Geometric intuition: from Hölder spaces to the Calderón-Zygmund estimate
|
|
- Timothy Fleming
- 6 years ago
- Views:
Transcription
1 Geometric intuition: from Hölder spaces to the Calderón-Zygmund estimate A survey of Lihe Wang s paper Michael Snarski December 5, 22 Contents Hölder spaces. Control on functions Control on sets Regularity results 4 3 Calderòn-Zygmund decomposition 4 4 Vitali covering lemma 5 5 The Hardy-Littlewood Maximal function 7 6 Proof of theorem 9 Abstract The following report is based on paper by Wang [?]. We include Wang s discussion of Hölder norms and supplement it with results from class and assignments. Then, we introduce the Hardy- Littlewood maximal function, the Calderòn-Zygmund decomposition, and the Vitali covering lemma. Finally, we include the proof of the Calderòn-Zygmund estimate ( ) D 2 u p c f p + u p. B B B Hölder spaces. Control on functions Let us first discuss the geometry of Hölder spaces. We are motivated by the discussion in Wang. Hölder norms given us intuition on the density of a function and what can be said about sets { f > λ} = {x : f(x) > λ} and their measure. Recall that the definition of Hölder norm C,α for α (, ] is u C,α u(x) u(y) = sup u(x) + sup x x,y x y α. () Note that, from homework assignment 4, question 5. (a), the space for which (??) holds for α = + ε, for any ε >, is only the space of constants. Moreover, from (b), this space is much larger than C ; continuously differentiable functions are not dense in C,α. Thus, there is an advantage at looking at this class of functions. Consider what the control on these functions looks like. For > x y > and α (, ), we have x y < x y α, and so f(x) f(y) x y α < f(x) f(y). x y
2 Figure : Graph of f(x) = x α for α = (left) and α = 2 (right). In particular, this makes it clear that C C,α. Taylor s theorem gives us the error on first order approximation, so one can interpret the Hölder norm as a sort of error estimate. However, it gives much more local information about the function than the L p norm. Indeed, because integrals do not detect sets of measure, and the L p norm does not tell us about any local behaviour of a function, we would not be able to bound D 2 u without additional information. Now consider the definition of density. Provided the limit exists, the metric density of a measurable set E at a point x is defined as lim r m(b(x, r) E). m(b(x, r)) With this definition in mind, consider the geometry of Hölder space. In his paper, Wang uses the following statements for motivation: Lemma. If u is a solution of (4) and h is a continuous harmonic function with h = u on B, then u(x) h(x) 2n x 2 ) f. (2) Corollary. For any < α <, there are positive universal contsants r < and ε > such that for a solution u of (4) in B, with u and f ε, there is a constant A such that u(x) A r α for all x B r. Taking B = B r, we can iterate this to obtain u(x) A k r kα for x B r k. In other words, we can get very precise local precision of u. This will be the guiding idea for the measure-theoretic bounds on the sizes of sets..2 Control on sets Equation (2) tells us that the function u is almost harmonic. In other words, we should be able to get some control over it, possibly by looking at averages over balls (we will see this when we introduce the maximal function). Let s look at how we can translate Lemma into set-theoretic language. We first include the following theorem. Theorem (Rudin 8.6). If f : Ω [, ) is a measurable function, µ is a σ finite positive measure on the Borel sigma algebra of Ω R d, ϕ : [, ] [, ] is monotonic, absolutely continuous on [, T ] for every T <, ϕ() = and ϕ(t) ϕ( ) as t, then (ϕ f)dµ = µ{x : f(x) > t}ϕ (t)dt. Ω Proof. Let E be the set of all (x, t) Ω [, ) such that f(x) > t. E is clearly a measurable set whenever f is a simple function, and its measurability follows from the approximation of any 2
3 measurable function by a sequence of increasing simple functions φ φ 2... f. t [, ), let E t be the slice E t = {x : (x, t) E}. We therefore have µ(e t ) = χ {x:f(x)>t} (x, t)dµ(x). Hence, µ{x : f(x) > t}ϕ (t)dt = = Ω = = Ω Ω f(x) Ω Ω χ {f>t} (x, t)ϕ (t)dµ(x)dt χ {f>t} (x, t)ϕ (t)dtdµ(x) ϕ (t)dtdµ(x) ϕ(f(x))dµ(x). For every Corollary 2. We have the relationship u p dx = p Ω t p m{x Ω : u(x) > t}dt. Proof. Apply the previous theorem with ϕ(t) = t p and f(x) = u(x). We obtain u(x) p dx = p t p µ{f > t}dt. Ω Thus, if Ω u p dx =, we should have that { u > λ} λ p, so that { u > λ} is small for λ large. In order to prove that any u L p, then, we should start by showing the decay of { u > λ}. We might hope to prove { u > λ } ε { u > } for some fixed ε > and λ >, and use the scaling to get u(rx) = r 2 f(rx) { u > λ λ} ε { u > λ}. Now compare with (). We might hope to use the above inequality to get some inductive estimate. Based on the fact that D 2 u L p if u is, we might guess that { D 2 u > λ λ} ε( { D 2 u > λ} + { f δ λ} }. However, the above is not true; the failure is due to the fact that the condition D 2 u(x ) is unstable. In order to get the desired bounds, we will have to use the maximal function. 3
