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内容简介

《群与对称》讲述了 numbers measure size， groups measure symmetry. the first statement comes as no surprise； after all， that is what numbers are for. the second will be exploited here in an attempt to introduce the vocabulary and some of the highlights of elementary group theory.
a word about content and style seems appropriate. in this volume， the emphasis is on examples throughout， with a weighting towards the symmetry groups of solids and patterns. almost all the topics have been chosen so as to show groups in their most natural role， acting on （or permuting） the members ora set， whether it be the diagonals of a cube， the edges of a tree， or even some collection of subgroups of the given group. the material is divided into twenty-eight short chapters， each of which introduces a new result or idea.a glance at the contents will show that most of the mainstays of a first course arc here. the theorems of lagrange， cauchy， and sylow all have a chapter to themselves， as do the classifcation of finitely generated abelian groups， the enumeration of the finite rotation groups and the plane crystallographic groups， and the nielsen-schreier theorem.

目录

preface
chapter 1 symmetries of the tetrahedron
chapter 2 axioms
chapter 3 numbers
chapter 4 dihedral groups
chapter 5 subgroups and generators
chapter 6 permutations
chapter 7 isomorphisms
chapter 8 plato‘s solids and cayley’s theorem
chapter 9 matrix groups
chapter 10 products
chapter 11 lagrange‘s theorem
chapter 12 partitions
chapter 13 cauehy’s theorem
chapter 14 coujugacy
chapter 15 quotient groups
chapter 16 homomorphisms
chapter 17 actions， orbits， and stabilizers
chapter 18 counting orbits
chapter 19 finite rotation groups
chapter 20 the sylow theorems
chapter 21 finitely generated abelian groups
chapter 22 row and column operations
chapter 23 automorphisms
chapter 24 the euclidean group
chapter 25 lattices and point groups
chapter 26 wallpaper patterns
chapter 27 free groups and presentations
chapter 28 trees and the nielsen-schreier theorem
bibliography
index

前言/序言

群与对称 [Groups and Symmetry] 电子书 下载 mobi epub pdf txt

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生物形态的对称

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内容未读，应该 不错吧

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（3）左右对称：或称两侧对称，是仅通过一个平面（正中矢面）将身体分为互相显镜像关系的两个部分（例如脊椎动物的外形）。在正中矢面内由身体前端至后端的轴称为头尾轴或纵轴，这个轴与身体长轴大都一致。在正中矢面内与头尾轴成直角并通过背腹的轴为背腹轴或矢状轴。还有与正中矢面成直角的轴称正中侧面轴（或内外轴）、该轴夹着正中矢面，彼此相等且具有方向相反的极性，如果将两侧的正中侧面轴合起来看成为一轴时，则称为横轴。在辐射对称中，如相当于海星的一根足的同型部分，称为副节（paramere），副节其本身成两侧对称。一般两侧对称的每一半为与同一轴相关而极向相反的同型部分，此称为对节或体辐。副节、对节等的同型部分，一般来看，仅相互方向不同，可认为这是与对外界的关系相同有着密切的联系。所以在个体发生或系统发生过程中其生活方式变化时，而与之相关的对

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（2）双辐射对称：只有两个辐射轴，彼此互成直角，形式上可以把它看成是从辐射对称向左右对称的过渡型（例如栉水母）；

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（2）双辐射对称：只有两个辐射轴，彼此互成直角，形式上可以把它看成是从辐射对称向左右对称的过渡型（例如栉水母）；

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生物形态的对称

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一般指图形和形态被点、线或平面区分为相等的部分而言。在生物形态上主要的对称分为下列各种：（1）辐射对称：与身体主轴成直角且互为等角的几个轴（辐射轴）均相等，如果通过辐射轴把含有主轴的身体切开时，则常可把身体分为显镜像关系的两个部分。例如海星可见有五个辐射轴。另外在高等植物的茎和花等，也常具有辐射对称的结构；

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