

1. The grading is by the pass/fail scheme.
2. There is no predefined minimum of problems/items to get “pass”. Everything is decided in a purely subjective matter. Please avoid writing nonsense just for the sake of writing something: if accumulated beyond certain critical mass, it may result in the unlikely “fail”.
3. All textbooks are at your disposition: actually, you are encouraged to consult as many of them as you feel like it. Solutions of the form “this is theorem 123 from [XXX]” are accepted, provided that they indeed are coherent (e.g., not based on a different set of definitions).
4. You are welcome to talk to each other during preparation, but please spare me from checking identical submissions. The art of writing math texts is in itself important and you can learn it only by trial and error/corrections.
5. The deadline for submission is August 3, 2018 as set by the Feinberg Graduate school. I can cover you for some extra time in case you have unexpected circumstances, but please don’t try to stretch the deadline beyond reasonable limits.
6. If you find an error, invalidating one of the exam problems (errare humanum est), please post a comment to this post, and I will try to correct the formulation (or cancel the problem outright if it is beyond repair, – things happened…)
7. All questions concerning the exam/notes can be posted here in the comments or (if you prefer) in an email to sergei.yakovenko@weizmann.ac.il  (that’s me, in case you didn’t notice). I am essentially around here until the deadline, so if you prefer a personal communication, you are welcome to lay an ambush when I am in my office (Zyskind 150, usually every day between 15:30 and 18:00). But in any case, I’d prefer to be notified about request for a meeting in advance.
8. The choice of the language for submission is yours. My preference is the pidgin English typeset in LaTeX, but this is by no means mandatory. On the other hand, if you plan to write in Mandarin or Basque, I’ll have to disappoint you. Please double check if your the language of your choice is covered.
9. Problems marked with asterisks are considerably more difficult then the rest. If you love challenges, this is for you to try, but don’t get frustrated if you fail. If not, just register them as subtle math results which are so close to a general math course.

Good luck, and … and good luck!

–Sergei.

## Sunday, May 6, 2018

### Lecture 7, May 6, 2018

Filed under: Calculus on manifolds course — Sergei Yakovenko @ 4:05
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## Integration of differential 1-forms. Differential k-forms and their integration

Integration of a 1-form over a smooth curve: use pull-back or approximate by the Riemann sums? Independence of the parameterization.

$\displaystyle \int_\gamma\mathrm df=f(\text{end})-f(\text{start})$.

Integral of $y\,\mathrm dx$ along a smooth closed curve: area bounded by the curve.

Multilinear forms on a linear n-space. Tensor product.

Symmetric and antisymmetric multilinear forms. Symmetrization and alternation. Determinant. Wedge product. Algebraic properties of the wedge product, action by adjoint operators.

Differential k-forms and their integration along smooth k-dimensional “pieces” (images of a cube).

### Lecture 6, Apr 29, 2018

Filed under: Calculus on manifolds course — Sergei Yakovenko @ 3:55
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## Differential 1-forms

Vectors vs. covectors, covector field = differential 1-form. Functoriality: how to pull them back by smooth maps. Examples: differentials of smooth functions. Module of differential 1-forms over the structure algebra of smooth functions. Lie derivation along a flow of vector field.

## Vector fields on manifolds and what they are good for

Various ways to formally introduce tangent vectors to an abstract object that apriori sits nowhere (via charts, derivations, etc.) How the tangent spaces get their structure of vector spaces. Flows and related stuff.

Commutator of vector fields. Mystery of the depressed order.

Flows and the Lie derivative. Lie bracket. Commutator revisited.

[Not covered in the class]: Commuting vector fields, commuting flows, common integral surfaces, involutive distributions, Frobenius theorem*.

The last couple of lectures deviated slightly from the lecture notes from the past years. For your convenience, here is a more digestible (?) version.

*The topic is briefly covered in the revised lecture notes. Please let me know (in comments), if you are interested in more explanations. Otherwise chances are that I will include the detailed proof (split into doable steps) as a problem for the exam (in which case you will learn the subject much better 😉 ).

### Lecture 4 (April 15, 2018)

Filed under: Calculus on manifolds course — Sergei Yakovenko @ 4:09
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## Smooth manifolds: howto and whatfor

Topological spaces with an extra smooth structure. Examples: subsets of the Euclidean spaces, Lie groups, quotient spaces. Examples.

How to work with manifolds. Charts, atlases.

How to simplify your thoughts once you know what they are about. Structural algebra of smooth functions, various definitions of the vector fields, functoriality.

### Lecture 3 (April 8, 2018)

Filed under: Calculus on manifolds course — Sergei Yakovenko @ 4:04
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## Vector fields in open domains of $\mathbb R^n$

Ordinary differential equations, differential operators of the first order, local analysis and global consequences.

## Differentiable maps

• Definition of differentiability at a point. Maps $f:U\to W$ between open subspaces of the Euclidean spaces $U\subseteq \mathbb R^n,\ W\subseteq\mathbb R^m$ smooth on their domain.
• Tangent spaces $T_a U$, tangent bundle $TU=\bigcup_{a\in U}T_a U\simeq U\times\mathbb R^n$.
• Differential of a smooth map: $\mathrm df:TU\to TW$.
• What is the derivative? (answer: exists only when $n=m=1$). Partial derivatives.
• How do we define functions “having more than one derivative”?

Algebraic formalism:

• Algebra $C^\infty(U)$ of functions infinitely smooth in a domain $U\subseteq\mathbb R^n$
• Pullback morphism of algebras $f^*:C^\infty(W)\to C^\infty(U)$.

Vector fields: smooth maps $v:U\to TU$, such that $v(a)\in T_a U$.
Lie (directional, flow) derivations $L_v:C^\infty(U)\to C^\infty(U)$. The Leibniz rule (algebra) and its meaning (“Any Leibniz linear map of $C^\infty(U)$ to itself is a Lie derivative along some vector field).
Commutator of two vector fields (to be discussed more in the future).
Push-forward of vector fields by smooth invertible maps.

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