2 (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?

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1 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 a rational number. Is A measurable? What is its Lebesgue measure? 3 (Due 9/5). Prove that every open set A is measurable (hint: represent it by a countable union of rectangles). Prove that every closed set A is measurable. 4 (Due 9/14). Let L denote the set of all Lebesgue measurable sets (in R). Prove that card(l) > C. 5 (Due 9/14). Let U denote the set of all open subsets U R. Prove that card(u) = C. Do the same for open sets in R 2. 6 (Due 9/14). Let = {1, 2, 3}. Construct all σ-algebras of. 7 (Due 9/14). Let = [0, 1] and G consist of all one-point sets, i.e. G = { {x}, x }. Describe the σ-algebra M(G). 8 (Due 9/14). Show that every Borel set in R is Lebesgue measurable, but not vice versa. 9 (Bonus). Does there exist an infinite σ-algebra which has only countably many members? 10 (Due 9/14). Show that the assumption µ(a 1 ) < in Theorem 2.10 is indispensable. Hint: consider the counting measure on N and take sets A n = {n, n + 1,...}. 11 (Due 9/14). Let R 2 be a rectangle. Verify that the collection of all subrectangles R is a semi-algebra. 12 (Due 9/21). Prove that A M if and only if χ A is measurable. 13 (Due 9/21). Let (, M) be a measurable space and f : [, ]. Prove that f is measurable iff f 1( [, x] ) is a measurable set for every x R. 1

2 14 (Due 9/21). Let (, M) be a measurable space and f : [, ]. Prove that f is measurable iff f 1( [, x] ) is a measurable set for every rational x Q. 15 (Bonus). Let (, M) be a measurable space and f : [, ] and g : [, ] two measurable functions. Prove that the sets are measurable. {x: f(x) < g(x)} and {x: f(x) = g(x)} 16 (Due 9/26). Show that the function f : R R defined by { x if x Q f(x) = 0 if x / Q is a Borel function. 17 (Due 9/26). Show that the function f : R R defined by { sin(1/x) if x 0 f(x) = 0 if x = 0 is a Borel function. 18 (Bonus). Let f : R R be a monotonically increasing function, i.e. f(x 1 ) f(x 2 ) for x 1 x 2. Show that f is a Borel function. 19 (Due 10/3). Let x 0 and µ = δ x0 the δ-measure. Assume that {x 0 } M. Show that for every measurable function f : [0, ] we have f dµ = f(x 0 ). 20 (Due 10/3). Let = N and µ the counting measure on the σ-algebra M = 2 N. Show that for every function f : [0, ] we have f dµ = f(n). 21 (Due 10/3). Let f : [0, ] and f dµ = 0. Show that µ{x: f(x) 0} = 0. 2 n=1

3 22 (Due 10/3). Let E be such that µ(e) > 0 and µ(e c ) > 0. Put f n = χ E if n is odd and f n = 1 χ E if n is even. What is the relevance of this example to Fatou s lemma? 23 (Due 10/3). Construct an example of a sequence of nonnegative measurable functions f n : [0, ) such that f(x) = lim n f n (x) exists pointwise, but f dµ < lim inf f n dµ. n 24 (Due 10/5). Let f, g L 1 (µ) be real-valued functions and f g. Show that f dµ g dµ. 25 (Due 10/5). Let f n : [0, ] be a sequence of measurable functions such that f 1 f 2 0 and lim n f n (x) = f(x) for every x. Suppose f 1 L 1 (µ). Show that lim n f n dµ = f dµ. 26 (Due 10/10). Let f be a function as above. Fix a y Y and define { f(x) if x \ N f(x) = y if x N. Show that f : Y is measurable. 27 (Due 10/10). Let f be a function as above and µ a complete measure. Show that { f(x) if x \ N f(x) =. any point of Y if x N Show that f : Y is measurable. 28 (Due 10/10). Suppose µ() <. Let a sequence {f n } of bounded complex measurable functions uniformly converge to f on. Prove that lim n f n dµ = f dµ. Show that the assumption µ() < cannot be omitted. 3

