The Relation of Cobordism to K-Theories

119 Pages·1966·4.367 MB·English

by  P. E. Conner, E. E. Floyd

Lecture Notes ni Mathematics
A collection of informal reports dna sranimes
Edited yb .A Dold, Heidelberg dna .B ,nnamkcE hcirUZ
28
.P .E Conner. ,E .E Floyd
ytisrevinU of Virginia, ellivsettolrahC
ehT noitaleR
of Cobordism to K-Theories
6691
m I
.galreV-regnirpS Berlin-Heidelberg-New kroY
All rights, especially that oft ranslation into foreign languages, reserved. It is also forbidden to reproduce this book, either
whole or in part, by photomechanical means (photostat, microfilm and/or microcard)or by other procedure without
written permission from Springer Verlag. © by Springer-Verlag Berlin • Heidelberg 1966.
Library of Congress Catalog Card Number 66-30143. Printed in Germany. Title No. 7348.
INTRODUCTION
These lectures treat certain topics relating K-theory and cobordism.
Since new connections are in the process of being discovered by various
workers, we make no attempt to be definitive but simply cover a few of
our favorite topics. If there is any unified theme it is that we treat
various generalizations of the Todd genus.
In Chapter I we treat the Thom isomorphism in K-theory. The
families U, SU, Sp of unitary, special unitary, symplectic groups
generate spectra MU, MSU, HSp of Thom spaces. In the fashion of
G. W. Whitehead [2B], each spectrum generates a generalized cohomology
theory and a generalized homology theory. The cohomology theories are
by t~*~ * .
denoted (.),~ (.), ~ ).( and are called cobordism theories;
U SU Sp ~U . _ SU . . Sp(.)
the homology theories are denoted by~.[ ),/~. ( ),~ and are
called bordism theories. The coefficient groups are, taking one case
as an example, given by~ U n =/~n (point),~U =~U (point) and are
U n n
related by~ U n = ~1. U -n . M°re°ver~U is Just the bordism group of all
n
bordism classes [M n] of closed weakly almost complex manifolds M n,
similarly for~ SU and~ Sp. On the other hand there are the
n n
Grothendieck-Atlyah-Hirzebruch periodic cohomology theories
K*(-),K0*(-) of K-theory. The main point of Chapter I, then, is to
define natural transformations
*(.) @...- Ko*(.)
US
).(*K
)'(* "@
:
U
of cohomology theories. Such transformations have been folk theorems
since the work of Atlyah-Hirzebruch [6], Dold [13], and others. It
should be noted that on the coefficient groups,
:/~-2n -2n
~c U ~-- K )tp( = Z
Kl U
is identified up to sign with the Todd genus Td : ~-- Z.
2n
In Chapter II we show among other things that the cobordlsm
theories determine the K-theories. For example, ~c generates a
ring homomorphism ~ U * ~-- Z and makes Z into a ~U-mOdule. It is
shown that
(X,A)~ ~. Z
K (X,A)~ /~U U
as Z2-graded modules. Similarly symplectic cobordism determines
KO t')- The isomorphisms are generated by /~c,/~respectively.
Various topics are treated along the way, in particular cobordism
characteristic classes.
There is the sphere spectrumS, whose homology groups are the
fr
framed bordlsm groups~. (.). The coefficient group
fr (polnt)~=O fr are Just the stable stems 7Tn+k(Sk), k large.
n n
The spectrum~is embedded in a natural way in MU, and one can thus
form MU//. In Chapter III we study the group
~U, fr = "77 (MU//) = VT-n+2k(MU(k)/s2k),
n n
k large. The elements of/~ U'fr are interpreted as bordism classes
n
[Mnj of compact ,U( fr)-manlfolds M ,n where roughly a ,U( fr)-manlfold
is a differentiable manifold M with a given complex structure on its
stable tangent bundle~ and a given compatible framing of~restricted
to the boundary ~ M. These bordism classes have Chern numbers and
hence a Todd genus
Td :~ U, fr
--@ Q, Q the rationals.
