Ultraproducts and the Foundations of Higher Order Fourier Analysis
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The Fundamental Theorem of Calculus for Lebesgue Integral
Divulgaciones Matem´aticasVol. 8 No. 1 (2000), pp. 75{85 The Fundamental Theorem of Calculus for Lebesgue Integral El Teorema Fundamental del C´alculo para la Integral de Lebesgue Di´omedesB´arcenas([email protected]) Departamento de Matem´aticas.Facultad de Ciencias. Universidad de los Andes. M´erida.Venezuela. Abstract In this paper we prove the Theorem announced in the title with- out using Vitali's Covering Lemma and have as a consequence of this approach the equivalence of this theorem with that which states that absolutely continuous functions with zero derivative almost everywhere are constant. We also prove that the decomposition of a bounded vari- ation function is unique up to a constant. Key words and phrases: Radon-Nikodym Theorem, Fundamental Theorem of Calculus, Vitali's covering Lemma. Resumen En este art´ıculose demuestra el Teorema Fundamental del C´alculo para la integral de Lebesgue sin usar el Lema del cubrimiento de Vi- tali, obteni´endosecomo consecuencia que dicho teorema es equivalente al que afirma que toda funci´onabsolutamente continua con derivada igual a cero en casi todo punto es constante. Tambi´ense prueba que la descomposici´onde una funci´onde variaci´onacotada es ´unicaa menos de una constante. Palabras y frases clave: Teorema de Radon-Nikodym, Teorema Fun- damental del C´alculo,Lema del cubrimiento de Vitali. Received: 1999/08/18. Revised: 2000/02/24. Accepted: 2000/03/01. MSC (1991): 26A24, 28A15. Supported by C.D.C.H.T-U.L.A under project C-840-97. 76 Di´omedesB´arcenas 1 Introduction The Fundamental Theorem of Calculus for Lebesgue Integral states that: A function f :[a; b] R is absolutely continuous if and only if it is ! 1 differentiable almost everywhere, its derivative f 0 L [a; b] and, for each t [a; b], 2 2 t f(t) = f(a) + f 0(s)ds: Za This theorem is extremely important in Lebesgue integration Theory and several ways of proving it are found in classical Real Analysis. -
[Math.FA] 3 Dec 1999 Rnfrneter for Theory Transference Introduction 1 Sas Ihnrah U Twl Etetdi Eaaepaper
Transference in Spaces of Measures Nakhl´eH. Asmar,∗ Stephen J. Montgomery–Smith,† and Sadahiro Saeki‡ 1 Introduction Transference theory for Lp spaces is a powerful tool with many fruitful applications to sin- gular integrals, ergodic theory, and spectral theory of operators [4, 5]. These methods afford a unified approach to many problems in diverse areas, which before were proved by a variety of methods. The purpose of this paper is to bring about a similar approach to spaces of measures. Our main transference result is motivated by the extensions of the classical F.&M. Riesz Theorem due to Bochner [3], Helson-Lowdenslager [10, 11], de Leeuw-Glicksberg [6], Forelli [9], and others. It might seem that these extensions should all be obtainable via transference methods, and indeed, as we will show, these are exemplary illustrations of the scope of our main result. It is not straightforward to extend the classical transference methods of Calder´on, Coif- man and Weiss to spaces of measures. First, their methods make use of averaging techniques and the amenability of the group of representations. The averaging techniques simply do not work with measures, and do not preserve analyticity. Secondly, and most importantly, their techniques require that the representation is strongly continuous. For spaces of mea- sures, this last requirement would be prohibitive, even for the simplest representations such as translations. Instead, we will introduce a much weaker requirement, which we will call ‘sup path attaining’. By working with sup path attaining representations, we are able to prove a new transference principle with interesting applications. For example, we will show how to derive with ease generalizations of Bochner’s theorem and Forelli’s main result. -
