fonseca i leoni g modern methods in the calculus of variations lp spaces sinhvienzone com

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Modern Methods in the Calculus
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of Variations:L Spaces

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Irene Fonseca
Giovanni Leoni

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Modern Methods
in the Calculus
p
of Variations: L Spaces

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Giovanni Leoni

Department of Mathematical Sciences
Carnegie Mellon University
Pittsburgh, PA 15213
USA


Department of Mathematical Sciences
Carnegie Mellon University
Pittsburgh, PA 15213
USA


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Irene Fonseca

ISBN: 978-0-387-35784-3

e-ISBN: 978-0-387-69006-3

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Library of Congress Control Number: 2007931775
Mathematics Subject Classification (2000): 49-00, 49-01, 49-02, 49J45, 28-01, 28-02, 28B20, 52A

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© 2007 Springer Science+Business Media, LLC
All rights reserved. This work may not be translated or copied in whole or in part without the
written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street,
New York, NY, 10013, USA), except for brief excerpts in connection with reviews or scholarly
analysis. Use in connection with any form of information storage and retrieval, electronic
adaptation, computer software, or by similar or dissimilar methodology now known or hereafter
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The use in this publication of trade names, trademarks, service marks, and similar terms, even if
they are not identified as such, is not to be taken as an expression of opinion as to whether or
not they are subject to proprietary rights.
Printed on acid-free paper.
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To our families

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Preface

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In recent years there has been renewed interest in the calculus of variations,
motivated in part by ongoing research in materials science and other disciplines. Often, the study of certain material instabilities such as phase transitions, formation of defects, the onset of microstructures in ordered materials,
fracture and damage, leads to the search for equilibria through a minimization
problem of the type
min {I (v) : v ∈ V} ,

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where the class V of admissible functions v is a subset of some Banach space V .
This is the essence of the calculus of variations: the identification of necessary and sufficient conditions on the functional I that guarantee the existence
of minimizers. These rest on certain growth, coercivity, and convexity conditions, which often fail to be satisfied in the context of interesting applications,
thus requiring the relaxation of the energy. New ideas were needed, and the introduction of innovative techniques has resulted in remarkable developments
in the subject over the past twenty years, somewhat scattered in articles,
preprints, books, or available only through oral communication, thus making
it difficult to educate young researchers in this area.
This is the first of two books in the calculus of variations and measure
theory in which many results, some now classical and others at the forefront
of research in the subject, are gathered in a unified, consistent way. A main
concern has been to use contemporary arguments throughout the text to revisit and streamline well-known aspects of the theory, while providing novel
contributions.
The core of this book is the analysis of necessary and sufficient conditions
for sequential lower semicontinuity of functionals on Lp spaces, followed by
relaxation techniques. What sets this book apart from existing introductory
texts in the calculus of variations is twofold: Instead of laying down the theory
in the one-dimensional setting for integrands f = f (x, u, u
), we work in N

dimensions and no derivatives are present. In addition, it is self-contained in

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the sense that, with the exception of fundamentally basic results in measure
theory that may be found in any textbook on the subject (e.g., Lebesgue
dominated convergence theorem), all the statements are fully justified and
proved. This renders it accessible to beginning graduate students with basic
knowledge of measure theory and functional analysis. Moreover, we believe

that this text is unique as a reference book for researchers, since it treats both
necessary and sufficient conditions for well-posedness and lower semicontinuity
of functionals, while usually only sufficient conditions are addressed.
The central part of this book is Part III, although Parts I and II contain
original contributions. Part I covers background material on measure theory,
integration, and Lp spaces, and it combines basic results with new approaches
to the subject. In particular, in contrast to most texts in the subject, we do
not restrict the context to σ-finite measures, therefore laying the basis for
the treatment of Hausdorff measures, which will be ubiquitous in the setting
of the second volume, in which gradients will be present. Moreover, we call
attention to Section 1.1.4, on “comparison between measures”, which is completely novel: The Radon–Nikodym theorem and the Lebesgue decomposition
theorem are proved for positive measures without our having first to introduce
signed measures, as is usual in the literature. The new arguments are based on
an unpublished theorem due to De Giorgi treating the case in which the two
measures in play are not σ-finite. Here, as De Giorgi’s theorem states, a diffuse
measure must be added to the absolutely continuous and singular parts of the
decomposition. Also, we give a detailed proof of the Morse covering theorem,
which does not seem to be available in other books on the subject, and we
derive as a corollary the Besicovitch covering theorem instead of proving it
directly.
Part II streamlines the study of convex functions, and the treatment of the
direct method of the calculus of variations introduces the reader to the close
connection between sequential lower semicontinuity properties and existence
of minimizers. Again here we present an unpublished theorem of De Giorgi,
the approximation theorem for real-valued convex functions, which provides
an explicit formula for the affine functions approximating a given convex function f . A major advantage of this characterization is that additional regularity
hypotheses on f are reflected immediately on the approximating affine functions.
In Part III we treat sequential lower semicontinuity of functionals defined
on Lp , and we separate the cases of inhomogeneous and homogeneous functionals. The latter are studied in Chapter 5, where

