Topics in Banach Space Theory

296 Pages·2006·3.58 MB·English

by  Fernando Albiac, Nigel J. Kalton

233
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S.Axler
K.A.Ribet
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(continued after index)
Fernando Albiac and Nigel J.Kalton
Topics in Banach Space
Theory
Fernando Albiac Nigel J.Kalton
Department ofMathematics Department ofMathematics
University ofMissouri University ofMissouri
Columbia,Missouri 65211 Columbia,Missouri 65211
USA USA
[email protected]. [email protected]
Editorial Board
S.Axler K.A.Ribet
Mathematics Department Mathematics Department
San Francisco State University University ofCalifornia,Berkeley
San Francisco,CA 94132 Berkeley,CA 94720-3840
USA USA
[email protected] [email protected]
Mathematics Subject Classification (2000):46B25
Library ofCongress Cataloging in Publication Data:2005933143
ISBN10:0-387-28141-X
ISBN13:978-0387-28141-4
Printed on acid-free paper.
© 2006 Springer Inc.
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Preface
This book grew out of a one-semester course given by the second author in
2001 and a subsequent two-semester course in 2004-2005, both at the Univer-
sity of Missouri-Columbia. The text is intended for a graduate student who
has already had a basic introduction to functional analysis; the aim is to give
a reasonably brief and self-contained introduction to classical Banach space
theory.
Banach space theory has advanced dramatically in the last 50 years and
webelievethatthetechniquesthathavebeendevelopedareverypowerfuland
shouldbewidelydisseminatedamongstanalystsingeneralandnotrestricted
to a small group of specialists. Therefore we hope that this book will also
prove of interest to an audience who may not wish to pursue research in this
area but still would like to understand what is known about the structure of
the classical spaces.
Classical Banach space theory developed as an attempt to answer very
natural questions on the structure of Banach spaces; many of these questions
date back to the work of Banach and his school in Lvov. It enjoyed, perhaps,
its golden period between 1950 and 1980, culminating in the definitive books
by Lindenstrauss and Tzafriri [138] and [139], in 1977 and 1979 respectively.
The subject is still very much alive but the reader will see that much of the
basic groundwork was done in this period.
We will be interested specifically in questions of the following type: given
two Banach spaces X and Y, when can we say that they are linearly isomor-
phic, or that X is linearly isomorphic to a subspace of Y? Such questions
date back to Banach’s book in 1932 [8] where they are treated as problems
of linear dimension. We want to study these questions particularly for the
classical Banach spaces, that is, the spaces c , (cid:1) (1 ≤ p ≤ ∞), spaces C(K)
0 p
of continuous functions, and the Lebesgue spaces L , for 1≤p≤∞.
p
At the same time, our aim is to introduce the student to the fundamental
techniques available to a Banach space theorist. As an example, we spend
much of the early chapters discussing the use of Schauder bases and basic
sequencesinthetheory.Thesimpleideaofextractingbasicsequencesinorder
VI Preface
to understand subspace structure has become second-nature in the subject,
and so the importance of this notion is too easily overlooked.
It should be pointed out that this book is intended as a text for graduate
students, not as a reference work, and we have selected material with an
eye to what we feel can be appreciated relatively easily in a quite leisurely
two-semester course. Two of the most spectacular discoveries in this area
during the last 50 years are Enflo’s solution of the basis problem [54] and
the Gowers-Maurey solution of the unconditional basic sequence problem
[71]. The reader will find discussion of these results but no presentation. Our
feeling, based on experience, is that detouring from the development of the
theorytopresentlengthyandcomplicatedcounterexamplestendstobreakup
theflowofthecourse.Wepreferthereforetopresentonlyrelativelysimpleand
easily appreciated counterexamples such as the James space and Tsirelson’s
space. We also decided, to avoid disruption, that some counterexamples of
intermediate difficulty should be presented only in the last optional chapter
and not in the main body of the text.
