Oct 032012
 

Number theory will be understood, not as a collection of tricks and isolated results, but as a coherent and interconnected theory.

算术基本定理最早的准确表述与证明, 应该是出自 Gauss 的名著算术研究. 但是, 在这之前很久, 人们似乎就已经知道这个定理的具体内容, 并且已经广泛使用.

很明显, 这个结果是重要的! \(\sqrt2\) 是无理数的经典证法也依仗它! 当然, 定理也是漂亮的!

Green(与 Tao 合作 Green–Tao theorem 的那位)的(博士)导师 Gowers, 写了两篇关于此定理的文章: Why isn’t the fundamental theorem of arithmetic obvious?  和 Proving the fundamental theorem of arithmetic.

定理不是显然的. 流行的证明, 大家都知道, 是利用 Bézout 恒等式还有归纳法. 罕见的一个证明来自英国大数学家 Hardy 的数论导引, 如今名为哈代数论, 的第二章的最后: 利用最小数原理.

陈省身说, 好的数学就是可以引导出很多后来发展的数学! 所以, 题外话是: 竞赛数学不是好的数学!

对于算术基本定理来说, 很遗憾, 在一般的数域中, 在更广泛的范围, 并不成立. 20世纪最伟大的数学家 Hilbert 曾经举个一个例子: 唯一分解在

\[H=\{1,5,9,13,17,\dotsc\}\]

中不成立. 众所周知的一个事情是: 如果唯一分解总是成立的的话, 著名的 Fermat 大定理就太 easy 了.

算术基本定理诱导了如唯一分解整环, Euclid 整环等等概念. Gauss 为了描述唯一分解成立的程度, 引进了类数的概念. 大致上, 类数越大, 一个数被分解成质数的分法个数就越多. 虚二次域 \(Q( \sqrt D)\) 的 Gauss 猜想已经解决, 但实二次域的情形, 现在还是木有证明的事情.

关于理想 ideal, 倒是有一个相应的唯一分解定理: \(O_k\) 的任意非零真理想可以唯一写成质理想的积, 不考虑顺序的话.

 Posted by at 11:44 am
Oct 032012
 

Bernoulli 不等式

设 \( x \geqslant -1\) 为任意实数, \( n\in\Bbb N^+\), 则有

\[ (1+x)^n \geqslant 1+nx, \]

成立, 其中当 \( n>1 \) 时等号成立的充分必要条件是 \( x=0\).

注意: Bernoulli 不等式对 \( x \geqslant -2 \) 仍然成立.

Bernoulli 不等式的推广

1. 设 \(a > 0, a+b \geqslant 0, n \in \Bbb N^+\), 则下式成立

\begin{equation}(a+b)^n \geqslant a^n +na^{n-1}b,\end{equation}

其中当 \( n>1 \) 时等号成立的充分必要条件是 \( b=0\).

由于 \( \frac ba \geqslant -1\), 于是

\[(1+ \frac ba)^n \geqslant 1+n\frac ba,\]

这就是所要证明的 \((1)\).  \(\Box\)

2. 设 \( a_i > -1(i = 1,2,\dotsc,n ) \) 且同号, 则下式成立

\begin{equation}\prod_{i=1}^n(1+a_i) \geqslant 1+ \sum_{i=1}^na_i.\end{equation}

  事实上, 记 \(A_n = \prod\limits_{i=1}^n(1+a_i) – (1+ \sum\limits_{i=1}^na_i) \), 容易验证

\[A_n\geqslant A_{n-1} \geqslant \dotsb \geqslant A_2 > A_1 =0.  \Box \]

 例题

1. 使用归纳法, 注意 \((1)\), 可以容易的证明算术几何平均不等式(Inequality of arithmetic and geometric means).