f (v(x)) dx
I(u) :=
E

with E a Lebesgue measurable subset of the Euclidean space RN , f : Rm →
(−∞, ∞] and v ∈ Lp (E; Rm ) for 1 ≤ p ≤ ∞. This material is intended for
an introductory graduate course in the calculus of variations, since it requires
only basic knowledge of measure theory and functional analysis. We treat both

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bounded and unbounded domains E, and we address most types of strong and
weak convergence. In particular, the setting in which the underlying conver

gence is that of (Cb (E)) is new.
Chapter 6 and Chapter 7 are devoted to integrands f = f (x, v) and f =
f (x, u, v), respectively, and are significantly more advanced, since the proofs

of the necessity parts are heavily hinged on the concept of multifunctions. An
important tool here is selection criteria, and the reader will benefit from a
comprehensive and detailed study of this subject.
Finally, Chapter 8 describes basic properties of Young measures and how
they may be used in relaxation theory.
The bibliography aims at giving the main references relevant to the contents of the book. It is by no means exhaustive, and many important contributions to the subject may have failed to be listed here.
To conclude, this text is intended as a graduate textbook as well as a reference for more-experienced researchers working in the calculus of variations,
and is written with the intention that readers with varied backgrounds may
access different parts of the text.
This book prepares the ground for a second volume, since it introduces
and develops the basic tools in the calculus of variations and in measure theory needed to address fundamental questions in the treatment of functionals
involving derivatives.
Finally, in a book of this length, typos and errors are almost inevitable.
The authors will be very grateful to those readers who will write to either
or indicating those that
they have found. A list of errors and misprints will be maintained and updated
at the web page />
Irene Fonseca
Giovanni Leoni

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Pittsburgh,
month 2007

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Contents

Part I Measure Theory and Lp Spaces
Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Measures and Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Measures and Outer Measures . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Radon and Borel Measures and Outer Measures . . . . . . . 22
1.1.3 Measurable Functions and Lebesgue Integration . . . . . . . 37
1.1.4 Comparison Between Measures . . . . . . . . . . . . . . . . . . . . . . 55
1.1.5 Product Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
1.1.6 Projection of Measurable Sets . . . . . . . . . . . . . . . . . . . . . . . 83
1.2 Covering Theorems and Differentiation of Measures in RN . . . . 90
1.2.1 Covering Theorems in RN . . . . . . . . . . . . . . . . . . . . . . . . . . 90
1.2.2 Differentiation Between Radon Measures in RN . . . . . . . 103
1.3 Spaces of Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
1.3.1 Signed Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
1.3.2 Signed Finitely Additive Measures . . . . . . . . . . . . . . . . . . . 119
1.3.3 Spaces of Measures as Dual Spaces . . . . . . . . . . . . . . . . . . 123
1.3.4 Weak Star Convergence of Measures . . . . . . . . . . . . . . . . . 129

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Lp Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
2.1 Abstract Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
2.1.1 Definition and Main Properties . . . . . . . . . . . . . . . . . . . . . . 139
2.1.2 Strong Convergence in Lp . . . . . . . . . . . . . . . . . . . . . . . . . . 148
2.1.3 Dual Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
2.1.4 Weak Convergence in Lp . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
2.1.5 Biting Convergence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
2.2 Euclidean Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
2.2.1 Approximation by Regular Functions . . . . . . . . . . . . . . . . 190
2.2.2 Weak Convergence in Lp . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
2.2.3 Maximal Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
2.3 Lp Spaces on Banach Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

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Part II The Direct Method and Lower Semicontinuity
The Direct Method and Lower Semicontinuity . . . . . . . . . . . . . 231
3.1 Lower Semicontinuity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
3.2 The Direct Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