4 2 Regularity results We have already seen the following. Theorem 2 (Evans, Thm. 4). For u H (Ω) a solution to (4), we have We use this to obtain the following. u H 2 (Ω) C f L 2 (Ω). Proposition. If { u = f B u = B, then D 2 u 2 C f 2. B B Evans motivates this bound as follows: if u = f, we have f 2 = ( u) 2 = = = n u xix i u xjx j i,j= n u xix ix j u xj i,j= n u xix j u xix j i,j= (u ) ( 2 ) u =... u n i j.. u n 3 Calderòn-Zygmund decomposition Though it will not be used here, we briefly present the decomposition. We first present the covering lemma. Lemma 2 (Calderón-Zygmund). Let f L (R n ) with f. There is an open set Ω which is a disjoint union of open cubes and its complement F a closed set such that: i. f(x) α a.e. on F ii. Ω = k Q k, where the cubes Q k have disjoint interior and α < f 2 n α (3) Q k Q k iii. m(ω) f α. Proof. Divide R n using a dyadic grid. Here, we see that there are only two cases { Q 2 k Q 2 n. Q 2 k Q 2n = bdry Start with a Q sufficiently large so that Q Q f(x)dx α for any x, where we are integrating with respect to Lebesgue measure. Let n be the dimension of the space. We get 2 n cubes at each stage of the decomposition. Stage I: two cases for each new cube 4
5 . m(q) Q f α 2. m(q) Q f > α. In the second case, we keep the cube and put it into our collection {Q k }, which will eventually become Ω. Note that since each new cube has half the length of the one of the previous generation (say Q ), we have f 2n f 2 n α, m(q) Q m(q ) Q so that (3) holds. In the first case, we keep on dividing and get a countable collection. At each stage, we look again, since almost every point of an L function is a Lebesgue point, we find that f α almost everywhere. Clearly, then, i. and ii. hold. It remains to see that m(ω) = k k m(q k ) f(x)dx α Q k = α f L (Ω) α f. We can now show the decomposition of f into good and bad parts, f = g + b. We do so as follows: { f(x) x F g(x) = m(q k ) Q k f(x)dx x Q k Ω. We notice that g(x) 2 α almost everywhere. Then b = f g and { x F b(x) = f(x) f x Q k Ω. Thus, we have that for every cube, b =. 4 Vitali covering lemma The Vitali covering lemma is another simple combinatorial-geometric proof about sets which allows one to consider disjoint unions of balls. It is often used in the proof of the Lebesgue-Radon-Nikodym theorem. We present the proof with a figure to give intuition, and afterwards we give a modified version of Vitali which Wang uses for his proof. The following proof is from Rudin s Real & Complex analysis. Theorem 3 (Vitali covering lemma). Suppose that Ω = n B(x i, r i ) is a finite collection of balls. There is then a set S = {i,..., i k } {,..., n} such that 5
6 Figure 2: Top: original set Ω, a disjoint union of balls. Bottom left: the final result, S = i, i 2. Bottom right: any y that was in a discarded ball will be within three times the radius of the ball that superceded it. (i) the balls B(x i, r i ), i S are disjoint, (ii) Ω i S B(x i, 3r i ), (iii) m(ω) 3 k i S m(b(x i, r i )). Proof. The proof is simple. Without loss of generality (re-label if necessary), assume the balls are ordered from largest to smallest radii, i.e. r r 2 r n. Let i =, and discard all balls which intersect B i. Let i 2 be the largest radius of the remaining balls, and repeat the same procedure. Since there is only a finite numbers of balls, this procedure ends at some i k. It is obvious that the balls are disjoint, so that (i) holds. Now, suppose that y Ω, so y B(x j, r j ) for some i. If j = i j for some i j S, we are done, so suppose the ball B(x j, r j ) was not chosen. This means it must have intersected some B(x i, r i ) before it got discarded. Since r i r j (otherwise B(x i, r i ) would ve been discarded), we must have y B(x i, 3r i ), which proves (ii). The last statement then follows easily, as by the basic properties of Lebesgue measure. (m(b(x, 3r i ))) = 3 k m(b(x, r i )) The modified Vitali is a little more complicated. Theorem 4 (Modified Vitali). Let < ε < and C D B be two measurable sets with C < ε B and satisfying the following property: for evrey x B with C B r (x) ε B r, B r (x) B D. Then D (2 n ε) C. Proof. First, C < ε B, so for almost all x C, we can find r x such that C B rx (x) = ε B rx. To see this, note that if we choose any x in the interior of C, we can certainly choose r x small enough so that B rx C and then C B rx (x) = B rx > ε B rx. Moreover, we can choose r x so big that C B rx (x) < ε B rx. Assuming C B rx (x) varies continuously with r x, we can invoke the intermediate value theorem to see there is an r x such that we have equality. Let then r x < 2 be such that C B rx (X) = ε B rx with C B r (x) < ε B r 6
7 Figure 3: Left: too small; center: too big; right: just right! for all r x < r < 2. The set C can clearly be written as a union of balls, so we can apply the Vitali covering lemma to obtain x, x 2,..., x n C which satisfy the above condition and such that B rxi B rxj =, i j, C n B 5rxi (x i ), and since C B, we also have C i B 5rxi (x i ) B. Moreover, since each B rxi (x i ) satisfies we have ε B rxi (x i ) = C B rxi (x i ), C B 5rxi (x i ) < ε B 5rxi (x i ) (5r i > r i ) = 5 n ε B rxi (x i ) = 5 n C B rxi (x i ). Moreover, since r xi < 2, we can say that B rxi (x i ) 4 n B rxi B. Thus, n C = B 5rxi (x i ) C n B 5rxi (x i ) C n 5 n 2 n n ε B rxi (x i ) ε B rxi (x i ) B n = ε2 n B rxi (x i ) B = 2 n D. 5 The Hardy-Littlewood Maximal function The Hardy-Littlewood function is one of the most important tools in analysis (or so I ve been read and been told). In some sense, it gives the worst behiaviour of a function. For f L p (R d ), the maximal function is defined as (Mf)(x) = sup r> where dm denotes the d-dimensional Lebesgue measure. f dm, m(b r ) B(x,r) 7
