Math 215a: Algebraic topology
UC Berkeley, Fall 2005
Instructor
Michael Hutchings
Email: [my last name minus the last
letter]@math.berkeley.edu
Office phone: 5106424329.
Office: 923 Evans.
Tentative office hours: Tu 25.
Spacetime coordinates
TuTh, 9:3011:00. As of 9/8, we are officially moving to 9
Evans.
Prerequisites
The only formal requirements are some basic algebra, pointset
topology, and "mathematical maturity". However, the more familiarity you have
with algebra and topology, the better.
Syllabus
Algebraic topology seeks to capture the "essence" of a topological
space in terms of various algebraic and combinatorial objects. We
will construct three such gadgets: the fundamental group, homology
groups, and the cohomology ring. We will apply these to prove various
classical results such as the classification of surfaces,
the Brouwer fixed point theorem, the Jordan curve theorem, the
Lefschetz fixed point theorem, and more.
An important topic related to algebraic topology is differential
topology, i.e. the study of smooth manifolds. In fact, I don't think
it really makes sense to study one without the other. So without
making differential topology a prerequisite, I will emphasize the
topology of manifolds, in order to provide more intuition and
applications.
No single textbook does all the things that I want to do in this
course. The official textbook is Algebraic Topology by
Hatcher. This is a very nice book, although it does not say much
about differential topology. Another very nice algebraic topology
text which also covers some differential topology is Geometry and
Topology by Bredon. Some further recommended books are listed
below.
Homework
Doing homework exercises is essential for learning the material, and
is an official course requirement. A reasonable effort on homework
will result in a good grade.
 HW#1, due Thursday 9/15 at the beginning of class. pdf ps
 HW#1a, due Thursday 9/22 at the beginning of class: Grade HW#1
according to the guidelines below. Here are
solutions.
 HW#2, due Thursday 9/29 at the beginning of class. pdf ps
 HW#2a, due Thursday 10/6 at the beginning of class: Grade HW#2.
Please be sure to STAPLE a separate sheet of comments with your name,
the gradee's name, and the grade! The more helpful comments you can
write, the better. Here are
solutions.
 HW#3, due Thursday 10/13 at the beginning of class. pdf
 HW#3a, due Thursday 10/20 at the beginning of class: Grade HW#3.
Here are
solutions.
 HW#4, due Thursday 10/27 at the beginning of class. pdf
 HW#4a, due Thursday 11/3 at the beginning of class: Grade HW#4.
Here are
solutions.
 HW#5, due Tuesday 11/15 at the beginning of class. pdf
 HW#5a, due Tuesday 11/22 at the beginning of class: Grade HW#5.
Here are
solutions.
 HW#6, due Tuesday 11/29 at the beginning of class. pdf (Note: some of the problems use the notion of
"fundamental class", which we will discuss in class on 11/22, and
which is explained on p. 236 of Hatcher.)
 HW#6a, due Tuesday 11/36 (that's a joke, please don't write to me
and complain) at the beginning of class: Grade HW#6.
Here are
solutions.
 HW#7, due never (because there won't be time to grade it): select
some interesting problems from section 3.3 of Hatcher and do them.
Homework grading policy. Since there are approximately fifty
students in the class, it is not possible for me to read all of the
homework carefully, and unfortunately I was not able to get a grader.
So we will try the following, which might even be more educational
than the traditional approach. Each homework assignment will be due
at the beginning of class on some particular day. At the end of that
lecture, everyone who handed in an assignment will be given a random
assignment from someone else to grade. (I will keep a record of who
gets which assignment.) After class, I will post solutions online to
help with grading (although of course these solutions are not unique).
If you can't make it to class on the day that the homework is due,
please slide your homework under my office door before class, together
with a note explaining how I can give you another assignment to grade.
To grade the assignment, please take a separate sheet of paper, and
label it with both your name and the name of the person whose
assignment you are grading. Write at most one page of commentary on
the assignment. If an answer is good, say so; if you are not
convinced by a proof, say why. Then give the whole assignment a grade
of 3 (excellent), 2 (good), or 1 (needs improvement). The grades
should not be taken too seriously; the important thing is to
communicate and critique mathematical arguments. (At the end of the
course, I will play some statistical games to adjust for the fact that
different people grade more generously than others.)
Finally, staple your commentary to the assignment, and hand in the
package to me in or before class one week after the assignment was
due. I will then look over all of the graded assignments and return
them in a subsequent class.
What I plan to do
 Introductory lecture.
 The fundamental group and covering spaces. (about 7 lectures)
 The fundamental group. (See Hatcher, sections
1.1 and 1.2.)
 Definition and basic examples: R^n, S^1 (by lifting to R),
products, S^n for n>1 (by using
smooth approximation to miss a point, or by cutting S^n into two
simply connected pieces whose intersection is connected).
 A more interesting example: the braid group. (Not in Hatcher.)
 Applications of the fundamental group of the circle:
 The Brouwer fixed point theorem and BorsukUlam theorem in
two dimensions.
 The socalled fundamental theorem of algebra.
 The Seifertvan Kampen theorem for the fundamental group of
the union of two spaces.
 The fundamental group of a CW complex with one 0cell has a
presentation with one generator for each 1cell and one relation
for each 2cell. (For details about the topology of CW complexes,
see the Appendix in Hatcher.)
 Dependence of the fundamental group on the base point.
 A homotopy equivalence induces an isomorphism on fudamental groups.
 The trefoil knot is