Ergodicity and Metric Transitivity
Chapter 25 Ergodicity and Metric Transitivity Section 25.1 explains the ideas of ergodicity (roughly, there is only one invariant set of positive measure) and metric transivity (roughly, the system has a positive probability of going from any- where to anywhere), and why they are (almost) the same. Section 25.2 gives some examples of ergodic systems. Section 25.3 deduces some consequences of ergodicity, most im- portantly that time averages have deterministic limits ( 25.3.1), and an asymptotic approach to independence between even§ts at widely separated times ( 25.3.2), admittedly in a very weak sense. § 25.1 Metric Transitivity Definition 341 (Ergodic Systems, Processes, Measures and Transfor- mations) A dynamical system Ξ, , µ, T is ergodic, or an ergodic system or an ergodic process when µ(C) = 0 orXµ(C) = 1 for every T -invariant set C. µ is called a T -ergodic measure, and T is called a µ-ergodic transformation, or just an ergodic measure and ergodic transformation, respectively. Remark: Most authorities require a µ-ergodic transformation to also be measure-preserving for µ. But (Corollary 54) measure-preserving transforma- tions are necessarily stationary, and we want to minimize our stationarity as- sumptions. So what most books call “ergodic”, we have to qualify as “stationary and ergodic”. (Conversely, when other people talk about processes being “sta- tionary and ergodic”, they mean “stationary with only one ergodic component”; but of that, more later. Definition 342 (Metric Transitivity) A dynamical system is metrically tran- sitive, metrically indecomposable, or irreducible when, for any two sets A, B n ∈ , if µ(A), µ(B) > 0, there exists an n such that µ(T − A B) > 0. -
Defining Physics at Imaginary Time: Reflection Positivity for Certain
Defining physics at imaginary time: reflection positivity for certain Riemannian manifolds A thesis presented by Christian Coolidge Anderson [email protected] (978) 204-7656 to the Department of Mathematics in partial fulfillment of the requirements for an honors degree. Advised by Professor Arthur Jaffe. Harvard University Cambridge, Massachusetts March 2013 Contents 1 Introduction 1 2 Axiomatic quantum field theory 2 3 Definition of reflection positivity 4 4 Reflection positivity on a Riemannian manifold M 7 4.1 Function space E over M ..................... 7 4.2 Reflection on M .......................... 10 4.3 Reflection positive inner product on E+ ⊂ E . 11 5 The Osterwalder-Schrader construction 12 5.1 Quantization of operators . 13 5.2 Examples of quantizable operators . 14 5.3 Quantization domains . 16 5.4 The Hamiltonian . 17 6 Reflection positivity on the level of group representations 17 6.1 Weakened quantization condition . 18 6.2 Symmetric local semigroups . 19 6.3 A unitary representation for Glor . 20 7 Construction of reflection positive measures 22 7.1 Nuclear spaces . 23 7.2 Construction of nuclear space over M . 24 7.3 Gaussian measures . 27 7.4 Construction of Gaussian measure . 28 7.5 OS axioms for the Gaussian measure . 30 8 Reflection positivity for the Laplacian covariance 31 9 Reflection positivity for the Dirac covariance 34 9.1 Introduction to the Dirac operator . 35 9.2 Proof of reflection positivity . 38 10 Conclusion 40 11 Appendix A: Cited theorems 40 12 Acknowledgments 41 1 Introduction Two concepts dominate contemporary physics: relativity and quantum me- chanics. They unite to describe the physics of interacting particles, which live in relativistic spacetime while exhibiting quantum behavior. -
Generalizations of the Riemann Integral: an Investigation of the Henstock Integral