2. (04 年国家队选拔) 设 \( n_1 ,n_2,\dotsc,n_k \) 是 \(k(k \geqslant 2) \) 个正整数, 且 \( 1<n_1 <n_2<\dotsc<n_k, \) 正整数 \(a,b\) 满足

\begin{equation}\prod_{i=1}^k(1- \frac{1}{n_i}) \leqslant \frac{a}{b} < \prod_{i=1}^{k-1}(1- \frac{1}{n_i}) .\end{equation}

证明  \( \prod\limits_{i=1}^kn_i \leqslant (4a)^ {2^k-1} \).

  这个问题其实不一定要用 Bernoulli 不等式来进行证明. 我 04 年首次见到这个题目的时候, 给出的解答就没有用到 Bernoulli 不等式.

3. \( n\in\Bbb N^+\), 则

\begin{equation}(1+ \frac1n)^n > \sum_{i=0}^n \frac1{i!}-\frac{\mathrm e}{2n}.\end{equation}

将 \((1+ \frac1n)^n \) 进行二项展开, 得

\[(1+ \frac1n)^n = 1+ 1+ \sum_{i=2}^n \frac1{i!}\prod_{k=1}^{i-1 }(1-\frac kn).\]

注意, 当 \(2\leqslant i\leqslant n\) 时, 有

\[\prod_{k=1}^{i-1} (1-\frac{k}{n})\geqslant1-\sum_{k=1}^{i-1}\frac{k}{n}=1-\frac{i(i-1)}{2n},\]

因此

\begin{equation}\begin{split}(1+ \frac1n)^n & \geqslant1+1+\sum_{i=2}^n \frac1{i!} (1-\frac{i(i-1)}{2n} )\\& =1+1+\sum_{i=2}^n(\frac1{i!} – \frac1{2n(i-2)!}) \\& = \sum_{i=0}^n\frac1{i!}-\frac1{2n}\sum_{i=0}^{n-2}\frac1{i!},\end{split}\end{equation}

由于 \(\sum\limits_{i=0}^{n-2}\frac 1{i!} < \mathrm e\), 所以, 这也就完成了我们的证明.  \(\Box\)

 Posted by at 11:16 am
Oct 022012
 
IMO 2012

IMO 2012

走向 IMO 数学奥林匹克试题集锦(2012) 出版

本书收集了 \(2011\) 年至 \(2012\) 年度中国数学奥林匹克的试题, 并对试题作详细地分析, 解答与评点. 试题包括: 全国高中数学联赛, 全国中学生数学冬令营, 国家队集训资料, 国家队选拔考, 女子奥林匹克, 西部奥林匹克, 东南地区数学奥林匹克以及国际数学奥林匹克等.

书名: 走向 IMO 数学奥林匹克试题集锦(2012)
ISBN: 978-7-5617-9860-7/G.5837
出版社: 华东师范大学出版社
作者: 2012年IMO中国国家集训队教练组
装帧:平装
开本:32开
出版日期: 2012.9
定价: 20 人民币元

 Posted by at 6:35 am
Oct 012012
 

北京大学数学科学学院的周健老师正在把 EGA 与 SGA 翻译成中文. 早在 \(05\) 年(也可能是 \(06\) 年, 记不清了), 我就听说他在进行这个工作. 嗯, 这是艰难的事业.

依照丘成桐先生的建议, 为方便广大青少年学习者, 设立EGA, SGA. 这里包括了 EGA 与 SGA 的中文译本, Bourbaki 的数学原理 看样子也会加进来. EGA 的相关资料也会收集整理, R. Godement, J.-P. Serre, P. Deligne, M. Raynaud 的几篇文章已经呈现在读者面前, Ngô Bảo Châu(吴宝珠)证明基本引理(fundamental lemma for automorphic forms)的文章也已经翻译完成.

翻译文档会即时更新在这里, 请仅仅使用在个人学习的用途上.

Sep 292012
 
Asymptote

Asymptote

Asymptote 是一门强大的矢量作图语言, 它是一个开源项目. 作者:Andy HammerlindlJohn Bowman, etc.