knotted and the Hopf link and Borromean rings are linked. (Not in
Hatcher.)
 Covering spaces. (See Hatcher, section 1.3.)
 Definition and discussion of basic examples, including the
relation between
finite covers of S^1 and permutations, coverings of the torus, the
covering of any genus g>1 surface by the hyperbolic plane, the
canonical double covering of any manifold by an oriented manifold,
and the nonexample of branched covers of Riemann surfaces.
 Lifting paths to a covering space.
 The lifting criterion for maps to a covering space.
 The classification of covering spaces over a reasonable space.
 An application to algebra: any subgroup of a free group is free.
 Brief introduction to higher homotopy groups: definition, easy
properties, introduction to pi_n(S^n) (which will be discussed more
later), why homotopy
groups are hard in general.
 Homology. (about 13 lectures; see Hatcher, chapter 2)
 Introducing singular homology.
 Warmup definition: simplicial homology of a Deltacomplex.
 Main definition: singular homology of a topological space.
 H_0 is a direct sum of Z's, one for each path component.
 Computation of the homology of a contractible space, using
cones over simplices.
 H_1 of a path connected space is the abelianization of pi_1.
(I prove this differently from Hatcher, without using the
classificaion of surfaces, with generalization to the
Hurewicz theorem in mind.)
 Homology is a homotopy invariant. (I prove this a bit
differently from Hatcher, using "acyclic models" to avoid explicitly
constructing a "triangulation" of a prism, by using homology theory
to show that a "triangulation" with the desired properties exists.
One can also avoid this issue by defining singular homology using
cubes instead of simplices.)
 MayerVietoris sequence:
 statement and simple examples, including the homology of S^n.
 A bit of homological algebra: a short exact sequence of chain
complexes induces a long exact sequence in homology.
 Proof of MayerVietoris sequence using subdivision lemma.
 Understanding the connecting homomorphism in the
MayerVietoris sequence.
 Application: proof of the generalized Jordan curve theorem.
 An interesting example: linking numbers.
 How to compute the homology of a CW complex.
 Relative homology; long exact sequences of a pair and a triple;
excision; isomorphism between relative homology and reduced homology
of the quotient
for "good pairs".
 Equivalence of simplicial and singular homology.
 Cellular homology of a CW complex.
 Extensive discussion (including some perspectives not in
Hatcher) of the degree of a map from a sphere to itself.
 Application to the "hairy ball theorem".
 Isomorphism between singular and cellular homology of a CW
complex.
 Examples including real and complex projective space.
 Euler characteristic, and Lefschetz fixed point theorem for
simplicial complexes.
 Homology with coefficients, Tor, and the universal coefficient
theorem.
 The Kuenneth formula for the homology of the product of two
spaces, via the EilenbergZilber theorem. (The approach I am using here
is different from that of Hatcher, and may be found in Bredon.)
 Cohomology. (about 7 lectures; see Hatcher, chapter 3)
 Definition of cohomology, and universal coefficient theorem for
cohomology.
 Definition of the cup product.
 Statement of Poincare duality (to be proved shortly).
 Statement of the geometric interpretation of cup product via
intersection theory in a smooth manifold. (The proof is beyond the scope
of this course, but I believe that the statement is the most
important thing to know about cup product.)
 Computation of the cohomology ring of projective spaces.
 Application: the BorsukUlam theorem and the Ham Sandwich Theorem.
 Main ideas in the proof of Poincare duality, including the
orientation bundle, compactly supported cohomology, direct limits,
and the cap
product.
 Application: the Lefschetz fixed point theorem on a smooth
manifold (which
actually counts fixed points with signs, rather than merely proving
that a fixed point exists). (This is not in Hatcher, but it is in BottTu or
Bredon.)
 PoincareLefschetz duality for a manifold with boundary.
 Application: the boundary of a 4n+1 dimensional oriented
compact manifold has signature zero.
 Alexander duality.
 Application: the Klein bottle does not embed in R^3 (and more
generally, a nonorientable closed nmanifold does not embed in
R^(n+1)).
 Concluding lecture.
What we actually did.
 (8/30) Introduced some of the problems that algebraic topology
can solve, or try to solve. Gave an overview of the classification of
manifolds. Algebraic topology can also be used to prove existence
theorems, such as the Brouwer fixed point theorem and the BorsukUlam
theorem. It can also be used to count things, as in the Lefschetz
fixed point theorem.
 (9/1) Defined the fundamental group. Computed the fundamental
group of the circle, by lifting paths to the real line. As
applications, outlined proofs of the Brouwer fixed point theorem in
two dimensions, the BorsukUlam theorem in two dimensions, and the
"fundamental theorem of algebra".
 (9/6) Discussed the extent to which the fundamental group depends
on the base point, and proved that the fundamental group is invariant
under homotopy equivalence. Computed the fundamental group of a
product, and of S^n for n>1 (in two different ways).
 (9/8) Explained the Seifertvan Kampen theorem for the
fundamental group of a union. (I didn't prove the whole thing; see
Hatcher and the other references for the complete proof.) Computed
examples, especially surfaces. The punchline, which will be explained
more next time, is that if X is a CW complex with one
0cell x_0, then pi_1(X,x_0) has a natural presentation with one
generator for each 1cell and one relation for each 2cell.
 (9/13) Explained the punchline from the previous lecture. (See
Hatcher for more details.) Had some fun with the Wirtinger
presentation of the fundamental group of a knot complement, and the
braid group. (These topics are not discussed much in Hatcher, and we
will not use them much later.)