Generalizations of the Riemann Integral: An Investigation of the Henstock Integral Jonathan Wells May 15, 2011 Abstract The Henstock integral, a generalization of the Riemann integral that makes use of the δ-fine tagged partition, is studied. We first consider Lebesgue’s Criterion for Riemann Integrability, which states that a func- tion is Riemann integrable if and only if it is bounded and continuous almost everywhere, before investigating several theoretical shortcomings of the Riemann integral. Despite the inverse relationship between integra- tion and differentiation given by the Fundamental Theorem of Calculus, we find that not every derivative is Riemann integrable. We also find that the strong condition of uniform convergence must be applied to guarantee that the limit of a sequence of Riemann integrable functions remains in- tegrable. However, by slightly altering the way that tagged partitions are formed, we are able to construct a definition for the integral that allows for the integration of a much wider class of functions. We investigate sev- eral properties of this generalized Riemann integral. We also demonstrate that every derivative is Henstock integrable, and that the much looser requirements of the Monotone Convergence Theorem guarantee that the limit of a sequence of Henstock integrable functions is integrable. This paper is written without the use of Lebesgue measure theory. Acknowledgements I would like to thank Professor Patrick Keef and Professor Russell Gordon for their advice and guidance through this project. I would also like to acknowledge Kathryn Barich and Kailey Bolles for their assistance in the editing process. Introduction As the workhorse of modern analysis, the integral is without question one of the most familiar pieces of the calculus sequence. -
Stability in the Almost Everywhere Sense: a Linear Transfer Operator Approach ∗ R
CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector J. Math. Anal. Appl. 368 (2010) 144–156 Contents lists available at ScienceDirect Journal of Mathematical Analysis and Applications www.elsevier.com/locate/jmaa Stability in the almost everywhere sense: A linear transfer operator approach ∗ R. Rajaram a, ,U.Vaidyab,M.Fardadc, B. Ganapathysubramanian d a Dept. of Math. Sci., 3300, Lake Rd West, Kent State University, Ashtabula, OH 44004, United States b Dept. of Elec. and Comp. Engineering, Iowa State University, Ames, IA 50011, United States c Dept. of Elec. Engineering and Comp. Sci., Syracuse University, Syracuse, NY 13244, United States d Dept. of Mechanical Engineering, Iowa State University, Ames, IA 50011, United States article info abstract Article history: The problem of almost everywhere stability of a nonlinear autonomous ordinary differential Received 20 April 2009 equation is studied using a linear transfer operator framework. The infinitesimal generator Available online 20 February 2010 of a linear transfer operator (Perron–Frobenius) is used to provide stability conditions of Submitted by H. Zwart an autonomous ordinary differential equation. It is shown that almost everywhere uniform stability of a nonlinear differential equation, is equivalent to the existence of a non-negative Keywords: Almost everywhere stability solution for a steady state advection type linear partial differential equation. We refer Advection equation to this non-negative solution, verifying almost everywhere global stability, as Lyapunov Density function density. A numerical method using finite element techniques is used for the computation of Lyapunov density. © 2010 Elsevier Inc. All rights reserved. 1. Introduction Stability analysis of an ordinary differential equation is one of the most fundamental problems in the theory of dynami- cal systems. -
ERGODIC THEORY and ENTROPY Contents 1. Introduction 1 2