4 29 (Due 10/10). Give an example of a sequence of complex measurable functions f n : C (i.e., defined on the entire ) such that f n dµ < n=1 but the series f(x) = n=1 f n diverges for some x. 30 (Due 10/12). Suppose µ() < and f n are measurable functions defined a.e. on. Prove that if f n f a.e. on, then f n f in measure. What happens if µ() =? 31 (Due 10/12). In Theorem 4.26, let A be the set of points which belong to infinitely many of the sets E k. Show that A = n=1 k=n E k. Use this fact to prove the theorem without any reference to integration. Hint: recall that every measure µ is countably subbaditive, i.e. for any E n M (not necessarily disjoint) µ( E n ) µ(e n ). 32 (Bonus). Prove that if f n f in measure, then there is a subsequence {f nk } of {f n } such that f nk f a.e. on. Hint: use Theorem (Bonus). Suppose f L 1 (µ). Prove that ε > 0 δ > 0 such that f dµ < ε whenever µ(e) < δ. E 34 (Due 10/19). Show that the counting measure on R k is not regular. 35 (Due 10/19). Find examples of Lebesgue measurable sets E 1, E 2 R such that µ(e 1 ) < inf{µ(a): E A, A closed} µ(e 2 ) > sup{µ(v ): V E, V open}. 36 (Due 10/19). Extend this theorem to R 2 : show that if µ is a translation invariant Borel measure on R 2 such that µ(r) < for at least one open rectangle R, then there exists a constant c such that µ (E) = cµ(e) for every Borel set E. 37 (Bonus). Let f : R [0, ] be a Borel function, f L 1 (µ), and consider the measure ρ(e) = f dµ, where µ is the Lebesgue measure. Prove that E ρ is regular. Hint: use the result of Exercise 33. 4

5 38 (Due 11/2). Let s: [a, b] R be a simple Lebesgue measurable function. Show that for every ε > 0 there is a step function ϕ: [a, b] R and a Lebesgue measurable set E [a, b] such that s(x) = ϕ(x) on E and µ ( [a, b] \ E ) < ε. Hint: use the regularity of µ. 39 (Due 11/2). Let f : [a, b] R be a Lebesgue measurable function. Show that for every ε > 0 there is a step function g : [a, b] R such that µ { x [a, b]: f(x) g(x) ε } < ε. Hint: first show that f M except for a set of small measure, then use the previous exercise. 40 (Bonus). Let f L 1 with respect to the Lebesgue measure µ on R. Prove that there is a sequence {g n } of step functions such that f g n dµ = 0. lim n Hint: use the previous exercise. R 41 (Due 11/2). Prove that if f : R is upper (lower) semicontinuous and is compact, then f is bounded above (below) and attains its maximum (minimum). 42 (Due 11/9). Find a function f(x) on [0, ) such that the improper Riemannian integral A f(x) dx = lim 0 A f(x) dx exists (is finite), but f is 0 not Lebesgue integrable. 43 (Due 11/14). Prove that the supremum of any collection of convex functions on (a, b) is convex on (a, b) (assume that the supremum is finite). Prove that a pointwise limit of a sequence of convex functions is convex. 44 (Due 11/14). Let ϕ be convex on (a, b) and ψ convex and nondecreasing on the range of ϕ. Prove that ψ ϕ is convex on (a, b). For ϕ > 0, show that the convexity of log ϕ implies the convexity of ϕ, but not vice versa. 45 (Bonus). Assume that ϕ is a continuous real function on (a, b) such that ( x + y ) ϕ ϕ(x) ϕ(y) for all x, y (a, b). Prove that ϕ is convex. (The conclusion does not follow if continuity is omitted from the hypotheses.) 5

6 46 (Due 11/21). Prove that equality in the Minkowski inequality holds if and only if there exist α, β 0, not both equal to 0, such that αf = βg a.e. 47 (Due 11/21). Suppose µ(ω) = 1 and suppose f and g are two positive measurable functions on Ω such that fg 1. Prove that f dµ g dµ 1. Ω Ω 48 (Due 11/21). Suppose µ(ω) = 1 and h: Ω [0, ] is measurable. Denote A = h dµ. Prove that Ω 1 + A2 1 + h2 dµ 1 + A. Ω 49 (Bonus). If µ is Lebesgue measure on [0, 1] and if h is a continuous function on [0, 1] such that h = f, then the inequalities in the previous exercise have a simple geometric interpretation. From this, conjecture (for general Ω) under what conditions on h equality can hold in either of the above inequalities, and prove your conjecture. 50 (Due 11/28). When does one get equality in fg 1 f g 1? 51 (Due 11/28). Suppose f : C is measurable and f > 0. Define ϕ(p) = f p dµ = f p p (0 < p < ) and consider the set E = {p: ϕ(p) < }. Each of the following questions is graded as a separate exercise. Question (c) and (e) are bonus problems. (a) Let r < p < s and r, s E. Prove that p E. (b) Prove that log ϕ is convex in the interior of E and that ϕ is continuous on E. (c) By (a), E is connected. Is E necessarily open? Closed? Can E consist of a single point? Can E be any connected subset of (0, )? (d) If r < p < s, prove that f p max ( f r, f s ). Show that this implies the inclusion L r µ L s µ L p µ. 6