8 Remark For f L, Mf is never in L. Here is a simple example that is easily generalized. Take d = and w.l.o.g. suppose f > and let x 2. By choosing a different radius, we will do worse than the supremum over all radii, and so in particular choosing r = x we have Mf(x) = sup f(x) dx <r< m(b r (x)) B r(x) f(x) dx 2 x 2 x B x (x) f(x) dx = C x. Even if we take the maximal function to be on x 2, the function still diverges due to the factor of x and therefore cannot be in L. There exist other types of maximal functions. For instance, if µ is any complex Borel measure and m denotes the d-dimensional Lebesgue measure, we first set (P r µ)(x) = and we define the maximal function Mµ for µ by µ(b(x, r)) m(b(x, r)), (Mµ)(x) = sup(p r µ)(x). r> It is easy to show that the function Mµ is lower semicontinuous, hence measurable. We now use the Vitali covering lemma to prove the following lemma: Theorem 5. For a complex Borel measure µ on R d and λ >, we have {x R d : (Mµ)(x) > λ} 3 d λ u, where u = µ (R d ) < is the total variation of µ. Proof. Let µ, λ be given. There is a compact subset K of the open set {Mµ > λ} (open by the previous claims). For every x K and open ball B, we have µ (B) > λm(b). Since K is compact, we can cover K by finitely many balls B, and by the Vitali covering lemma there are balls B,..., B n such that m(k) 3 d n m(b i ) 3 d λ n µ (B i ) 3 d λ µ where the last inequality follows by disjointness of the balls. The result now follows by taking the supremum over all compact subset K {Mµ > λ}. Definition The space Weak L is the space of measurable f for which λm{ f > λ} < for < λ <. Remark Any function f L is in weak L. To see this, let λ >, and let E = { f > λ}. We then have λm(e) = λdm f dm f dm = f. E E R d Taking λ to the other side yields the result. Corollary 3. For every f L (R d ) and λ >, {Mf > λ} 3 d λ f. 8
9 Proof. We can use the previous theorem since for any measurable set E R d, the mapping E fdm defines a complex Borel measure µ. Writing µ = fdm, we see that the definition of the maximal function for a measure agrees precisely with the Hardy-Littlewood maximal function. The above are called weak (,) estimates and give à-priori control over Mf in L. On the other hand, we have strong (p,p) type estimates, which tell us that if f L p for p >, then Mf L p as well. Proof. If Mf > λ, then by definition, there is a ball of radius r centered at x such that f dm > λ B(x, r). B(x,r) By the Vitali covering lemma, there are disjoint balls B (x, r ),..., B n (x n, r n ) such that Hence, E n B(x i, 5r i ) {x : Mf(x) > λ}. {Mf > λ} 3 d n 3 d λ B i f dm. Note that this is the same weak estimate we obtained in the previous corollary. We now apply it to obtain strong estimates. Define { f(x) f(x) > λ β(x) = 2 otherwise. Clearly then, β(x) L p for f L p, and we can apply the above weak estimate to β: {Mf > λ} 2 3d f dm. λ f >λ/2 Hence, Mf p p = = p = p Mf(x) R d Mf(x) Mf(x) 3 d 2p pλ p dλdm λ p dmdλ R d λ p m({mf > λ})dλ R d λ p f dmdλ f >λ/2 = C p f p p <. (Fubini) 6 Proof of theorem We are now in a position to prove the estimate. 9
10 Theorem 6 (Calderón-Zygmund). For u satisfying we have the following estimate for p (, ): D 2 u p C B where D denotes the total derivative operator. We first prove the following lemma Lemma 3. If u is a solution of u(x) = f(x), x < 2, (4) ( ) f p + u p, B 2 B 2 u = f in a domain Ω which contains B 4, the balls of radius 4, and {M( f 2 ) δ 2 } {M D 2 u 2 } B, (5) then there is a constant N so that for any ε >, there is a δ > depending only on ε such that {M D 2 u 2 > N 2 } B < ε B. (6) Proof. Since the set in (5) is not empty, there is x B such that, for any r >, we have D 2 u 2 2 n, f 2 2 n δ 2. (7) B(r,x ) B(r,x ) In particular, this allows us to recenter and obtain D 2 u 2, f 2 δ 2. B 4 B 4 Hence (why?) u u B4 2 C. B 4 Letting v be a solution to the corresponding system { v = on B 4 v = u ( u) B4 x u B4 on B 4, we have by minimality of harmonic functions for the Dirichlet energy, v 2 u u B4 2 C. B 4 B 4 Now, by typical regularity results, we have that v C, so we can bound the supremum of D 2 v: Meanwhile, D 2 v 2 L (B 3) N 2. D 2 (u v) 2 C f 2 Cδ 2. B 3 B 4 Using the weak L estimate proved in Corollary 3, we have the following bound: λ {x B 3 : M D 2 (u v) > λ} C N 2 D 2 (u v) 2 B 3 C N 2 f 2 Cδ 2 B 4
11 by the above. Now, taking N 2 = max(4n 2, 2 n ), we have that {x B : M D 2 u 2 > N 2 } {x B : M B3 D 2 (u v) 2 > N 2 }. How to see this? Well, if y B 3, then certainly D 2 u(y) 2 = D 2 u(y) 2 2 D 2 v(y) D 2 v(y) 2 2 D 2 u(y) D 2 v(y) 2 + 2N 2. For any x {x B : M B3 D 2 (u v) 2 > N 2 }, r 2, we have B 2 (x) B 3 and therefore D 2 u 2 2M B3 ( D 2 (u v) 2 )(x) + 2N 2 4N 2. sup r 2 B r(x) On the other hand, for r > 2, we have D 2 u 2 B r since B(x,r) B(x,r) by assumption. Finally, Wang shows that B(x,2r) D 2 u 2 2 n, D 2 u 2 2 n, f 2 2 n δ 2 B(x,r) {x B : M( D 2 u 2 ) > N 2 } {x B : M( D 2 (u v) 2 > N 2 } C N 2 f 2 Cδ 2 /N 2 < Cδ 2 = ε B where δ 2 = ε B C by choice. Corollary 4. Suppose u is a solution of (4) in a domain Ω which contains the ball B(x, 4r) for some x, r. Let B = (x, r). If {M( D 2 u 2 ) > N 2 } B ε B then Proof. By the contrapositive, if then the assumption B {M( D 2 u 2 ) > } {M( D 2 u 2 > δ 2 }. {M D 2 u 2 > N 2 } B ε B {M( f 2 ) δ 2 } {M D 2 u 2 } B must be false. In other words, the intersection is empty, and so the original set is entirely contained in the set where M( f 2 ) > δ 2 and/or M( D 2 u 2 ) >. From Wang: The moral of Corollary 2 is that the set {x : M( D 2 u 2 ) > } is bigger than the set {x : M( D 2 u 2 ) > N 2 } modulo {M(f 2 ) > δ 2 } if {x : M( D 2 u 2 ) > N 2 } B = ε B. As in the Vitali lemma, we will cover a good portion of the set {x : M( D 2 u 2 ) > N 2 } by disjoint balls so that in each of the balls, the density is ε. Corollary 5. Assuming that u is a solution of (4) in Ω B 4, with then for ε = 2 n ε, we have i. {x B : M( D 2 u 2 ) > N 2 } ε B, {x B : M( D 2 u 2 ) > N 2 } ε ( {x B : M( D 2 u 2 ) > } + {x B : M( f 2 ) > δ 2 } ).