(9/15) Introduced covering spaces and examples. Discussed
the path lifting lemma and homotopy lifting lemma. Showed that a
covering space determines both a right action of the fundamental group
of the base on the fiber over the base point, and (given a choice of a
base point upstairs in the fiber over the base point downstairs) a
subgroup of the fundamental group of the base.

(9/20) Studied examples of covering spaces (of the torus and the
figure eight) and their associated subgroups of the fundamental group
of the base and right actions of the fundamental group of the base on
the fiber. Explained the Lifting Criterion.
 (9/22) Explained the classification of path connected covering
spaces of a reasonable space in terms of subgroups of the fundamental
group. (There is another classification of covering spaces in terms
of right actions of the fundamental group, which I didn't have time to
explain.) As an application, proved that any subgroup of a free group
is free.
 (9/27) Introduced the simplicial homology of a Deltacomplex.
(From now on, the algebra part of the course will be linear algebra
instead of group theory, which is a lot easier, although not as easy
as you might think.)
 (9/29) Defined the singular homology of a topological space.
Showed that H_0 of a (nonempty) path connected space is Z. Computed
the homology of a contractible space. (I departed from the book
here.) Started to explain why H_1 of a path connected space is the
abelianization of pi_1.
 (10/4) Completed the proof that H_1 is the abelianization of
pi_1. Proved homotopy invariance of homology. (I did this
differently from the book, without having to explicitly construct the
prism operator; the argument I gave is in Greenberg and Harper, and
in greater generality in Bredon.)

(10/6) Introduced the MayerVietoris sequence, used it to compute some
examples of homology, and discussed how a short exact sequence of
chain complexes induces a long exact sequence in homology.

(10/11) Proved (most of) the Subdivision Lemma. Used it to derive the
MayerVietoris sequence.

(10/13) Explained the JordanBrouwer separation theorem. Elaborated on this
by discussing the linking number of two smooth knots.

(10/18) Introduced relative homology. Excision and applications.
Isomorphism between simplicial and singular homology of a
Deltacomplex, using the Five Lemma.

(10/20) Discussed the degree of a map from a sphere to itself, from
various points of view. Introduced the cellular homology of a CW complex.

(10/25) Discussed the cellular homology of a CW complex a lot more.
Introduced the Euler characteristic of a finite CW complex.

(10/27) Proof that the Euler characteristic is a topological
invariant. Generalization to the Lefschetz number of a map.
Statement and sketch of proof of Lefschetz fixed point theorem for
a finite simplicial complex.

(11/1) Homology with coefficients, the universal coefficient theorem,
and Tor. Cf. Hatcher, section 3.A. If you are curious, you can read
about the more general theory of derived functors in Lang's Algebra.