2n
It is proved that given a compact (U, fr)-manifold ~n, there is a
closed weakly almost complex manifold having the same Chern numbers
M2n 2n
as if and only if Td [M J is an integer; this makes use of re-
cent theorems of Stong [23J and Hattori [15]. There is a diagram
0 _.,~U ,__ ~ ,U fr ~ #i fr
--~ 0
2n 2n 2n-i
Q
which gives rise to a homomorphism
:~ fr
2n-i -~ q/z.
This turns out to coincide with a well-known homomorphism of Adams,
e :~ fr
c 2n-1 > Q/Z.
We are thus able to give a cobordism interpretation of the results of
Adams [13] on c. e It should be pointed out that Chapter III is in
large part independent of Chapter II.
It is to be noted that we have omitted spin cobordism completely;
this is because of our ignorance. However the recent work of Anderson-
Brown-Feterson is a notable example of the application of K-theory to
cobordlsm.
CONTENTS
Chapter I. The Thom Isomorphism in K-theory ............... I
.I Exterior Algebra ..................................... I
2. Tensor products of exterior algebras ............... 5
3. Application to bundles ............................. 11
4. Thom classes of line bundles ....................... 18
5. Cobordism and homomorphisms into K-theory .......... 25
6. The homomorphism~c ................................ 30
Chapter II. Cobordism Characteristic Classes ............ 38
7. A theorem of Dold ................................. 39
8. Characteristic classes in cobordism ............... 48
9. Characteristic classes in K-theory ................ 52
10. A cobordism interpretation for K* (X) ............. 59
11. ~appings into spheres ............................. 65
Chapter III. U-Manifolds with Framed Boundaries ......... 69
12. The U-bordism groups~ U 70
o e e o o e o o e e e o o e o e e o e e o o m e e o e
13. Characteristic numbers from K-theory .............. 78
14. The theorem of Stong and Hattori .................. 82
15. U-manifolds with stably framed boundaries ......... 91
16. The bo r dism groups~ U. 'fr ......................... 96
SU
17. The groups~' ................................. 105
18. The image f~fr in n.SU 108
O ~
4 , 1 e e e ~ e l o e o o o o 0 e o m o e e t * o e e
Bibliography ............................................ 111
CHAPTER .I THE THOM ISOMORPHISM IN K-THEORY.
Given a U(n)-bundle ~- ever a finite CW complex X there is con-
structed an element ~(~) s K(M(~)) where M(~) is the Them space
of -~ ; we call ~ (~) the Them class of "{ . Similarly given an
SU(4k)-bundle there is constructed a Them class t(~) s KO(M(~)),
and given an SU(4k + 2)-bundle there is constructed a class
s(~) s KSp(M(~)). These Them classes give rise to isomorphisms
KO )X( ~ KO ) E ) (M(
KO(X) ~ KSp(M(~))
in the three cases. Formulas for the Chern character ch ~(~) are
obtalned.
No claims for originality are made in this chapter; the methods
have been well-known since the work of Atiyah-Hirzebruch [6], Dold [15],
and others [7]. However since the results are needed explicitly in the
later chapters we include an exposition. A deviation from the standard
treatment is made in that exterior algebra is used in all cases, thus
avoiding the use of Clifford algebras.
The chapter terminates with the setting up of homomorphlsms
SU(. ) > K0 ).( and/~U(. ) --9 K ).( of cohomology theories, where
*lf
~* ['), )'t denote the cohomology theories based on the spectra
SU U
MSU, MU.
i. Exterior algebra
We fix in this section a complex inner product space V
of dimension n, and we also fix a unit vector -o ~ /kn~. If n = 4k + 2,
we make the exterior algebra AV into a quaternlonlc vector space. If
2
n = 4k then a real vector subspace RV of /~V is selected so that AV
is identified with the complexification of RV. The special unitary
group SU(n) operates in a quaternionic linear fashion on AV in the
first case, in a real linear fashion on RV in the second case.
Fix, then, the complex inner product space V of dimension n.
To fit with quaternionic notation, the complex numbers are taken to
act on the right and the inner product 4,> is taken conjugate linear
in the first variable and complex linear in the n->second.