Let us describe the contents of the book in more detail. Chapters 1-3 are
intended to introduce the reader to the methods of bases and basic sequences
and to study the structure of the sequence spaces (cid:1) for 1 ≤ p < ∞ and c .
p 0
We then turn to the structure of the classical function spaces. Chapters 4
and 5 concentrate on C(K)-spaces and L (µ)-spaces; much of the material
1
in these chapters is very classical indeed. However, we do include Miljutin’s
theorem that all C(K)-spaces for K uncountable compact metric are linearly
isomorphicinChapter4;thissection(Section4.4)andthefollowingone(Sec-
tion4.5)onC(K)-spacesforK countablecanbeskippedifthereaderismore
interested in the L -spaces, as they are not used again. Chapters 6 and 7
p
deal with the basic theory of L -spaces. In Chapter 6 we introduce the no-
p
tions of type and cotype. In Chapter 7 we present the fundamental ideas of
Maurey-Nikishin factorization theory. This leads into the Grothendieck the-
ory of absolutely summing operators in Chapter 8. Chapter 9 is devoted to
problemsassociatedwiththeexistenceofcertaintypesofbases.InChapter10
we introduce Ramsey theory and prove Rosenthal’s (cid:1) -theorem; we also cover
1
Tsirelson space, which shows that not every Banach space contains a copy of
(cid:1) for some p, 1 ≤ p < ∞, or c . Chapters 11 and 12 introduce the reader
p 0
to local theory from two different directions. In Chapter 11 we use Ram-
sey theory and infinite-dimensional methods to prove Krivine’s theorem and
Dvoretzky’stheorem,whileinChapter12weusecomputationalmethodsand
the concentration of measure phenomenon to prove again Dvoretzky’s theo-
rem. Finally Chapter 13 covers, as already noted, some important examples
which we removed from the main body of the text.
The reader will find all the prerequisites we assume (without proofs) in
the Appendices. In order to make the text flow rather more easily we decided
to make a default assumption that all Banach spaces are real. That is, unless
otherwise stated, we treat only real scalars. In practice, almost all the results
Preface VII
in the book are equally valid for real or complex scalars, but we leave to the
reader the extension to the complex case when needed.
Thereareseveralbookswhichcoversomeofthesamematerialfromsome-
what different viewpoints. Perhaps the closest relatives are the books by Di-
estel [39] and Wojtaszczyk [221], both of which share some common themes.
Two very recent books, namely, Carothers [23] and Li and Queff´elec [126],
alsocoversomesimilartopics.Wefeelthatthestudentwillfinditinstructive
to compare the treatments in these books. Some other texts which are highly
relevant are [10], [78], [149], and [56]. If, as we hope, the reader is inspired to
learn more about some of the topics, a good place to start is the Handbook of
the Geometry of Banach Spaces,editedbyJohnsonandLindenstrauss[90,92]
whichisacollectionofarticleson thedevelopmentofthetheory;thishasthe
advantage of being (almost) up to date at the turn of the century. Included is
an article by the editors [91] which gives a condensed summary of the basic
theory.
ThefirstauthorgratefullyacknowledgesGobiernodeNavarraforfunding,
andwantstoexpresshisdeepgratitudetoSheilaJohnsonforallherpatience
andunconditionalsupportforthedurationofthisproject.Thesecondauthor
acknowledges support from the National Science Foundation and wishes to
thankhiswifeJenniferforhertolerancewhilehewasworkingonthisproject.