7 (e) Assume that f r < for some r < and prove that f p f as p. 52 (Due 12/5). Let µ() = 1. Each of the following questions is graded as a separate exercise. (a) Prove that f r f s if 0 < r < s. (b) Under what condition does it happen that 0 < r < s and f r = f s <? (c) Prove that L r µ L s µ if 0 < r < s. If = [0, 1] and µ is the Lebesgue measure, show that L r µ L s µ. 53 (Bonus). For some measures, the relation r < s implies L r (µ) L s (µ); for others, the inclusion is reversed; and there are some for which L r (µ) does not contain L s (µ) if r s. Give examples of these situations, and find conditions on µ under which these situations will occur. 54 (Due 12/5). (a) Show that π 2 0 x sin x dx < π 2 ; 2 (b) Show that [ 1 0 x 1 2 (1 x) 1 3 dx ] (Bonus). Suppose 1 < p < and f L p ((0, )) relative to the Lebesgue measure. Define F (x) = 1 x Prove Hardy s inequality x 0 f(t) dt F p p p 1 f p (0 < x < ). which shows that the mapping f F carries L p into L p. [Hint: assume first that f 0 and f C c ((0, )), i.e. the support of f is a finite closed interval [a, b] (0, ). Then use integration by parts: A ε F p (x) dx = p A ε F p 1 xf (x) dx where ε < a and A > b. Note that xf = f F, and apply Hölder inequality to F p 1 f dx.] 7

8 56 (Due 1/16). Let (, M, µ) be a measure space. For f L 1 µ define µ f (E) = f dµ. Show: (a) µ f is a complex measure; (b) µ f = µ f, assuming that f is real-valued; (c) µ f = µ f, now for a general f L 1 (µ). 57 (Bonus). Prove that the space M(, M) with the norm µ is a Banach space, i.e. it is a complete metric space (every Cauchy sequence converges to a limit). Hint: given a Cauchy sequence of complex measures {µ n } you need to construct the limit measure µ and prove that µ n µ 0 as n. 58 (Due 1/25). Let µ be a positive measure on (, M) and f, g L 1 µ. Prove that (a) µ f is concentrated on A if and only if µ{x A c : f(x) 0} = 0; (b) µ f µ g if and only if µ{x : f(x)g(x) 0} = 0; (c) if f 0, then E µ µ f f(x) > 0 for µ a.e. x. 59 (Due 1/25). Let λ be a positive measure on (, M). Prove that λ is concentrated on A if and only if λ(a c ) = 0. Give a counterexample to this statement in the case of a complex measure λ. 60 (Due 2/20). Let f be a real-valued and integrable function on [0, 1]. Prove that there exists c [0, 1] such that f dm = f dm. [0,c] 61 (Due 2/20). Let f, f k L 1 m[0, 1], for k = 1, 2,.... Suppose f k (x) f(x) a.e. on [0, 1], and f [0,1] k dm f dm. Show that [0,1] f k f dm 0. [0,1] Hint: look at f + f k f f k. 8 [c,1]

9 62 (Due 2/20). Do the previous exercise with L 1 replaced by L (Due 2/20). Let = [0, 1] and µ the Lebesgue measure. Show that L () L 1 (), but L () L 1 () (in the sense g Φ g ). (Hint: Use the following consequence of the Hahn-Banach theorem: If is a Banach space and A is a closed subspace of, with A, then there exists f with f 0, and f(x) = 0 for all x A.) 64 (Due 2/20). Suppose µ is a positive measure on and µ() <. Let f L µ and f > 0. Define α n = f n dµ (n = 1, 2, 3,... ). Prove that α n+1 lim = f. n α n (Hint: Start with f = 1, use Hölder inequality and the result of Exercise 51 (e).) 65 (Due 3/20). Let µ be a complex Borel measure on R and assume that its symmetric derivative (Dµ)(x) exists at some x 0 R. Does it follow that its distribution function F (x) = µ((, x)) is differentiable at x 0? 66 (Due 3/20). Let f L 1 m(r k ). Show that f(x) (Mf)(x) at every Lebesgue point x of f. 67 (Due 3/20). Let f(x) = { 0 if x < 0 or x > 1 1 if 0 < x < 1. Is it possible to define f(0) and f(1) such that 0 and 1 become Lebesgue points of f? 68 (Bonus). Construct a function f : R R such that f(0) = 0 and 0 is a Lebesgue point of f, but for every ε > 0 m { x R: x < ε and f(x) 1 } > 0, i.e. f is essentially discontinuous at 0. 9