Asymptote 源于2002 年加拿大Alberta 大学的一个项目, 由Alberta 大学的 Bowman 和 Toronto大学的 Hammerlindl 发起. 当初的目的是希望把大名鼎鼎的科学绘图语言METAPOST 存储实数的方式由低精度的定点数改为浮点数, 以便更适应数据绘图的需要, 于是他们决定重新写一个绘图引擎.

  • Asymptote 的功能受 MetaPost 启发, 输出 PostScript 矢量图. 对 MetaPost 有所加强, 数学能力更强, 可以输出嵌入 3D prc 的 PDF 图形;
  • Asymptote 使用 \(\rm\LaTeX\) 生成图形中的文字和数学公式, 并可借此嵌入外部 EPS 图形;
  • Asymptote 语法受 C++ 启发, 并糅合了 Java,Python 等语言的一些元素, 支持模块化,基于对象的编程风格.

国内有 ctex 网站在大力推广Asymptote, 在这里可以找到许多相关的文档, 并可以与网友交流.

Sep 262012
 

刘培杰工作室出版了很多书, 估计快 100 本了. 不能否认, 好书确实有一些. 在平面几何, 不等式方面有些书确实不错; 数论方面, 也还行. 如果我现在是一个中学生,应该会很喜欢初等数论难题集. 再版一些好书, 题目收集的很完整, 这些都值得肯定!

不过, 在我看来, 刘培杰也不能免俗, 这土地的风气也在这些作品得到极大的体现: 出书只是他敛财的手段. 资料收集的很完整, 制作成书的细节则是需要改进的. 具体的表现有:

  • 追求数量, 而严重忽视质量;
  • 小错误过多, 比盗版书严重 10 倍不止, 估计没有经过校对;
  • 装订, 纸张都很糟糕, 封面单一丑陋;
  • 特贵; 可能是最贵的
  • 很多书厚度惊人, 仅仅是平庸的堆砌制作;
  • 版权

不少著作, 本来可以成为传世经典. 举一例足以: 厚厚的两本几何瑰宝! 这书仅仅是简单的拼凑. 砍掉大半, 再另外加进一些内容, 重新组织, 有一条主线, 体现出思想, 方能成为最上品.

不要把书都写成百科全书, 写成词典. 对词典来说, 每个字都具同等地位, 但数学的定理却不是.

我相信, 国外应该有很多我们从未听说的经典. 搜刮全世界, 找好书, 再译成中文. 而不是一窝蜂, 10本大同小异的不等式, 再10本厚度可砸死人的几何, 接着10本非常初级的数论, 10本雷同的方程,…

我认为一个严肃的数学工作者, 绝对不应该把书交给他来出版. 潘承洞, 于秀源的 “阶的估计” 是一本好书, 拒绝交给刘培杰, 即将由高等教育出版社再版, 是非常正确的明智之举.

我本人绝对不会买他的书. 其实也没啥新书, 重印居多.

 Posted by at 12:01 am
Sep 252012
 

把研究与性做类比的言论不少. 随便举两个例子:

Feynman:”physics is like sex: sure, it may give some practical results, but that’s not why we do it.”

Hawking(霍金)有一次作报告, 有人问到关于做研究的快乐, 他回答道,”跟做爱差不多, 不过前者更持久.”

好像, 我本人早些年有一句话:”做数学题就像做爱一样快乐.”

数学让人喜爱, 让人讨厌, 都是基于同样的理由: 需要思考! 需要死很多的脑细胞!

喜欢一件事物, 是因为带来的快乐; 讨厌一件事情, 则是因为痛苦.

 Posted by at 1:31 pm
Sep 212012
 

The Shaw Prize in Mathematical Sciences 2012 is awarded to Maxim Kontsevich for his pioneering works in algebra, geometry and mathematical physics and in particular deformation quantization, motivic integration and mirror symmetry.