12 ii. {x B : M( D 2 u 2 ) > N 2 } ε ( {x B : M( D 2 u 2 ) > λ 2 } + {x B : M( f 2 ) > λ 2 δ 2 } ) iii. {x B : M( D 2 u 2 ) > (N 2 ) i } k ε i {x B : M( f 2 ) > δ 2 (N 2 ) k i } +ε k {x B : M( f 2 ) > }. Proof. The important thing is to notice the resemblance between this and how we dealt with the geometry of functions. The proof is straightforward from what we have proven. First, set C = {x B : M( D 2 u 2 ) > N 2 } D = {x B : M( D 2 u 2 ) > } {x B : M( f 2 ) > δ 2 }. Now, we have that C ε B by assumption. To see that the second condition for the modified Vitali lemma is satisfied, we need to show that B r C ε B r implies B r C D. This follows immediately from the main lemma 3 that B r C ε B r D c B r =. To see ii., we need only iterate the above result on the equation (λ u) = λ f. Here, we are using the scaling property of the Laplacian: u(rx) = r 2 f(rx). Finally, repeating the same thing over and over on λ = N, N 2,... we get further local information on the geometry of these sets. We are now in a position to prove the estimate. We do so for p > 2. Theorem 7. If u = f in B 4, then D 2 u p C f p + u p. B B 4 Proof. The first step of the proof is to assume that f p is small so that we can obtain {x B : M D 2 u 2 > N 2 } ε B. Indeed, this is the crucial bound that makes everything work. We can always obtain this, however, by multiplying f by a small constant and then pulling it out into the larger one. The trick is to write D 2 u p = ( D 2 u 2 ) p/2 2
13 and show that M( D 2 u 2 ) L p/2 (B ), so that D 2 u L p (B ). Suppose then that f p = δ. We then have that (why?) (N ) ip {M( f 2 ) > δ 2 (N ) 2i pn p } δ p (N ) f p p C. Hence, D 2 u p (M( D 2 u ) ) p/2 dx B B = p λ p {x B : M D 2 u 2 λ 2 } dλ ( ) p B + (N ) kp {x B : M D 2 u 2 > (N ) 2k } p ( B + k= N kp k (N ) ip {M( f 2 ) > δ 2 (N ) 2i } C. ε i {x B : M f 2 δ 2 N 2(k i) } + Taking ε so that N p ε <, the sum converges and thus the theorem is proved. References k= N kp εk {x : M( D 2 u 2 ) } [] Lihe Wang, A geometric approach to the Calderón-Zygmund estimates. University of Iowa, lwang/cccalderon.pdf. ) 3
Regularity Theory. Lihe Wang
Regularity Theory Lihe Wang 2 Contents Schauder Estimates 5. The Maximum Principle Approach............... 7.2 Energy Method.......................... 3.3 Compactness Method...................... 7.4 Boundary
More informationHARMONIC ANALYSIS. Date:
HARMONIC ANALYSIS Contents. Introduction 2. Hardy-Littlewood maximal function 3. Approximation by convolution 4. Muckenhaupt weights 4.. Calderón-Zygmund decomposition 5. Fourier transform 6. BMO (bounded
More informationLebesgue s Differentiation Theorem via Maximal Functions
Lebesgue s Differentiation Theorem via Maximal Functions Parth Soneji LMU München Hütteseminar, December 2013 Parth Soneji Lebesgue s Differentiation Theorem via Maximal Functions 1/12 Philosophy behind
More informationCHAPTER 6. Differentiation
CHPTER 6 Differentiation The generalization from elementary calculus of differentiation in measure theory is less obvious than that of integration, and the methods of treating it are somewhat involved.
More informationMATH 202B - Problem Set 5
MATH 202B - Problem Set 5 Walid Krichene (23265217) March 6, 2013 (5.1) Show that there exists a continuous function F : [0, 1] R which is monotonic on no interval of positive length. proof We know there
More informationJUHA KINNUNEN. Harmonic Analysis
JUHA KINNUNEN Harmonic Analysis Department of Mathematics and Systems Analysis, Aalto University 27 Contents Calderón-Zygmund decomposition. Dyadic subcubes of a cube.........................2 Dyadic cubes
More information3 (Due ). Let A X consist of points (x, y) such that either x or y is a rational number. Is A measurable? What is its Lebesgue measure?
MA 645-4A (Real Analysis), Dr. Chernov Homework assignment 1 (Due ). Show that the open disk x 2 + y 2 < 1 is a countable union of planar elementary sets. Show that the closed disk x 2 + y 2 1 is a countable
More informationMaximal Functions in Analysis
Maximal Functions in Analysis Robert Fefferman June, 5 The University of Chicago REU Scribe: Philip Ascher Abstract This will be a self-contained introduction to the theory of maximal functions, which
More information2 (Bonus). Let A X consist of points (x, y) such that either x or y is a rational number. Is A measurable? What is its Lebesgue measure?
MA 645-4A (Real Analysis), Dr. Chernov Homework assignment 1 (Due 9/5). Prove that every countable set A is measurable and µ(a) = 0. 2 (Bonus). Let A consist of points (x, y) such that either x or y is
More informationReal Analysis Notes. Thomas Goller
Real Analysis Notes Thomas Goller September 4, 2011 Contents 1 Abstract Measure Spaces 2 1.1 Basic Definitions........................... 2 1.2 Measurable Functions........................ 2 1.3 Integration..............................
More informationThe Lebesgue Integral
The Lebesgue Integral Brent Nelson In these notes we give an introduction to the Lebesgue integral, assuming only a knowledge of metric spaces and the iemann integral. For more details see [1, Chapters
More informationREAL ANALYSIS I HOMEWORK 4
REAL ANALYSIS I HOMEWORK 4 CİHAN BAHRAN The questions are from Stein and Shakarchi s text, Chapter 2.. Given a collection of sets E, E 2,..., E n, construct another collection E, E 2,..., E N, with N =
More informationLebesgue Measure on R n
CHAPTER 2 Lebesgue Measure on R n Our goal is to construct a notion of the volume, or Lebesgue measure, of rather general subsets of R n that reduces to the usual volume of elementary geometrical sets
More informationMATHS 730 FC Lecture Notes March 5, Introduction
1 INTRODUCTION MATHS 730 FC Lecture Notes March 5, 2014 1 Introduction Definition. If A, B are sets and there exists a bijection A B, they have the same cardinality, which we write as A, #A. If there exists
More information1/12/05: sec 3.1 and my article: How good is the Lebesgue measure?, Math. Intelligencer 11(2) (1989),
Real Analysis 2, Math 651, Spring 2005 April 26, 2005 1 Real Analysis 2, Math 651, Spring 2005 Krzysztof Chris Ciesielski 1/12/05: sec 3.1 and my article: How good is the Lebesgue measure?, Math. Intelligencer
More informationJUHA KINNUNEN. Real Analysis
JUH KINNUNEN Real nalysis Department of Mathematics and Systems nalysis, alto University Updated 3 pril 206 Contents L p spaces. L p functions..................................2 L p norm....................................