(11/3) Explained the Kuenneth formula for the homology of a Cartesian
product, following the approach in Bredon. It is instructive to
compare this with the proof for CW complexes in Hatcher, section 3.B.
 (11/8) Introduced cohomology. Proved the universal coefficient
theorem for cohomology.
 (11/10) Introduced the cup product on cohomology.
 (11/15) Used the cup product to prove the BorsukUlam
theorem. Discussed the Kuenneth formula for cohomology.
 (11/17) Explained the product structure on the cohomology
of a Cartesian product. Introduced orientations of manifolds.
 (11/22) More about orientation. Explained the
fundamental class of an Roriented compact manifold. More
generally showed that if A is a compact subset of an ndimensional
manifold M, then H_n(M,MA;R) is canonically isomorphic to the
sections of the Rorientation bundle over A. (This is in Bredon.)
Gave more details about direct limits.
 (11/29) Discussed the main ideas in the proof of Poincare
duality, including the cap product and compactly supported cohomology.
 (12/1) Brief review of smooth manifolds. Stated duality
between cup product and intersection of submanifolds. (See sections 1
and 2 of this handout. The rest of
the handout uses material which you can learn about in 215b.)
 (12/6) Proved the Lefschetz fixed point theorem on a
smooth manifold (cf. Bredon or above handout). Corollary:
PoincareHopf index theorem.
 (12/8) Introduced PoincareLefschetz duality for a
manifold with boundary. Used this to show that
the boundary of a compact oriented (4k+1)dimensional manifold has
signature zero. Didn't have time to explain Alexander duality, but
you should read about this in section 3.3 of Hatcher.
References
The following are some related books which are sitting on my shelf,
along with my opinions about them. Some of these may be useful now,
while others will be more useful after you have taken 215a.
 C. Adams, The knot book. Every topologist should know a
little knot theory, if for no other reason than to be able to explain
something about what you do to nonmathematicians. In the course I
will do a couple of examples of simple applications of algebraic
topology to knot theory. This book is an easy read. Why study hard
algebraic topology, if you haven't learned this easier stuff first?
 Bott and Tu, Differential forms in algebraic topology.
This book gives a beautiful tour through many topics including de Rham
theory, Cech cohomology, basic homotopy theory, spectral sequences,
Chern classes. The path taken by this book is very different from
most other algebraic topology textbooks. The basic concepts are
introduced using manifolds and differential forms; this sacrifices
some generality, but is more relevant for many geometric applications,
and allows for much more intuitive explanations.
 G. Bredon, Geometry and Topology. This is a nice algebraic
topology text with a welcome emphasis on manifolds. It provides a
nice alternate perspective on the basic material covered in Hatcher.
 J. Dieudonne, A history of algebraic and differential
topology, 19001960. This book is very interesting if you are
already familiar with the basic concepts of algebraic topology and
want to learn how people originally thought of them. Don't be put off
by the word "history" in the title; it gives excellent summary
explanations of many mathematical topics.
 Gompf and Stipcisz, 4manifolds and Kirby calculus. This
book leads up to some current research topics in 4dimensional
topology, and has lots of pictures. You should be able to understand
this after taking 215.
 Greenberg and Harper, Algebraic topology: a first course
. This was the primary textbook when I took algebraic topology.
It provides a nice concise development of singular homology theory.
 Guillemin and Pollack, Differential topology. This is a
gorgeous book on basic differential topology. I highly recommend
reading this, and the prerequisites are minimal.
 A. Hatcher, Algebraic topology. This is a very nice text. I
wish it would do a bit more with manifolds, because I think that this
adds a lot of insight. But, no single book can do everything.
 W.S. Massey has written at least three algebraic topology texts at
different levels, of which I have seen and enjoyed two. If I recall
correctly, he gives nice treatments of 2manifolds, the fundamental
group, and cubical singular homology.
 J.P. May, A concise course in algebraic topology.
Provides a nice summary of algebraic topology from a certain advanced
viewpoint. (For example, some pretty heavy category theory appears
before the fundamental group of a surface is computed.)
 J. McCleary, A user's guide to spectral sequences . Too
advanced for 215a, but might be good for 215b. A very useful book
about spectral sequences.
 Milnor and Stasheff, Characteristic classes. A beautiful
treatment of vector bundles and characteristic classes. We won't get
to this material in 215a, but I have taught it in 215b.
 J. Munkres, Topology (2nd edition) . This book gives a
clear and gentle treatement which should be good for beginners. The
first part of the book covers pointset topology. We will be assuming
basic pointset topology in the course (although this book does more
of it than we will need). The second part of the book introduces the
beginnings of algebraic topology.
 D. Rolfsen, Knots and links. After reading the Adams
book, if you want to see some more serious applications of algebraic
topology to knot theory, this book is a classic.
 E. Spanier, Algebraic topology. Kind of heavy going, but a
good reference.
 M. Spivak, A comprehensive introduction to differential
geometry, volume I. This book gives an obsessively detailed
explanation of the basic concepts of differential topology. The other
four volumes in the series (which are not really about topology) are
also an interesting read. The way I read them was to start with the
last chapter of volume 5 and look things up in earlier chapters as needed.
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