ERGODIC THEORY AND ENTROPY JACOB FIEDLER Abstract. In this paper, we introduce the basic notions of ergodic theory, starting with measure-preserving transformations and culminating in as a statement of Birkhoff's ergodic theorem and a proof of some related results. Then, consideration of whether Bernoulli shifts are measure-theoretically iso- morphic motivates the notion of measure-theoretic entropy. The Kolmogorov- Sinai theorem is stated to aid in calculation of entropy, and with this tool, Bernoulli shifts are reexamined. Contents 1. Introduction 1 2. Measure-Preserving Transformations 2 3. Ergodic Theory and Basic Examples 4 4. Birkhoff's Ergodic Theorem and Applications 9 5. Measure-Theoretic Isomorphisms 14 6. Measure-Theoretic Entropy 17 Acknowledgements 22 References 22 1. Introduction In 1890, Henri Poincar´easked under what conditions points in a given set within a dynamical system would return to that set infinitely many times. As it turns out, under certain conditions almost every point within the original set will return repeatedly. We must stipulate that the dynamical system be modeled by a measure space equipped with a certain type of transformation T :(X; B; m) ! (X; B; m). We denote the set we are interested in as B 2 B, and let B0 be the set of all points in B that return to B infinitely often (meaning that for a point b 2 B, T m(b) 2 B for infinitely many m). Then we can be assured that m(B n B0) = 0. This result will be proven at the end of Section 2 of this paper. In other words, only a null set of points strays from a given set permanently. -
Chapter 6. Integration §1. Integrals of Nonnegative Functions Let (X, S, Μ
Chapter 6. Integration §1. Integrals of Nonnegative Functions Let (X, S, µ) be a measure space. We denote by L+ the set of all measurable functions from X to [0, ∞]. + Let φ be a simple function in L . Suppose f(X) = {a1, . , am}. Then φ has the Pm −1 standard representation φ = j=1 ajχEj , where Ej := f ({aj}), j = 1, . , m. We define the integral of φ with respect to µ to be m Z X φ dµ := ajµ(Ej). X j=1 Pm For c ≥ 0, we have cφ = j=1(caj)χEj . It follows that m Z X Z cφ dµ = (caj)µ(Ej) = c φ dµ. X j=1 X Pm Suppose that φ = j=1 ajχEj , where aj ≥ 0 for j = 1, . , m and E1,...,Em are m Pn mutually disjoint measurable sets such that ∪j=1Ej = X. Suppose that ψ = k=1 bkχFk , where bk ≥ 0 for k = 1, . , n and F1,...,Fn are mutually disjoint measurable sets such n that ∪k=1Fk = X. Then we have m n n m X X X X φ = ajχEj ∩Fk and ψ = bkχFk∩Ej . j=1 k=1 k=1 j=1 If φ ≤ ψ, then aj ≤ bk, provided Ej ∩ Fk 6= ∅. Consequently, m m n m n n X X X X X X ajµ(Ej) = ajµ(Ej ∩ Fk) ≤ bkµ(Ej ∩ Fk) = bkµ(Fk). j=1 j=1 k=1 j=1 k=1 k=1 In particular, if φ = ψ, then m n X X ajµ(Ej) = bkµ(Fk). j=1 k=1 In general, if φ ≤ ψ, then m n Z X X Z φ dµ = ajµ(Ej) ≤ bkµ(Fk) = ψ dµ. -
2. Convergence Theorems
2. Convergence theorems In this section we analyze the dynamics of integrabilty in the case when se- quences of measurable functions are considered. Roughly speaking, a “convergence theorem” states that integrability is preserved under taking limits. In other words, ∞ if one has a sequence (fn)n=1 of integrable functions, and if f is some kind of a limit of the fn’s, then we would like to conclude that f itself is integrable, as well R R as the equality f = limn→∞ fn. Such results are often employed in two instances: A. When we want to prove that some function f is integrable. In this case ∞ we would look for a sequence (fn)n=1 of integrable approximants for f. B. When we want to construct and integrable function. In this case, we will produce first the approximants, and then we will examine the existence of the limit. The first convergence result, which is somehow primite, but very useful, is the following. Lemma 2.1. Let (X, A, µ) be a finite measure space, let a ∈ (0, ∞) and let fn : X → [0, a], n ≥ 1, be a sequence of measurable functions satisfying (a) f1 ≥ f2 ≥ · · · ≥ 0; (b) limn→∞ fn(x) = 0, ∀ x ∈ X. Then one has the equality Z (1) lim fn dµ = 0. n→∞ X Proof. Let us define, for each ε > 0, and each integer n ≥ 1, the set ε An = {x ∈ X : fn(x) ≥ ε}. ε Obviously, we have An ∈ A, ∀ ε > 0, n ≥ 1. One key fact we are going to use is the following. -
Lecture 26: Dominated Convergence Theorem