2012 年的邵逸夫奖, 数学奖颁予法国高等科学研究所的教授马克西姆·康采维奇, 以表彰他在代数, 几何和数学物理上的开创性工作.

Maxim Kontsevich 也是 1998 年的菲尔兹奖(Fields Medal)得主.

颁奖典礼已经于 9 月 17 日进行.

邵逸夫奖的官网是 shawprize, 在这里可以找到获奖得主及其工作的简单介绍, 此外, 还有颁奖视频.

on the Prize in Mathematical Sciences 2012

Traditionally the interaction between mathematics and theoretical physics has been concerned with topics ranging from dynamical systems and partial differential equations to differential geometry to probability theory. For the last two decades, modern algebra and algebraic geometry (which is the study of the solutions of systems of polynomial equations in several variables via algebraic methods) have taken a central position in this interaction.  Physical insights and intuition, especially from string theory, have led to a number of unexpected and striking predictions in both classical and modern algebraic geometry.  Thanks to the efforts of many mathematicians new techniques and theories have been developed and some of these conjectures have been proven.

Maxim Kontsevich has led the way in a number of these developments.  Among his many achievements is his early work on Witten’s conjecture concerning the topology and geometry of the moduli (that is parameter) spaces of all algebraic curves of a given genus, his solution of the problem of deformation quantization, his work in mirror symmetry and in a different direction the theory of motivic integration.

Quantization is the process of passing from classical to quantum mechanics and it has been realized by different mathematical theories.  One of these is the algebraic theory of deformation quantization.  This takes place on a Poisson manifold (that is a manifold with a Poisson bracket on functions) for which there are two natural algebras, the classical observables which are the functions under point-wise multiplication and the Poisson algebra where the multiplication comes from the Poisson structure.  The problem is to give a formal deformation in powers of a parameter h, in which the zeroth order term is the classical algebra of observables and the next order term is the given Poisson algebra.  The construction of such a deformation was carried out in special cases (Weyl, Moyal, Fedosov…) but the general case proved formidable. It was resolved brilliantly by Kontsevich using ideas from quantum field theory.

The discovery by physicists of mirror pairs of Calabi–Yau manifolds has led to a rich and evolving mathematical theory of mirror symmetry.  The physics predicts that there is a relation between the symplectic geometry (that is a geometry coming from classical mechanics) on such a manifold and the algebraic/complex geometry of the mirror manifold.  When carried out in certain examples for which explicit computations can be made, this led to some remarkable predictions in classical enumerative geometry, concerning the counting of curves in higher dimensional spaces.  Some of these predictions have since been proven. Kontsevich introduced homological mirror symmetry which predicts that further refined objects associated with the symplectic geometry of the manifold are related to ones associated with the complex geometry of its mirror.  These conjectures and their generalizations have been proven in significant special cases.  From the beginning Kontsevich has played a leading role in the development of the mathematical theory of mirror symmetry.  He continues to revisit the original formulation and to provide clearer conceptual answers to the mathematical question:   “What is mirror symmetry?”

Motivic integration is another invention of Kontsevich.  It is an integration theory which applies in the setting of algebraic geometry.  Unlike the usual integral from calculus whose value is a number, the motivic integral has its values in a large ring which is built out of the collection of all varieties (the zero sets of polynomial equations).  It satisfies many properties similar to the usual integral and while appearing to be quite abstract, when computed and compared in different settings it yields some far reaching information about algebraic varieties as well as their singularities.  It has been used to resolve some basic questions about invariants of Calabi–Yau varieties and it is also central to many recent developments concerning the uniform structure of counting points on varieties over finite fields and rings.

Through his technical brilliance in resolving central problems, his conceptual insights and very original ideas, Kontsevich has played a substantial role in shaping modern algebra, algebraic geometry and mathematical physics and especially the connections between them.

