Functional Analysis

Download A (terse) introduction to Lebesgue integration by John Franks PDF

By John Franks

This booklet presents a student's first come across with the techniques of degree concept and useful research. Its constitution and content material replicate the assumption that tricky recommendations might be brought of their easiest and so much concrete kinds. regardless of using the notice ``terse'' within the identify, this article may additionally were referred to as A (Gentle) advent to Lebesgue Integration. it's terse within the experience that it treats just a subset of these strategies commonly present in a considerable graduate-level research path. The booklet emphasizes the incentive of those thoughts and makes an attempt to regard them easily and concretely. particularly, little point out is made from normal measures except Lebesgue till the ultimate bankruptcy and a focus is proscribed to $R$ instead of $R^n$. After developing the first principles and effects, the textual content strikes directly to a few functions. bankruptcy 6 discusses classical genuine and intricate Fourier sequence for $L^2$ capabilities at the period and indicates that the Fourier sequence of an $L^2$ functionality converges in $L^2$ to that functionality. bankruptcy 7 introduces a few recommendations from measurable dynamics. The Birkhoff ergodic theorem is said with out facts and effects on Fourier sequence from bankruptcy 6 are used to turn out that an irrational rotation of the circle is ergodic and that the squaring map at the advanced numbers of modulus 1 is ergodic. This booklet is acceptable for a complicated undergraduate direction or for the beginning of a graduate direction. The textual content presupposes that the coed has had a regular undergraduate direction in actual research

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Show there exists a family {fn }n∈N of step functions such that for each n and each x ∈ [a, b] the value of fn (x) is either 0 or 1 and yet the function defined by g(x) = max{fn (x)}n∈N is not Riemann integrable. (4) Prove that if f and g are bounded Riemann integrable functions on an interval [a, b], then so is f g. In particular, if r ∈ R, then rf is a bounded Riemann integrable function on [a, b]. 1. Introduction In the previous section we studied two definitions of integration that were based on two important facts: (1) There is only one obvious way to define the integral of step functions assuming we want it to satisfy certain basic properties, and (2) these properties force the definition for the integral for more general functions which are uniformly approximated by step functions (regulated integral) or squeezed between step functions whose integrals are arbitrarily close (Riemann integral).

3. A bounded function f : [a, b] → R is Riemann integrable if and only if, for every ε > 0 there are step functions v0 and u0 such that v0 (x) ≤ f (x) ≤ u0 (x) for all x ∈ [a, b] and b a b u0 (x) dx − v0 (x) dx ≤ ε. a Proof. Suppose the functions v0 ∈ L(f ) and u0 ∈ U(f ) have integrals within ε of each other. 1. This implies b u∈U(f ) b u(x) dx − sup inf v∈L(f ) a a v(x) dx ≤ ε. Since this is true for all ε > 0, we conclude that f is Riemann integrable. Conversely, if f is Riemann integrable, then from the properties of the infimum there exists a step function u0 ∈ U(f ) such that b a b u0 (x) dx < u(x) dx + inf u∈U(f ) Thus, a b b u0 (x) dx − a ε = 2 f (x) dx < a b a ε f (x) dx + .

N=1 • Regularity: If A ∈ M(I), then µ(A) = inf{µ(U ) | U is open and A ⊂ U }. • Increasing sequences: If An ∈ M(I) for n ∈ N satisfy An ⊂ An+1 , then ∞ µ An = lim µ(An ). n=1 n→∞ • Decreasing sequences: If An ∈ M(I) for n ∈ N satisfy An ⊃ An+1 , then ∞ An = lim µ(An ). 1. (The Cantor middle third set). Recursively define a nested sequence {Jn }∞ n=0 of closed subsets of I = [0, 1]. Each Jn consists of a finite union of closed intervals. We define J0 to be I and let Jn be the union of the closed intervals obtained by deleting the open middle third interval from each of the intervals in Jn−1 .

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