Columbia, Missouri, Fernando Albiac
November 2005 Nigel Kalton
Contents
1 Bases and Basic Sequences ................................ 1
1.1 Schauder bases.......................................... 1
1.2 Examples: Fourier series.................................. 6
1.3 Equivalence of bases and basic sequences ................... 10
1.4 Bases and basic sequences: discussion ...................... 15
1.5 Constructing basic sequences.............................. 19
1.6 The Eberlein-S˘mulian Theorem ........................... 23
Problems ................................................... 25
2 The Classical Sequence Spaces............................. 29
2.1 The isomorphic structure of the (cid:1) -spaces and c ............ 29
p 0
2.2 Complemented subspaces of (cid:1) (1≤p<∞) and c .......... 33
p 0
2.3 The space (cid:1) ............................................ 36
1
2.4 Convergence of series .................................... 38
2.5 Complementability of c .................................. 44
0
Problems ................................................... 48
3 Special Types of Bases..................................... 51
3.1 Unconditional bases ..................................... 51
3.2 Boundedly-complete and shrinking bases ................... 53
3.3 Nonreflexive spaces with unconditional bases................ 59
3.4 The James space J ...................................... 62
3.5 A litmus test for unconditional bases....................... 66
Problems ................................................... 69
4 Banach Spaces of Continuous Functions ................... 73
4.1 Basic properties ......................................... 73
4.2 A characterization of real C(K)-spaces ..................... 75
4.3 Isometrically injective spaces.............................. 79
4.4 Spaces of continuous functions on uncountable compact
metric spaces ........................................... 87
X Contents
4.5 Spaces of continuous functions on countable compact metric
spaces.................................................. 95
Problems ................................................... 98
5 L1(µ)-Spaces and C(K)-Spaces.............................101
5.1 General remarks about L (µ)-spaces .......................101
1
5.2 Weakly compact subsets of L (µ)..........................103
1
5.3 Weak compactness in M(K)..............................112
5.4 The Dunford-Pettis property..............................115
5.5 Weakly compact operators on C(K)-spaces..................118
5.6 Subspaces of L (µ)-spaces and C(K)-spaces.................120
1
Problems ...................................................122
6 The Lp-Spaces for 1 ≤ p < ∞..............................125
6.1 Conditional expectations and the Haar basis ................125
6.2 Averaging in Banach spaces...............................131
6.3 Properties of L .........................................142
1
6.4 Subspaces of L .........................................148
p
Problems ...................................................161
7 Factorization Theory ......................................165
7.1 Maurey-Nikishin factorization theorems ....................165
7.2 Subspaces of L for 1≤p<2 .............................173
p
7.3 Factoring through Hilbert spaces ..........................180
7.4 The Kwapien´-Maurey theorems for type-2 spaces ............187
Problems ...................................................191
8 Absolutely Summing Operators............................195
8.1 Grothendieck’s Inequality.................................196
8.2 Absolutely summing operators ............................205
8.3 Absolutely summing operators on L (µ)-spaces..............213
1
Problems ...................................................217
9 Perfectly Homogeneous Bases and Their Applications .....221
9.1 Perfectly homogeneous bases..............................221
9.2 Symmetric bases ........................................227
9.3 Uniqueness of unconditional basis .........................229
9.4 Complementation of block basic sequences..................231
9.5 The existence of conditional bases .........................235
9.6 Greedy bases ...........................................240
Problems ...................................................244
Contents XI
10 (cid:1)p-Subspaces of Banach Spaces ............................247
10.1 Ramsey theory..........................................247
10.2 Rosenthal’s (cid:1) theorem...................................251
1
10.3 Tsirelson space..........................................254
Problems ...................................................259
11 Finite Representability of (cid:1)p-Spaces .......................263
11.1 Finite representability....................................263
11.2 The Principle of Local Reflexivity .........................272
11.3 Krivine’s theorem .......................................275
Problems ...................................................285
12 An Introduction to Local Theory ..........................289
12.1 The John ellipsoid.......................................289
12.2 The concentration of measure phenomenon .................293
12.3 Dvoretzky’s theorem .....................................296
12.4 The complemented subspace problem ......................301
Problems ...................................................306
13 Important Examples of Banach Spaces.....................309
13.1 A generalization of the James space........................309
13.2 Constructing Banach spaces via trees ......................314
13.3 Pe(cid:3)lczyn´ski’s universal basis space..........................316
13.4 The James tree space ....................................317
A Fundamental Notions......................................327
B Elementary Hilbert Space Theory .........................331
C Main Features of Finite-Dimensional Spaces ...............335
D Cornerstone Theorems of Functional Analysis .............337
D.1 The Hahn-Banach Theorem...............................337
D.2 Baire’s Theorem and its consequences......................338
E Convex Sets and Extreme Points ..........................341
F The Weak Topologies......................................343
G Weak Compactness of Sets and Operators .................347
List of Symbols ................................................349
References.....................................................353
Index..........................................................365