Lecture 26: Dominated Convergence Theorem Continuation of Fatou's Lemma. + + Corollary 0.1. If f 2 L and ffn 2 L : n 2 Ng is any sequence of functions such that fn ! f almost everywhere, then Z Z f 6 lim inf fn: X X Proof. Let fn ! f everywhere in X. That is, lim inf fn(x) = f(x) (= lim sup fn also) for all x 2 X. Then, by Fatou's lemma, Z Z Z f = lim inf fn 6 lim inf fn: X X X If fn 9 f everywhere in X, then let E = fx 2 X : lim inf fn(x) 6= f(x)g. Since fn ! f almost everywhere in X, µ(E) = 0 and Z Z Z Z f = f and fn = fn 8n X X−E X X−E thus making fn 9 f everywhere in X − E. Hence, Z Z Z Z f = f 6 lim inf fn = lim inf fn: X X−E X−E X χ Example 0.2 (Strict inequality). Let Sn = [n; n + 1] ⊂ R and fn = Sn . Then, f = lim inf fn = 0 and Z Z 0 = f dµ < lim inf fn dµ = 1: R R Example 0.3 (Importance of non-negativity). Let Sn = [n; n + 1] ⊂ R and χ fn = − Sn . Then, f = lim inf fn = 0 but Z Z 0 = f dµ > lim inf fn dµ = −1: R R 1 + R Proposition 0.4. If f 2 L and X f dµ < 1, then (a) the set A = fx 2 X : f(x) = 1g is a null set and (b) the set B = fx 2 X : f(x) > 0g is σ-finite. -
MEASURE and INTEGRATION: LECTURE 3 Riemann Integral. If S Is Simple and Measurable Then Sdµ = Αiµ(
MEASURE AND INTEGRATION: LECTURE 3 Riemann integral. If s is simple and measurable then N � � sdµ = αiµ(Ei), X i=1 �N where s = i=1 αiχEi . If f ≥ 0, then � �� � fdµ = sup sdµ | 0 ≤ s ≤ f, s simple & measurable . X X Recall the Riemann integral of function f on interval [a, b]. Define lower and upper integrals L(f, P ) and U(f, P ), where P is a partition of [a, b]. Set − � � f = sup L(f, P ) and f = inf U(f, P ). P P − A function f is Riemann integrable ⇐⇒ − � � f = f, − in which case this common value is � f. A set B ⊂ R has measure zero if, for any � > 0, there exists a ∞ ∞ countable collection of intervals { Ii}i =1 such that B ⊂ ∪i =1Ii and �∞ i=1 λ(Ii) < �. Examples: finite sets, countable sets. There are also uncountable sets with measure zero. However, any interval does not have measure zero. Theorem 0.1. A function f is Riemann integrable if and only if f is discontinuous on a set of measure zero. A function is said to have a property (e.g., continuous) almost ev erywhere (abbreviated a.e.) if the set on which the property does not hold has measure zero. Thus, the statement of the theorem is that f is Riemann integrable if and only if it is continuous almost everywhere. Date: September 11, 2003. 1 2 MEASURE AND INTEGRATION: LECTURE 3 Recall positive measure: a measure function µ: M → [0, ∞] such ∞ �∞ that µ (∪i=1Ei) = i=1 µ(Ei) for Ei ∈ M disjoint. -
Lecture Notes for Math 522 Spring 2012 (Rudin Chapter
Lebesgue integral We will define a very powerful integral, far better than Riemann in the sense that it will allow us to integrate pretty much every reasonable function and we will also obtain strong convergence results. That is if we take a limit of integrable functions we will get an integrable function and the limit of the integrals will be the integral of the limit under very mild conditions. We will focus only on the real line, although the theory easily extends to more abstract contexts. In Riemann integral the basic block was a rectangle. If we wanted to integrate a function that was identically 1 on an interval [a; b], then the integral was simply the area of that rectangle, so 1×(b−a) = b−a. For Lebesgue integral what we want to do is to replace the interval with a more general subset of the real line. That is, if we have a set S ⊂ R and we take the indicator function or characteristic function χS defined by ( 1 if x 2 S, χ (x) = S 0 else. Then the integral of χS should really be equal to the area under the graph, which should be equal to the \size" of S. Example: Suppose that S is the set of rational numbers between 0 and 1. Let us argue that its size is 0, and so the integral of χS should be 0. Let fx1; x2;:::g = S be an enumeration of the points of S. Now for any > 0 take the sets −j−1 −j−1 Ij = (xj − 2 ; xj + 2 ); then 1 [ S ⊂ Ij: j=1 −j The \size" of any Ij should be 2 , so it seems reasonable to say that the \size" of S is less than the sum of the sizes of the Ij's.