There is the graded exterior algebra AV = Akv with A°V = C
o
and A1V = V. The inner product on V can be extended to an inner pro-
duct on AV by
i) if j ~ k then AJv is orthogonal to /kkv,
ii) if X I = ^ u -.. k ^ u and Y = Yl ^ ^ "'" Yk where
Ur,V s z V, then
<X,Y> = det I <Ur,V > s -l
If I e ,...,e n is an orthonormal basis for V then the erl A ... A erk
with I < r -., -~ k r form an orthonormal basis for Ik~. There is also
a canonical anti-lsomorphism :~o AV >--- AV with
o<(v la "''^v k) = v ka'''^v I = (-1)k(k-1)/2Vl^''" ^v k-
It is clear that (o is unitary.
DEFINITION. By an SU-structure for V we shall mean a unit vector
*"3( s AnV; suppose an SU-structure has been fixed for V. Define a real
linear map :~q Akv ~ An-kv as follows: fix X 8 Akv and let Y vary
over k/ n-kv so that <o-,X A Y> is a linear map An-kv >--- C; define
6~ X ~ to be the unique element of /kn-kv such that
~"C'X,Y.. b = <o-,X It Y> , all Y 8 An-kv.
3
It is then seen that the above equation holds for all Y ¢ ~V.
The map~is conjugate linear. For
/..'~(Xa),Y> = a <o'-,X ^ Y> = /~i~X)a,Y.>
and T(Xa) = (~XJ~.
Fix an orthonormal basis el,-.-,e n of V such that the given SU-
structure is ~= I A e ... A e n. By a monomial of AV we mean an
+
element X = - erl A --- ^ erk where I ~ r ...< r k. it is seen that
if X and Y are monomials, then
i_~ ifY=X
~X,Y ~ = if Y -X
0 otherwise.
Moreover given a monomial X there is a unique monomial X with X ~ X =
(1.1J If X is a monomial then ~X is the unique monomial X with
XAX=(r . ~
This is readily seen from the definition of ~.
tl.2) We have ~2X = (-l)ktn-k)x rE__of X ~ /kkv.
proof. It is sufficient to prove tl.2) for monomials. For X a
monomial, ~X is the unique monomial with A X ~X = .~-~ Then
~X A X = (-i) kin-k) o-and ~2X = (-l)k(n-k)x.
Define an operator ~: A~ > /~n-kv b_y~ = ~. Then/~is
conjugate linear.
(1.3) We have %c2X = t-ljntn-1)/2X for X s /~V.
rroof. It is seen from (1.2) that ~2X = (-l)rX where
r = k(k - 2'/)1 + (n - k)(n - k - 1)/2 + k(n - k)
= k(n - 1)/2 + (n - k)(n - 1)/2 = n(n - 1)/2.
The remark follows.
4
We now identify U(n) with the group of linear maps g : V --@ V
with ~gu, gv) = ~u,v~ for all u, v ~ V. Then UQn) scts on AV by
g(v ^ ... I ^Vk) = gv I A... ~ gv k. Identify the special unitary group
SU(n) with the set of all g s Urn) for which g(o-) =6--.
(1.4) If g s SU(n), then g~ = ~g and /~g = g/~ .
Froof. From ~X,Y~ = <~-,Xa Y~we get
~x, gY> : <~-,gx^ gY~ : ~gx, gY ~,
hence g ~ = ~ g. It follows immediately that g~ = /ag.
(1.5) THEOREM. Consider the complex inner product space V of
dimension n, with given SU-structure ~-s ~nv. If n = 4k + 2 then AV
becomes a right quaternionic vector space by defining Y.J = ~(Y) for
Y 6 ~V. Moreover SU(n) acts on AV in a quaternionic linear fashion.
If n = 4k, let R(V) be all X s AV with ~X = X and R_~V) all X with
/~X = -X; then
AV = RV + R (V)
is a splitting into real vector subspaces and multiplication by i takes
RV into R (V) and R (V) into RV. Moreover SU(n) acts n~_o RV in a real
linear fashion.
Proof. Consider the case n = 4k + 2. It follows from (1.3) that
~2 = -1. Also/~is conjugate linear so that
xlj = /~(xi~ = -(/~x~i = -xji.
It follows that there is defined an action of the quaternions H on AV,
and ~V is a quaternionic vector space. Consider g s SU(n). Then
gtxj~ = g/~(x) =/~g(x) = (gx)j