More informationMeasures. Chapter Some prerequisites. 1.2 Introduction
Lecture notes Course Analysis for PhD students Uppsala University, Spring 2018 Rostyslav Kozhan Chapter 1 Measures 1.1 Some prerequisites I will follow closely the textbook Real analysis: Modern Techniques
More informationII - REAL ANALYSIS. This property gives us a way to extend the notion of content to finite unions of rectangles: we define
1 Measures 1.1 Jordan content in R N II - REAL ANALYSIS Let I be an interval in R. Then its 1-content is defined as c 1 (I) := b a if I is bounded with endpoints a, b. If I is unbounded, we define c 1
More informationMTH 404: Measure and Integration
MTH 404: Measure and Integration Semester 2, 2012-2013 Dr. Prahlad Vaidyanathan Contents I. Introduction....................................... 3 1. Motivation................................... 3 2. The
More informationMeasure Theory. John K. Hunter. Department of Mathematics, University of California at Davis
Measure Theory John K. Hunter Department of Mathematics, University of California at Davis Abstract. These are some brief notes on measure theory, concentrating on Lebesgue measure on R n. Some missing
More informationTHE HARDY LITTLEWOOD MAXIMAL FUNCTION OF A SOBOLEV FUNCTION. Juha Kinnunen. 1 f(y) dy, B(x, r) B(x,r)
Appeared in Israel J. Math. 00 (997), 7 24 THE HARDY LITTLEWOOD MAXIMAL FUNCTION OF A SOBOLEV FUNCTION Juha Kinnunen Abstract. We prove that the Hardy Littlewood maximal operator is bounded in the Sobolev
More informationA VERY BRIEF REVIEW OF MEASURE THEORY
A VERY BRIEF REVIEW OF MEASURE THEORY A brief philosophical discussion. Measure theory, as much as any branch of mathematics, is an area where it is important to be acquainted with the basic notions and
More information(1) Consider the space S consisting of all continuous real-valued functions on the closed interval [0, 1]. For f, g S, define
Homework, Real Analysis I, Fall, 2010. (1) Consider the space S consisting of all continuous real-valued functions on the closed interval [0, 1]. For f, g S, define ρ(f, g) = 1 0 f(x) g(x) dx. Show that
More informationSHARP INEQUALITIES FOR MAXIMAL FUNCTIONS ASSOCIATED WITH GENERAL MEASURES
SHARP INEQUALITIES FOR MAXIMAL FUNCTIONS ASSOCIATED WITH GENERAL MEASURES L. Grafakos Department of Mathematics, University of Missouri, Columbia, MO 65203, U.S.A. (e-mail: loukas@math.missouri.edu) and
More informationLebesgue Measure on R n
8 CHAPTER 2 Lebesgue Measure on R n Our goal is to construct a notion of the volume, or Lebesgue measure, of rather general subsets of R n that reduces to the usual volume of elementary geometrical sets
More informationLebesgue Integration: A non-rigorous introduction. What is wrong with Riemann integration?
Lebesgue Integration: A non-rigorous introduction What is wrong with Riemann integration? xample. Let f(x) = { 0 for x Q 1 for x / Q. The upper integral is 1, while the lower integral is 0. Yet, the function
More informationTHEOREMS, ETC., FOR MATH 515
THEOREMS, ETC., FOR MATH 515 Proposition 1 (=comment on page 17). If A is an algebra, then any finite union or finite intersection of sets in A is also in A. Proposition 2 (=Proposition 1.1). For every
More informationIntegration on Measure Spaces
Chapter 3 Integration on Measure Spaces In this chapter we introduce the general notion of a measure on a space X, define the class of measurable functions, and define the integral, first on a class of
More informationx 0 + f(x), exist as extended real numbers. Show that f is upper semicontinuous This shows ( ɛ, ɛ) B α. Thus
Homework 3 Solutions, Real Analysis I, Fall, 2010. (9) Let f : (, ) [, ] be a function whose restriction to (, 0) (0, ) is continuous. Assume the one-sided limits p = lim x 0 f(x), q = lim x 0 + f(x) exist
More informationANALYSIS OF FUNCTIONS (D COURSE - PART II MATHEMATICAL TRIPOS)
ANALYSIS OF FUNCTIONS (D COURS - PART II MATHMATICAL TRIPOS) CLÉMNT MOUHOT - C.MOUHOT@DPMMS.CAM.AC.UK Contents. Integration of functions 2. Vector spaces of functions 9 3. Fourier decomposition of functions
More informationLebesgue Integration on R n
Lebesgue Integration on R n The treatment here is based loosely on that of Jones, Lebesgue Integration on Euclidean Space We give an overview from the perspective of a user of the theory Riemann integration
More informationThe Lebesgue Integral
The Lebesgue Integral Having completed our study of Lebesgue measure, we are now ready to consider the Lebesgue integral. Before diving into the details of its construction, though, we would like to give
More informationAnalysis Comprehensive Exam Questions Fall F(x) = 1 x. f(t)dt. t 1 2. tf 2 (t)dt. and g(t, x) = 2 t. 2 t
Analysis Comprehensive Exam Questions Fall 2. Let f L 2 (, ) be given. (a) Prove that ( x 2 f(t) dt) 2 x x t f(t) 2 dt. (b) Given part (a), prove that F L 2 (, ) 2 f L 2 (, ), where F(x) = x (a) Using
More information(2) E M = E C = X\E M
10 RICHARD B. MELROSE 2. Measures and σ-algebras An outer measure such as µ is a rather crude object since, even if the A i are disjoint, there is generally strict inequality in (1.14). It turns out to
More informationLecture Notes in Advanced Calculus 1 (80315) Raz Kupferman Institute of Mathematics The Hebrew University
Lecture Notes in Advanced Calculus 1 (80315) Raz Kupferman Institute of Mathematics The Hebrew University February 7, 2007 2 Contents 1 Metric Spaces 1 1.1 Basic definitions...........................