4

Convex Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
4.1 Convex Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
4.2 Separating Theorems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
4.3 Convex Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
4.4 Lipschitz Continuity in Normed Spaces . . . . . . . . . . . . . . . . . . . . . 262
4.5 Regularity of Convex Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
4.6 Recession Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
4.7 Approximation of Convex Functions . . . . . . . . . . . . . . . . . . . . . . . 293
4.8 Convex Envelopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
4.9 Star-Shaped Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

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Part III Functionals Defined on Lp

Integrands f = f (z) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
5.1 Well-Posedness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326

5.2 Sequential Lower Semicontinuity . . . . . . . . . . . . . . . . . . . . . . . . . . 331
5.2.1 Strong Convergence in Lp . . . . . . . . . . . . . . . . . . . . . . . . . . 331
5.2.2 Weak Convergence and Weak Star Convergence in Lp . . 334
5.2.3 Weak Star Convergence in the Sense of Measures . . . . . . 340
 


5.2.4 Weak Star Convergence in Cb E; Rm
. . . . . . . . . . . . . 350
5.3 Integral Representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354
5.4 Relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364
5.4.1 Weak Convergence and Weak Star Convergence in Lp ,
1 ≤ p ≤ ∞ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
5.4.2 Weak Star Convergence in the Sense of Measures . . . . . . 369
5.5 Minimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373

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5

6

Integrands f = f (x, z) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
6.1 Multifunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
6.1.1 Measurable Selections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
6.1.2 Continuous Selections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395
6.2 Integrands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
6.2.1 Equivalent Integrands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
6.2.2 Normal and Carath´eodory Integrands . . . . . . . . . . . . . . . . 404
6.2.3 Convex Integrands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413
6.3 Well-Posedness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
6.3.1 Well-Posedness, 1 ≤ p < ∞ . . . . . . . . . . . . . . . . . . . . . . . . . 428
6.3.2 Well-Posedness, p = ∞ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435

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6.4 Sequential Lower Semicontinuity . . . . . . . . . . . . . . . . . . . . . . . . . . 436
6.4.1 Strong Convergence in Lp , 1 ≤ p < ∞ . . . . . . . . . . . . . . . . 436
6.4.2 Strong Convergence in L∞ . . . . . . . . . . . . . . . . . . . . . . . . . 442
6.4.3 Weak Convergence in Lp , 1 ≤ p < ∞ . . . . . . . . . . . . . . . . . 445
6.4.4 Weak Star Convergence in L∞ . . . . . . . . . . . . . . . . . . . . . . 448
6.4.5 Weak Star Convergence in the Sense of Measures . . . . . . 449
6.5 Integral Representation in Lp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464
6.6 Relaxation in Lp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

6.6.1 Weak Convergence and Weak Star Convergence in Lp ,
1 ≤ p ≤ ∞ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
6.6.2 Weak Star Convergence in the Sense of Measures in L1 . 478
Integrands f = f (x, u, z) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485
7.1 Convex Integrands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485
7.2 Well-Posedness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
7.3 Sequential Lower Semicontinuity . . . . . . . . . . . . . . . . . . . . . . . . . . 491
7.3.1 Strong–Strong Convergence . . . . . . . . . . . . . . . . . . . . . . . . . 491
7.3.2 Strong–Weak Convergence 1 ≤ p, q < ∞ . . . . . . . . . . . . . 492
7.4 Relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

8

Young Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517
8.1 The Fundamental Theorem for Young Measures . . . . . . . . . . . . . 518
8.2 Characterization of Young Measures . . . . . . . . . . . . . . . . . . . . . . . 532
8.2.1 The Homogeneous Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533
8.2.2 The Inhomogeneous Case . . . . . . . . . . . . . . . . . . . . . . . . . . . 538
8.3 Relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540

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Functional Analysis and Set Theory . . . . . . . . . . . . . . . . . . . . . . . 549
A.1 Some Results from Functional Analysis . . . . . . . . . . . . . . . . . . . . . 549

A.1.1 Topological Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549
A.1.2 Metric Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552
A.1.3 Topological Vector Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . 554
A.1.4 Normed Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 558
A.1.5 Weak Topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 560
A.1.6 Dual Pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563
A.1.7 Hilbert Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566
A.2 Wellorderings, Ordinals, and Cardinals . . . . . . . . . . . . . . . . . . . . . 567

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Part IV Appendix

B

Notes and Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573

Notation and List of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587

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Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595

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Part I

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Measure Theory and Lp Spaces

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1
Measures

Galileo Galilei (1564–1642)

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Measure what is measurable, and
make measurable what is not so.
(Misura ci`
o che `e misurabile, e
rendi misurabile ci`
o che non lo `e)

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This chapter covers a wide range of properties of measures. Those that we consider basic and well known (for example the Lebesgue dominated convergence
theorem) will only be stated, and the reader is referred to classical textbooks
such as [DB02], [EvGa92], [Fol99], [Rao04], [Ru87], [Z67]. The reader should
be warned that in some of these books outer measures are called measures.
Results that are difficult to find in the literature, that are new, or that
may be presented in a more contemporary way will be proved in this text.

