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Ralph Chill Harmonic analysis – in Banach spaces – January 15, 2014 c by R. Chill Contents 1 Bochner-Lebesgue and Bochner-Sobolev spaces ................ 1 1.1 The Bochner integral . .1 1.2 Bochner-Lebesgue spaces . .5 1.3 The convolution . .7 1.4 Bochner-Sobolev spaces . 11 2 The Fourier transform ......................................... 15 2.1 The Fourier transform in L1 ................................ 15 2.2 The Fourier transform on L2 ............................... 21 2.3 The Fourier transform on ................................ 22 S 2.4 The Fourier transform on 0 ............................... 24 2.5 Elliptic and parabolic equationsS in RN ...................... 29 2.6 The Marcinkiewicz multiplier theorem . 29 3 Singular integrals ............................................. 31 3.1 The Marcinkiewicz interpolation theorem . 31 3.2 The Hardy-Littlewood maximal operator . 34 3.3 The Rubio de Francia extrapolation theorem . 44 3.4 Calderon-Zygmund operators . 51 3.5 The Hilbert transform . 58 3.6 Covering and decomposition . 63 References ........................................................ 65 Index ............................................................. 67 v Chapter 1 Bochner-Lebesgue and Bochner-Sobolev spaces 1.1 The Bochner integral Let X and Y be Banach spaces, and let (Ω, ;µ) be a measure space. A A function f : Ω X is called step function, if there exists a sequence (An) ! ⊆ of mutually disjoint measurable sets and a sequence (xn) X such that A P ⊆ f = 1A xn. A function f : Ω X is called mesurable, if there exists a n n ! sequence ( fn) of step functions fn : Ω X such that fn f pointwise µ- almost everywhere. ! ! Remark 1.1. Note that there may be a difference to the definition of mesura- bility of scalar valued functions. Measurability of a function is here depend- ing on the measure µ. However, if the measure space (Ω, ;µ) is complete in the sense that µ(A) = 0 and B A implies B , then theA above definition of measurability and the classical⊆ definition of2 measurability A coincide. Note that one may always consider complete measure spaces. Lemma 1.2. If f : Ω X is measurable, then f : Ω R is measurable. More generally, if f : Ω X! is measurable and if g :kXk Y! is continuous, then g f : Ω Y is measurable.! ! ◦ ! Proof. This is an easy consequence of the definition of measurability and the continuity of g. Note that in particular the norm : X R is continuous. k · k ! Lemma 1.3. If f : Ω X and g : Ω K are measurable, then f g : Ω X is ! ! ! measurable. Similarly, if f : Ω X and g : Ω X0 are measurable, then g; f X0;X : Ω K is measurable. ! ! h i ! Proof. For the proof it suffices to use the definition of measurability and to show that the (duality) product of two step functions is again a step function. This is, however, straightforward. Theorem 1.4 (Pettis). A function f : Ω X is measurable if and only if x0; f is measurable for every x X (we say that! f is weakly measurable) andh if therei 0 2 0 1 2 1 Bochner-Lebesgue and Bochner-Sobolev spaces exists a µ-null set N such that f (Ω N) is separable (we say that f is almost separably valued). 2 A n For the following proof of Pettis’ theorem, see Hille &Phillips [Hille and Phillips (1957)]. Proof. Sufficiency. Assume that f is measurable. Then f is weakly measurable by Lemma 1.2. Moreover, by definition, there exists a sequence ( fn) of test functions and a µ-null set N such that 2 A fn(t) f (t) for all t Ω N: ! 