More informationRecall that if X is a compact metric space, C(X), the space of continuous (real-valued) functions on X, is a Banach space with the norm
Chapter 13 Radon Measures Recall that if X is a compact metric space, C(X), the space of continuous (real-valued) functions on X, is a Banach space with the norm (13.1) f = sup x X f(x). We want to identify
More informationChapter 6. Integration. 1. Integrals of Nonnegative Functions. a j µ(e j ) (ca j )µ(e j ) = c X. and ψ =
Chapter 6. Integration 1. Integrals of Nonnegative Functions Let (, S, µ) be a measure space. We denote by L + the set of all measurable functions from to [0, ]. Let φ be a simple function in L +. Suppose
More informationSingular Integrals. 1 Calderon-Zygmund decomposition
Singular Integrals Analysis III Calderon-Zygmund decomposition Let f be an integrable function f dx 0, f = g + b with g Cα almost everywhere, with b
More informationconsists of two disjoint copies of X n, each scaled down by 1,
Homework 4 Solutions, Real Analysis I, Fall, 200. (4) Let be a topological space and M be a σ-algebra on which contains all Borel sets. Let m, µ be two positive measures on M. Assume there is a constant
More informationg 2 (x) (1/3)M 1 = (1/3)(2/3)M.
COMPACTNESS If C R n is closed and bounded, then by B-W it is sequentially compact: any sequence of points in C has a subsequence converging to a point in C Conversely, any sequentially compact C R n is
More information1 Topology Definition of a topology Basis (Base) of a topology The subspace topology & the product topology on X Y 3
Index Page 1 Topology 2 1.1 Definition of a topology 2 1.2 Basis (Base) of a topology 2 1.3 The subspace topology & the product topology on X Y 3 1.4 Basic topology concepts: limit points, closed sets,
More informationMeasurable functions are approximately nice, even if look terrible.
Tel Aviv University, 2015 Functions of real variables 74 7 Approximation 7a A terrible integrable function........... 74 7b Approximation of sets................ 76 7c Approximation of functions............
More informationMATH MEASURE THEORY AND FOURIER ANALYSIS. Contents
MATH 3969 - MEASURE THEORY AND FOURIER ANALYSIS ANDREW TULLOCH Contents 1. Measure Theory 2 1.1. Properties of Measures 3 1.2. Constructing σ-algebras and measures 3 1.3. Properties of the Lebesgue measure
More informationRegularity for Poisson Equation
Regularity for Poisson Equation OcMountain Daylight Time. 4, 20 Intuitively, the solution u to the Poisson equation u= f () should have better regularity than the right hand side f. In particular one expects
More informationIntroduction to Real Analysis Alternative Chapter 1
Christopher Heil Introduction to Real Analysis Alternative Chapter 1 A Primer on Norms and Banach Spaces Last Updated: March 10, 2018 c 2018 by Christopher Heil Chapter 1 A Primer on Norms and Banach Spaces
More information02. Measure and integral. 1. Borel-measurable functions and pointwise limits
(October 3, 2017) 02. Measure and integral Paul Garrett garrett@math.umn.edu http://www.math.umn.edu/ garrett/ [This document is http://www.math.umn.edu/ garrett/m/real/notes 2017-18/02 measure and integral.pdf]
More informationDuality of multiparameter Hardy spaces H p on spaces of homogeneous type
Duality of multiparameter Hardy spaces H p on spaces of homogeneous type Yongsheng Han, Ji Li, and Guozhen Lu Department of Mathematics Vanderbilt University Nashville, TN Internet Analysis Seminar 2012
More informationHerz (cf. [H], and also [BS]) proved that the reverse inequality is also true, that is,
REARRANGEMENT OF HARDY-LITTLEWOOD MAXIMAL FUNCTIONS IN LORENTZ SPACES. Jesús Bastero*, Mario Milman and Francisco J. Ruiz** Abstract. For the classical Hardy-Littlewood maximal function M f, a well known
More informationProbability and Measure
Part II Year 2018 2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2018 84 Paper 4, Section II 26J Let (X, A) be a measurable space. Let T : X X be a measurable map, and µ a probability
More informationTHEOREMS, ETC., FOR MATH 516
THEOREMS, ETC., FOR MATH 516 Results labeled Theorem Ea.b.c (or Proposition Ea.b.c, etc.) refer to Theorem c from section a.b of Evans book (Partial Differential Equations). Proposition 1 (=Proposition
More informationMath212a1413 The Lebesgue integral.
Math212a1413 The Lebesgue integral. October 28, 2014 Simple functions. In what follows, (X, F, m) is a space with a σ-field of sets, and m a measure on F. The purpose of today s lecture is to develop the
More informationHIGHER INTEGRABILITY WITH WEIGHTS
Annales Academiæ Scientiarum Fennicæ Series A. I. Mathematica Volumen 19, 1994, 355 366 HIGHER INTEGRABILITY WITH WEIGHTS Juha Kinnunen University of Jyväskylä, Department of Mathematics P.O. Box 35, SF-4351
More informationMeasure and integration
Chapter 5 Measure and integration In calculus you have learned how to calculate the size of different kinds of sets: the length of a curve, the area of a region or a surface, the volume or mass of a solid.
More informationHomework 11. Solutions
Homework 11. Solutions Problem 2.3.2. Let f n : R R be 1/n times the characteristic function of the interval (0, n). Show that f n 0 uniformly and f n µ L = 1. Why isn t it a counterexample to the Lebesgue
More informationReminder Notes for the Course on Measures on Topological Spaces
Reminder Notes for the Course on Measures on Topological Spaces T. C. Dorlas Dublin Institute for Advanced Studies School of Theoretical Physics 10 Burlington Road, Dublin 4, Ireland. Email: dorlas@stp.dias.ie
More informationREAL AND COMPLEX ANALYSIS
REAL AND COMPLE ANALYSIS Third Edition Walter Rudin Professor of Mathematics University of Wisconsin, Madison Version 1.1 No rights reserved. Any part of this work can be reproduced or transmitted in any
More information(U) =, if 0 U, 1 U, (U) = X, if 0 U, and 1 U. (U) = E, if 0 U, but 1 U. (U) = X \ E if 0 U, but 1 U. n=1 A n, then A M.