Mathematical Sciences Selection Committee
The Shaw Prize

17 September 2012, Hong Kong

Autobiography of Maxim Kontsevich

I was born in 1964 in a suburb of Moscow, close to a big forest. My father is a well-known specialist in Korean language and history, my mоther was an engineer (she is retired now), and my elder brother is a specialist in computer vision.  The apartment where I grew up was very small and full of books – about half of them in Korean or Chinese.

I became interested in mathematics at age 10 – 11, mainly because of the influence of my brother. Several books at popular level were very inspiring. Also, my brother was subscribed to the famous monthly “Kvant” magazine containing many wonderful articles on mathematics and physics addressed to high-school kids, sometimes explaining even new results or unresolved problems.  Also, I used to take part in Olympiads at various levels and was very successful.

In the Soviet Union, some schools had special classes for gifted children, with an additional four hours per week devoted to extra-curricular education (usually in mathematics or physics) taught by university students who had passed through the same system themselves. At age 13 – 15 I was attending such a school in Moscow, and from 1980 till 1985 was studying mathematics at Moscow State University. Because of my previous training in High School, I never attended regular courses, but instead went to several graduate and research-level seminars where I learned a huge amount of material. My tutor was Israel Gelfand, one of the greatest mathematicians of the 20th Century. His weekly seminar, on Mondays, was completely unpredictable, and covered the whole spectrum of mathematics. Outstanding mathematicians, both Soviet and visitors from abroad, gave lectures. In a sense, I grew up in these seminars, and also had the great luck to witness the birth of conformal field theory and string theory in the mid-80s. The interaction with theoretical physics remains vitally important for me even now. After graduating from university, I became a researcher at the Institute for Information Transmission Problems. Simultaneously, I began to learn to play the cello and for several years enjoyed the good company of my musician friends with whom I played some obscure pieces of baroque and renaissance music.

In 1988, I went abroad for the first time, to Poland and France. Also in 1988, I wrote a short article concerning two different approaches to string theory, and maybe because of this result, was invited to visit the Max Planck Institute for Mathematics in Bonn for three months in 1990. At the end of my stay there was an annual informal meeting of mostly European mathematicians, called Arbeitstatgung, where the latest hot results were presented. The opening lecture by Michael Atiyah was about a new surprising conjecture of Witten concerning matrix models and the topology of moduli spaces of algebraic curves. In two days I came up with an idea of how to relate moduli spaces but with a completely new type of matrix model, and explained it to Atiyah. People at MPIM were very impressed and invited me to come back the following year. During the next 3 – 4 years I was visiting mostly Bonn, and also IAS in Princeton and Harvard. My then future wife Ekaterina, whom I met in Moscow, accompanied me, and in 1993 we were married. In Bonn I finished several works which became very well-known: one on Vassiliev invariants, and another on quantum cohomology (with Yu Manin, whose seminar I had attended back in Moscow).  Scientifically, a very important moment for me was Spring 1993 when I came to the idea of homological mirror symmetry, which was an opening of a grand new perspective. In 1994, I accepted an offer from Berkeley, but one year later I moved to IHES in France, where I continue to work. In 1999 my wife and I were granted French citizenship (keeping our Russian citizenship as well), and in 2001 our son was born.

For a few years I visited simultaneously Rutgers University, where my teacher Gelfand moved to after the perestroika, and IAS in Princeton. During the last six years I have regularly visited the University of Miami.

In my work I often change subjects, moving from Feynman graphs to abstract algebra, differential geometry, dynamical systems, finite fields. Still, mirror symmetry remains the major line. The interaction during the last two decades between mathematics and theoretical physics has been an amazing chain of breakthroughs. I am very happy to be a participant in this dialogue, not only absorbing mathematical ideas from string theory, but also giving something back, like a recent wall-crossing formula which I discovered with my long-term collaborator Yan Soibelman, and which became a very important tool in the hands of physicists, simultaneously answering questions concerning supersymmetric particles, and solving the classical problem about asymptotics for equations depending on small parameter.

17 September 2012