10 69 (Due 3/20). Let α > 0, β > 0 and { x f(x) = α cos π for 0 < x 1 x β 0 for x = 0 Show that if α > 1 and α > β, then f is absolutely continuous on [0, 1]. What about α β? 70 (Due 3/20). Let f, g be absolutely continuous on [a, b]. Show that (a) fg is absolutely continuous on [a, b]. (b) if f(x) 0 for all x [a, b], then 1/f is absolutely continuous on [a, b]. (c) Does (a) remain true if [a, b] is replaced by R? 71 (Due 3/27). A function f : [a, b] C is said to be Lipschitz continuous if L > 0 such that f(x) f(y) L x y for all x, y [a, b]. Prove that if f is Lipschitz continuous, then f is absolutely continuous and f L a.e. Conversely, if f is absolutely continuous and f L a.e., then f is Lipschitz continuous (with that constant L). 72 (Due 3/27). Let f : [a, b] R be absolutely continuous. Prove that f dm. For an extra credit: is the equality always true? [a,x] V x a 73 (Due 3/27). Let f : [0, 1] R be absolutely continuous on [δ, 1] for each δ > 0 and continuous and of bounded variation on [0, 1]. Prove that f is absolutely continuous on [0, 1]. [Hint: use the result of the previous exercise to verify that f L 1 ([0, 1]).] 74 (Bonus). Let f : [a, b] C be absolutely continuous. Prove that f is absolutely continuous and d dx f(x) f (x) for a.e. x [a, b]. [Hint: first, use triangle inequality in the form p q p q, for complex numbers p, q, to show that f is AC. Then prove the above inequality for x [a, b] that are Lebesgue points for both f and f ; use Theorem ] 75 (Due 4/10). Given measure spaces (, M, µ) and (Y, N, λ) with σ-finite measures, show that µ λ is the unique measure on M N such that (µ λ)(a B) = µ(a)λ(b) for all measurable rectangles A B in Y. 10

11 76 (Due 4/10). Let a n 0 for n = 1, 2,..., and for t 0 let Prove that N(t) = #{n : a n > t} a n = n=1 0 N(t) dt For an extra credit, find a formula for n=1 φ(a n) in terms of N(t) as defined above, if φ : [0, ) [0, ) is locally absolutely continuous (i.e., AC on every finite interval), non-decreasing, and φ(0) = 0 (Hint: use the above fact with φ(x) = x). 77 (Due 4/10). Let f L 1 ([0, 1]) and f 0. Show that 1 0 f(y) dy x y 1/2 is finite for a.e. x [0, 1] and integrable. 78 (Due 4/10). Use Fubini s theorem and the relation to prove that 1 x = 0 e xt dt for x > 0 A sin x lim A 0 x dx = π 2 79 (Due 4/17). Suppose f L 1 (R) and g L p (R) for some 1 p. Show that f g exists at a.e. x R and f g L p (R), and prove that f g p f 1 g p. Hints: the case p = is simple and can be treated separately. If p <, then use Hölder inequality and argue as in the proof of the previous theorem (see the classnotes). 80 (Due 4/17). Let f L 1 (R) and g(x) = f(y) e (x y)2 dy. R Show that g L p (R), for all 1 p, and estimate g p in terms of f 1. You can use the following standard fact: e x2 dx = π. 11

12 81 (Bonus). Let E = [1, ) and f L 2 m(e). Also assume that f 0 a.e. and define g(x) = f(y) e xy dy. Show that g L 1 (E) and E g 1 c f 2 for some c < 1. Estimate the minimal value of c the best you can. 82 (Due 4/24). Let f : R R be Lebesgue measurable. Prove that (a) A = {(x, y) R 2 y < f(x)} is Lebesgue measurable (in the twodimensional sense) (b) Let f 0. Is it always true that f dm equals the Lebesgue measure R of A? (c) {(x, y) R 2 y = f(x)} is a null set. 83 (Due 4/24). Let f : R 2 R be such that f x is Borel-measurable for every x R and f y is continuous for every y R. Prove that f is Borel-measurable. (See hint on p. 176 in Rudin.) 12

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