1. Abstract Integration The main reference for this section is Rudin s Real and Complex Analysis. The purpose of developing an abstract theory of integration is to emphasize the difference between the
More informationCourse 212: Academic Year Section 1: Metric Spaces
Course 212: Academic Year 1991-2 Section 1: Metric Spaces D. R. Wilkins Contents 1 Metric Spaces 3 1.1 Distance Functions and Metric Spaces............. 3 1.2 Convergence and Continuity in Metric Spaces.........
More informationMATH41011/MATH61011: FOURIER SERIES AND LEBESGUE INTEGRATION. Extra Reading Material for Level 4 and Level 6
MATH41011/MATH61011: FOURIER SERIES AND LEBESGUE INTEGRATION Extra Reading Material for Level 4 and Level 6 Part A: Construction of Lebesgue Measure The first part the extra material consists of the construction
More informationReview of measure theory
209: Honors nalysis in R n Review of measure theory 1 Outer measure, measure, measurable sets Definition 1 Let X be a set. nonempty family R of subsets of X is a ring if, B R B R and, B R B R hold. bove,
More information(b) If f L p (R), with 1 < p, then Mf L p (R) and. Mf L p (R) C(p) f L p (R) with C(p) depending only on p.
Lecture 3: Carleson Measures via Harmonic Analysis Much of the argument from this section is taken from the book by Garnett, []. The interested reader can also see variants of this argument in the book
More informationRemarks on the Gauss-Green Theorem. Michael Taylor
Remarks on the Gauss-Green Theorem Michael Taylor Abstract. These notes cover material related to the Gauss-Green theorem that was developed for work with S. Hofmann and M. Mitrea, which appeared in [HMT].
More informationMeasures and Measure Spaces
Chapter 2 Measures and Measure Spaces In summarizing the flaws of the Riemann integral we can focus on two main points: 1) Many nice functions are not Riemann integrable. 2) The Riemann integral does not
More informationA List of Problems in Real Analysis
A List of Problems in Real Analysis W.Yessen & T.Ma December 3, 218 This document was first created by Will Yessen, who was a graduate student at UCI. Timmy Ma, who was also a graduate student at UCI,
More information8 Singular Integral Operators and L p -Regularity Theory
8 Singular Integral Operators and L p -Regularity Theory 8. Motivation See hand-written notes! 8.2 Mikhlin Multiplier Theorem Recall that the Fourier transformation F and the inverse Fourier transformation
More informationL p Spaces and Convexity
L p Spaces and Convexity These notes largely follow the treatments in Royden, Real Analysis, and Rudin, Real & Complex Analysis. 1. Convex functions Let I R be an interval. For I open, we say a function
More informationMATH6081A Homework 8. In addition, when 1 < p 2 the above inequality can be refined using Lorentz spaces: f
MATH68A Homework 8. Prove the Hausdorff-Young inequality, namely f f L L p p for all f L p (R n and all p 2. In addition, when < p 2 the above inequality can be refined using Lorentz spaces: f L p,p f
More informationReview of Multi-Calculus (Study Guide for Spivak s CHAPTER ONE TO THREE)
Review of Multi-Calculus (Study Guide for Spivak s CHPTER ONE TO THREE) This material is for June 9 to 16 (Monday to Monday) Chapter I: Functions on R n Dot product and norm for vectors in R n : Let X
More informationREVIEW OF ESSENTIAL MATH 346 TOPICS
REVIEW OF ESSENTIAL MATH 346 TOPICS 1. AXIOMATIC STRUCTURE OF R Doğan Çömez The real number system is a complete ordered field, i.e., it is a set R which is endowed with addition and multiplication operations
More informationReal Analysis Math 131AH Rudin, Chapter #1. Dominique Abdi
Real Analysis Math 3AH Rudin, Chapter # Dominique Abdi.. If r is rational (r 0) and x is irrational, prove that r + x and rx are irrational. Solution. Assume the contrary, that r+x and rx are rational.
More informationWEIGHTED REVERSE WEAK TYPE INEQUALITY FOR GENERAL MAXIMAL FUNCTIONS
GEORGIAN MATHEMATICAL JOURNAL: Vol. 2, No. 3, 995, 277-290 WEIGHTED REVERSE WEAK TYPE INEQUALITY FOR GENERAL MAXIMAL FUNCTIONS J. GENEBASHVILI Abstract. Necessary and sufficient conditions are found to
More informationU e = E (U\E) e E e + U\E e. (1.6)
12 1 Lebesgue Measure 1.2 Lebesgue Measure In Section 1.1 we defined the exterior Lebesgue measure of every subset of R d. Unfortunately, a major disadvantage of exterior measure is that it does not satisfy
More informationSelçuk Demir WS 2017 Functional Analysis Homework Sheet
Selçuk Demir WS 2017 Functional Analysis Homework Sheet 1. Let M be a metric space. If A M is non-empty, we say that A is bounded iff diam(a) = sup{d(x, y) : x.y A} exists. Show that A is bounded iff there
More informationThe Caratheodory Construction of Measures
Chapter 5 The Caratheodory Construction of Measures Recall how our construction of Lebesgue measure in Chapter 2 proceeded from an initial notion of the size of a very restricted class of subsets of R,
More informationProduct measures, Tonelli s and Fubini s theorems For use in MAT4410, autumn 2017 Nadia S. Larsen. 17 November 2017.
Product measures, Tonelli s and Fubini s theorems For use in MAT4410, autumn 017 Nadia S. Larsen 17 November 017. 1. Construction of the product measure The purpose of these notes is to prove the main
More informationLebesgue measure and integration
Chapter 4 Lebesgue measure and integration If you look back at what you have learned in your earlier mathematics courses, you will definitely recall a lot about area and volume from the simple formulas
More informationA note on some approximation theorems in measure theory
A note on some approximation theorems in measure theory S. Kesavan and M. T. Nair Department of Mathematics, Indian Institute of Technology, Madras, Chennai - 600 06 email: kesh@iitm.ac.in and mtnair@iitm.ac.in
More informationHomework 1 Real Analysis
Homework 1 Real Analysis Joshua Ruiter March 23, 2018 Note on notation: When I use the symbol, it does not imply that the subset is proper. In writing A X, I mean only that a A = a X, leaving open the
More informationConnectedness. Proposition 2.2. The following are equivalent for a topological space (X, T ).