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1.1.1 Measures and Outer Measures

Definition 1.1. Let X be a nonempty set. A collection M ⊂ P (X) is an
algebra if
(i) ∅ ∈ M;
(ii) if E ∈ M then X \ E ∈ M;
(iii) if E1 , E2 ∈ M then E1 ∪ E2 ∈ M.

M is said to be a σ-algebra if it satisfies (i)–(ii) and
∞
(iii)
if {En } ⊂ M then n=1 En ∈ M.

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1 Measures

To highlight the dependence of the σ-algebra M on X we will sometimes
use the notation M (X). If M is a σ-algebra then the pair (X, M) is called
a measurable space. For simplicity we will often apply the term measurable
space only to X.
Using De Morgan’s laws and (ii) and (iii)
, it follows that a σ-algebra is
closed under countable intersection. In particular, if E, F ∈ M then E ∩ F ∈
M, and this leads to the notion of restriction of M to a set E ⊂ X (not
necessarily measurable), i.e., the induced σ-algebra
ME := {E ∩ F : F ∈ M} .

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Example 1.2. (i) In view of (i) and (ii), every algebra contains X. Hence the
smallest algebra (respectively σ-algebra) is {∅, X} and the largest is the
collection P (X) of all subsets of X.
(ii) If X = [0, 1), the family M of all finite unions of intervals of the type
[a, b) ⊂ [0, 1) is an algebra but not a σ-algebra. Indeed,

∞ 

1
0,
= {0} ∈
/ M.
E :=
n
n=1

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Let X be a nonempty set. Given any subset F ⊂ P (X) the smallest (in
the sense of inclusion) σ-algebra that contains F is given by the intersection
of all σ-algebras on X that contain F.
If X is a topological space, then the Borel σ-algebra B (X) is the smallest
σ-algebra containing all open subsets of X. The elements of B (X) are called
Borel sets. Unless indicated otherwise, in the sequel it is understood that the
Euclidean space RN , N ≥ 1, and the extended real line R := [−∞, ∞] are endowed with the Borel σ-algebras associated to the respective usual topologies:
In RN we consider the Euclidean norm


2
2
|x| := (x1 ) + . . . + (xN )

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with x = (x1 , . . . , xN ), and we take as basis of open sets in R the collection
of all intervals of the form (a, b), (a, ∞], [−∞, b) with a, b ∈ R.

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Remark 1.3. If X is a topological space and Y ⊂ X then
B (Y ) = B (X) Y .

(1.1)

Indeed, B (X) Y is a σ-algebra in Y that contains
{A ∩ Y : A is open in X} = {A
: A
is open in Y } .

By definition of B (Y ) we deduce that B (Y ) ⊂ B (X) Y . Conversely, let
N := {F ⊂ X : F ∩ Y ∈ B (Y )} .
Then N is a σ-algebra in X that contains all open sets in X. Hence B (X) ⊂ N
and so B (Y ) ⊃ B (X) Y .

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1.1 Measures and Integration

Definition 1.4. Let X be a nonempty set, let M ⊂ P (X) be an algebra, and
let µ : M → [0, ∞].
(i) µ is a (positive) finitely additive measure if
µ (∅) = 0,

µ (E1 ∪ E2 ) = µ (E1 ) + µ (E2 )

for all E1 , E2 ∈ M with E1 ∩ E2 = ∅.
(ii) µ is a (positive) countably additive measure if

∞
∞




En =
µ (En )
µ (∅) = 0, µ
n=1

n=1

for every
∞ countable collection {En } ⊂ M of pairwise disjoint sets such
that n=1 En ∈ M.

