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Jean-Claude Hausmann - Mod Two Homology and Cohomology

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Jean-Claude Hausmann Mod Two Homology and Cohomology

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Cohomology and homology modulo 2 helps the reader grasp more readily the basics of a major tool in algebraic topology. Compared to a more general approach to (co)homology this refreshing approach has many pedagogical advantages:

1. It leads more quickly to the essentials of the subject,
2. An absence of signs and orientation considerations simplifies the theory,
3. Computations and advanced applications can be presented at an earlier stage,
4. Simple geometrical interpretations of (co)chains.

Mod 2 (co)homology was developed in the first quarter of the twentieth century as an alternative to integral homology, before both became particular cases of (co)homology with arbitrary coefficients.

The first chapters of this book may serve as a basis for a graduate-level introductory course to (co)homology. Simplicial and singular mod 2 (co)homology are introduced, with their products and Steenrod squares, as well as equivariant cohomology. Classical applications include Brouwers fixed point theorem, Poincar duality, Borsuk-Ulam theorem, Hopf invariant, Smith theory, Kervaire invariant, etc. The cohomology of flag manifolds is treated in detail (without spectral sequences), including the relationship between Stiefel-Whitney classes and Schubert calculus. More recent developments are also covered, including topological complexity, face spaces, equivariant Morse theory, conjugation spaces, polygon spaces, amongst others. Each chapter ends with exercises, with some hints and answers at the end of the book.

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Springer International Publishing Switzerland 2014
Jean-Claude Hausmann Mod Two Homology and Cohomology Universitext 10.1007/978-3-319-09354-3_1
1. Introduction
Jean-Claude Hausmann 1
(1)
University of Geneva, Geneva, Switzerland
Jean-Claude Hausmann
Email:
Picture 1 homology first occurred in 1908 in a paper of Tietze [196] (see also [40, pp. 4142]). Several results were first established using this Picture 2 approach, like the linking number for submanifolds in Picture 3 (see Sect. ), as well as Alexander duality [7]. One argument in favor of the choice of the Picture 4 homology was its simplicity, as J.W. Alexander says in his introduction: The theory of connectivity [homology] may be approached from two different angles depending on whether or not the notion of sense [orientation] is developed and taken into consideration. We have adopted the second and somewhat simpler point of view in this discussion in order to condense the necessary preliminaries as much as possible. A treatment involving the idea of sense would be somewhat more complicated but would follow along much the same lines.
Besides being simpler than its integral counterpart, Picture 5 homology sometimes gives new theorems. The first historical main example is the generalization of Poincar duality to all closed manifolds, whether orientable or not, a result obtained by Veblen and Alexander in 1913 [200]. As a consequence, the Euler characteristic of a closed odd-dimensional manifold vanishes.
The discoveries of Stiefel-Whitney classes in 19361938 and of Steenrod squares in 19471950 gave Picture 6 cohomology the status of a major tool in algebraic topology, providing for instance the theory of spin structures and Thoms work on the cobordism ring.
These notes are an introduction, at graduate students level, of Picture 7 (co)homology (there will be essentially no other). They include classical applications (Brouwer fixed point theorem, Poincar duality, Borsuk-Ulam theorem, Smith theory, etc) and less classical ones (face spaces, topological complexity, equivariant Morse theory, etc). The cohomology of flag manifolds is treated in details, including for Grassmannians the relationship between Stiefel-Whitney classes and Schubert calculus. Some original applications are given in Chap..
Our approach is different than that of classical textbooks, in which Picture 8 (co)homology is just a particular case of (co)homology with arbitrary coefficients. Also, most authors start with a full account of homology before approaching cohomology. In these notes, Picture 9 (co)homology is treated as a subject by itself and we start with cohomology and homology together from the beginning. The advantages of this approach are the following.
  • The definition of a (co)chain is simple and intuitive: an (say, simplicial) m-cochain is a set of Picture 10 -simplexes; an m-chain is a finite set of Picture 11 -simplexes. The concept of cochain is simpler than that of chain (one less word in the definition Picture 12 ), more flexible and somehow more natural. We thus tend to consider cohomology as the main concept and homology as a (useful) tool for some arguments.
  • Working with Picture 13 and its standard linear algebra is much simpler than working with Picture 14 . For instance, the Kronecker pairing has an intuitive geometric interpretation occurring at the beginning which shows in an elementary way that cohomology is the dual of homology. Several computations, like the homology of surfaces, are quite easy and come early in the exposition. Also, the cohomology ring is commutative . The cup square Picture 15 is a linear map and may be also non-trivial in odd degrees, leading to important invariants.
  • The absence of sign and orientation considerations is an enormous technical simplification (even of importance in computer algorithms computing homology). With much lighter computations and technicalities, the ideas of proofs are more apparent.
We hope that these notes will be, for students and teachers, a complement or companion to textbooks like those of Hatcher [82] or Munkres [155]. From our teaching experience, starting with Picture 16 (co)homology and taking advantage of its above mentioned simplicity is a great help to grasp the ideas of the subject. The technical difficulties of signs and orientations for finer theories, like integral (co)homology, may then be introduced afterwards, as an adaptation of the more intuitive Picture 17 (co)homology.
Not in this book The following tools are not used in these notes.
  • Augmented (co)chain complexes. The reduced cohomology is defined as for the unique map Simplicial approximation Spectral - photo 18 is defined as for the unique map Simplicial approximation Spectral sequences except - photo 19 for the unique map Picture 20 .
  • Simplicial approximation.
  • Spectral sequences (except in the proof of Proposition ).
Also, we do not use advanced homotopy tools, like spectra, completions, etc. Because of this, some prominent problems using Picture 21 cohomology are only briefly surveyed, like the work by Adams on the Hopf-invariant-one problem (p. 353), the Sullivans conjecture (pp. 240 and 353) and the Kervaire invariant (Sect. ).
Prerequisites The reader is assumed to have some familiarity with the following subjects:
  • general point set topology (compactness, connectedness, etc).
  • elementary language of categories and functors.
  • simple techniques of exact sequences, like the five lemma.
  • elementary facts about fundamental groups, coverings and higher homotopy groups (not much used).
  • elementary techniques of smooth manifolds.
Acknowledgments
A special thank is due to Volker Puppe who provided several valuable suggestions and simplifications. Michel Zisman, Pierre de la Harpe, Samuel Tinguely and Matthias Franz have carefully read several sections of these notes. The author is also grateful for useful comments to Jim Davis, Rebecca Goldin, Andr Haefliger, Tara Holm, Allen Knutson, Jrme Scherer, Dirk Schtz, Andras Szenes, Vladimir Turaev, Paul Turner, Claude Weber and Sad Zarati.
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