Connectedness 1 Motivation Connectedness is the sort of topological property that students love. Its definition is intuitive and easy to understand, and it is a powerful tool in proofs of well-known results.
More informationRudin Real and Complex Analysis - Harmonic Functions
Rudin Real and Complex Analysis - Harmonic Functions Aaron Lou December 2018 1 Notes 1.1 The Cauchy-Riemann Equations 11.1: The Operators and Suppose f is a complex function defined in a plane open set
More information18.175: Lecture 2 Extension theorems, random variables, distributions
18.175: Lecture 2 Extension theorems, random variables, distributions Scott Sheffield MIT Outline Extension theorems Characterizing measures on R d Random variables Outline Extension theorems Characterizing
More informationProblem set 1, Real Analysis I, Spring, 2015.
Problem set 1, Real Analysis I, Spring, 015. (1) Let f n : D R be a sequence of functions with domain D R n. Recall that f n f uniformly if and only if for all ɛ > 0, there is an N = N(ɛ) so that if n
More informationMAT 570 REAL ANALYSIS LECTURE NOTES. Contents. 1. Sets Functions Countability Axiom of choice Equivalence relations 9
MAT 570 REAL ANALYSIS LECTURE NOTES PROFESSOR: JOHN QUIGG SEMESTER: FALL 204 Contents. Sets 2 2. Functions 5 3. Countability 7 4. Axiom of choice 8 5. Equivalence relations 9 6. Real numbers 9 7. Extended
More informationConstruction of a general measure structure
Chapter 4 Construction of a general measure structure We turn to the development of general measure theory. The ingredients are a set describing the universe of points, a class of measurable subsets along
More informationvan Rooij, Schikhof: A Second Course on Real Functions
vanrooijschikhof.tex April 25, 2018 van Rooij, Schikhof: A Second Course on Real Functions Notes from [vrs]. Introduction A monotone function is Riemann integrable. A continuous function is Riemann integrable.
More informationMathematics for Economists
Mathematics for Economists Victor Filipe Sao Paulo School of Economics FGV Metric Spaces: Basic Definitions Victor Filipe (EESP/FGV) Mathematics for Economists Jan.-Feb. 2017 1 / 34 Definitions and Examples
More informationFUNDAMENTALS OF REAL ANALYSIS by. IV.1. Differentiation of Monotonic Functions
FUNDAMNTALS OF RAL ANALYSIS by Doğan Çömez IV. DIFFRNTIATION AND SIGND MASURS IV.1. Differentiation of Monotonic Functions Question: Can we calculate f easily? More eplicitly, can we hope for a result
More informationMaths 212: Homework Solutions
Maths 212: Homework Solutions 1. The definition of A ensures that x π for all x A, so π is an upper bound of A. To show it is the least upper bound, suppose x < π and consider two cases. If x < 1, then
More informationMAT1000 ASSIGNMENT 1. a k 3 k. x =
MAT1000 ASSIGNMENT 1 VITALY KUZNETSOV Question 1 (Exercise 2 on page 37). Tne Cantor set C can also be described in terms of ternary expansions. (a) Every number in [0, 1] has a ternary expansion x = a
More informationTHE INVERSE FUNCTION THEOREM
THE INVERSE FUNCTION THEOREM W. PATRICK HOOPER The implicit function theorem is the following result: Theorem 1. Let f be a C 1 function from a neighborhood of a point a R n into R n. Suppose A = Df(a)
More informationPart V. 17 Introduction: What are measures and why measurable sets. Lebesgue Integration Theory
Part V 7 Introduction: What are measures and why measurable sets Lebesgue Integration Theory Definition 7. (Preliminary). A measure on a set is a function :2 [ ] such that. () = 2. If { } = is a finite
More information2 Lebesgue integration
2 Lebesgue integration 1. Let (, A, µ) be a measure space. We will always assume that µ is complete, otherwise we first take its completion. The example to have in mind is the Lebesgue measure on R n,
More informationThe reference [Ho17] refers to the course lecture notes by Ilkka Holopainen.
Department of Mathematics and Statistics Real Analysis I, Fall 207 Solutions to Exercise 6 (6 pages) riikka.schroderus at helsinki.fi Note. The course can be passed by an exam. The first possible exam
More informationBoth these computations follow immediately (and trivially) from the definitions. Finally, observe that if f L (R n ) then we have that.
Lecture : One Parameter Maximal Functions and Covering Lemmas In this first lecture we start studying one of the basic and fundamental operators in harmonic analysis, the Hardy-Littlewood maximal function.
More informationIntegral Jensen inequality
Integral Jensen inequality Let us consider a convex set R d, and a convex function f : (, + ]. For any x,..., x n and λ,..., λ n with n λ i =, we have () f( n λ ix i ) n λ if(x i ). For a R d, let δ a
More informationDaniel Akech Thiong Math 501: Real Analysis Homework Problems with Solutions Fall Problem. 1 Give an example of a mapping f : X Y such that
Daniel Akech Thiong Math 501: Real Analysis Homework Problems with Solutions Fall 2014 Problem. 1 Give an example of a mapping f : X Y such that 1. f(a B) = f(a) f(b) for some A, B X. 2. f(f 1 (A)) A for
More informationTools from Lebesgue integration
Tools from Lebesgue integration E.P. van den Ban Fall 2005 Introduction In these notes we describe some of the basic tools from the theory of Lebesgue integration. Definitions and results will be given
More informationMath 720: Homework. Assignment 2: Assigned Wed 09/04. Due Wed 09/11. Assignment 1: Assigned Wed 08/28. Due Wed 09/04
Math 720: Homework Do, but don t turn in optional problems There is a firm no late homework policy Assignment : Assigned Wed 08/28 Due Wed 09/04 Keep in mind there is a firm no late homework policy Starred
More informationReal Analysis II, Winter 2018
Real Analysis II, Winter 2018 From the Finnish original Moderni reaalianalyysi 1 by Ilkka Holopainen adapted by Tuomas Hytönen January 18, 2018 1 Version dated September 14, 2011 Contents 1 General theory
More informationSummary of Real Analysis by Royden
Summary of Real Analysis by Royden Dan Hathaway May 2010 This document is a summary of the theorems and definitions and theorems from Part 1 of the book Real Analysis by Royden. In some areas, such as
More information