Definition 1.5. Let X be a nonempty set, let M ⊂ P (X) be a σ-algebra.

om

(i) A map µ : M → [0, ∞] is called a (positive) measure if
∞

∞




µ (∅) = 0, µ
En =
µ (En )
n=1

.C

n=1

Zo

ne

for every countable collection {En } ⊂ M of pairwise disjoint sets. The
triple (X, M, µ) is said to be a measure space.
(ii) Given a measure µ : M → [0, ∞], a set E ∈ M has σ-finite µ measure if it
can be written as a countable union of measurable sets of finite measure;
µ is said to be σ-finite if X has σ-finite µ measure; µ is said to be finite

if µ (X) < ∞. Analogous definitions can be given for finitely additive
measures.
(iii) A measure µ is said to be a probability measure if µ (X) = 1.

en

Exercise 1.6. (i) Let X be a nonempty set. Show that the function µ :
P (X) → [0, ∞] defined by

card E if E is finite,
µ (E) :=
∞
otherwise,

nh
Vi

Si

5

is a measure. Here card is the cardinality. The measure µ is called the
counting measure. Prove that µ is finite (respectively σ-finite) if and only
if X is finite (respectively denumerable).
(ii) Given a sequence {xn } of nonnegative numbers we introduce µ : P (N) →
[0, ∞] as


µ (E) :=
xn , E ⊂ N.

n∈E

Prove that µ is a σ-finite measure.

SinhVienZone.com

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6

1 Measures

If (X, M, µ) is a measure space then the restriction of µ to a subset E ∈ M
is the measure µE : M → [0, ∞] defined by
(µE) (F ) := µ (F ∩ E) ,

Often, when there is no possibility of confusion, we use the same notation µE
to denote the restriction of the measure µ to the σ-algebra ME.
Among properties of measures we single out the following monotone convergence results.
Proposition 1.7. Let (X, M, µ) be a measure space.
(i) If {En } is an increasing sequence of subsets of M then
∞



µ
En = lim µ (En ) .
n→∞

n=1

n=1

om

(ii) If {En } is a decreasing sequence of subsets of M and µ (E1 ) < ∞ then

∞

En = lim µ (En ) .
µ
n→∞

ne

.C

Proof. (i) If µ (En ) = ∞ for some n ∈ N then both sides in (i) are ∞ and
there is nothing to prove. Thus assume that µ (En ) < ∞ for all n ∈ N and
define
Fn := En \ En−1 ,

en

Zo

where E0 := ∅. Since {En } is 
an increasing
sequence, it follows that the sets
∞

∞
Fn are pairwise disjoint with n=1 En = n=1 Fn , and so by the properties
of measures we have
∞

∞

l
∞








µ
En = µ
Fn =
µ (Fn ) = lim
µ (Fn )
n=1

n=1

= lim

l→∞

nh
Vi

Si

F ∈ M.

l



l→∞

n=1

n=1

(µ (En ) − µ (En−1 )) = lim µ (El ) .
l→∞

n=1

(ii) Apply part (i) to the increasing sequence {E1 \ En }.

Example 1.8. Without the hypothesis µ (E1 ) < ∞, property (ii) may be false.
Indeed, consider the counting measure defined in Exercise 1.6 with X := N and
define En := {n, n + 1, . . . , }. Then {En } is a decreasing sequence, µ (En ) = ∞
for all n ∈ N, but
∞



µ
En = µ (∅) = 0
= lim µ (En ) = ∞.
n=1

SinhVienZone.com

n→∞

/>

1.1 Measures and Integration

7

It turns out that for a finitely additive measure, property (i) is equivalent
to countable additivity. Indeed, we have the following proposition.
Proposition 1.9. Let X be a nonempty set, let M ⊂ P (X) be an algebra,
and let µ : M → [0, ∞] be a finitely additive measure. Then µ is countably
additive if and only if
∞



µ
En = lim µ (En )
(1.2)
n→∞

n=1

∞
for every increasing sequence {En } ⊂ M such that n=1 En ∈ M.
In addition, if µ is finite then µ is countably additive if and only if
lim µ (En ) = 0

(1.3)

n→∞

for every decreasing sequence {En } ⊂ M such that

∞
n=1

En = ∅.

om

Proof. If µ is countably additive, then (1.2) and (1.3) follow as in the proof of
Proposition 1.7. Suppose now that 
(1.2) holds. Let {Fn } ⊂ M be a sequence
∞
of mutually disjoint sets such that n=1 Fn ∈ M, and define
n



.C

En :=

Fk .

k=1

ne

Then by (1.2) we have
∞

∞





µ
Fk = µ
En = lim µ (En )
n=1
n