2 n Hence, [ f (Ω N) fn(Ω): n ⊆ n Since for every step function fn the range is countable, the set on the right- hand side of this inclusion is separable, and hence f is almost separably valued. Necessity. Assume that f is weakly measurable and almost separably val- ued. We first show that f is measurable. By assumption, there exists a k k µ-null set and a sequence (xn) in X such that D := xn : n N is dense in f (Ω N). By the Hahn-Banach theorem, there existsf a sequence2 g (x ) in X n n0 ∗ such that xn0 = 1 and xn0 ;xn = xn . Since f is weakly measurable, xn0 ; f is measurablek k for everyhn. Asi a consequence,k k sup x ; f is measurable.jh Butij n jh n0 ij supn xn0 ; f = f on Ω N by the choice of the sequence (xn0 ) and the density of D jhin theij f (Ωk k N). Sincen our measure space (Ω, ;µ) is supposed to be complete, we obtainn that f is measurable. In a similarA way, one shows that k k f x is measurable for every x X, and in particular for x = xn. k −Nowk fix m N and define 2 2 Am1 := f x1 inf f xk ; fk − k ≤ 1 k m k − kg ≤ ≤ Am2 := f x2 inf f xk Am1; fk − k ≤ 1 k m k − kg n ≤ ≤ Am3 := f x3 inf f xk (Am1 Am2); fk − k ≤ 1 k m k − kg n [ ≤ ≤ : : : : m 1 [− Amm := f xm inf f xk ( Amk): fk − k ≤ 1 k m k − kg n ≤ ≤ k=1 Then (Amn)1 n m is a family of measurable, mutually disjoint sets such that Sm ≤ ≤ 1 n=1 Amn = Ω. Define 1 We are grateful to Anton Claußnitzer for the definition of the sets Amn and the functions fm. 1.1 The Bochner integral 3 Xm fm := 1Amn xn: n=1 Then ( fm) is a sequence of step functions, ( fm f )m is decreasing pointwise everywhere, and since D is dense in f (Ω kN),− k n lim fm(t) f (t) = 0 for every t Ω N: m !1 k − k 2 n that is, fm f µ-almost everywhere. As a consequence, f is measurable. ! Corollary 1.5. If ( fn) is a sequence of measurable functions Ω X such that fn f pointwise µ-almost everywhere, then f is measurable. ! ! Proof. We assume that this corollary is known in the scalar case, that is, when X = K. By Pettis’s theorem (Theorem 1.4), for all n there exists a µ-null set Nn 2 A such that fn(Ω Nn) is separable. Moreover there exists a µ-null set N0 Ω n S 2 such that fn(t) f (t) for all t Ω N0. Let N := n 0 Nn; as a countable union of µ-null sets, N!is a µ-null set.2 n ≥ Then f (restricted to Ω N) is the pointwise limit everywhere of the se- n quence ( fn). In particular f is weakly measurable. Moreover, f (Ω N) is separable since n [ f (Ω N) fn(Ω N); n ⊆ n n and since fn(Ω N) is separable. The claim follows from Pettis’ theorem. n R A measurable function f : Ω X is called integrable if f dµ < . ! Ω k k 1 P Lemma 1.6. For every integrable step function f : Ω X, f = 1A xn the series P ! n n n xnµ(An) converges absolutely and its limit is independent of the representation of f . P Proof. Let f = 1A xn be an integrable step function. The sets (An) are n n ⊆ A mutually disjoint and (xn) X. Then ⊆ X Z xn µ(An) = f dµ < : n k k Ω k k 1 P Let f : Ω X be an integrable step function, f = n 1An xn. We define the Bochner integral! (for integrable step functions) by Z X f dµ := xn µ(An): Ω n Lemma 1.7. (a) For every integrable function f : Ω X there exists a sequence ! ( fn) of integrable step functions Ω X such that fn f and fn f pointwise ! k k ≤ k k ! 4 1 Bochner-Lebesgue and Bochner-Sobolev spaces µ-almost everywhere. (b) Let f : Ω X be integrable. Let ( fn) be a sequence of integrable step functions ! such that fn f and fn f pointwise µ-almost everywhere. Then k k ≤ k k ! Z x := lim fn dµ exists n !1 Ω and Z x f dµ. k k ≤ Ω k k Proof. (a) Let f : Ω X be integrable. Then f : Ω R is integrable. ! k k ! Therefore there exists a sequence (gn) of integrable step functions such that 0 gn f and gn f pointwise µ-almost everywhere. ≤ ≤ k k ! k k Since f is measurable, there exists a sequence ( f˜n) of step functions Ω X ! such that f˜n f pointwise µ-almost everywhere. Put ! f˜n gn f := : n 1 f˜n + k k n (b) For every integrable step function g : Ω X one has ! Z Z g dµ g dµ. Ω ≤ Ω k k Hence, for every n, m Z Z fn fm dµ fn fm dµ, Ω − ≤ Ω k − k R and by Lebesgue’s dominated convergence theorem the sequence ( fn dµ) R Ω is a Cauchy sequence. When we put x = limn fn dµ then !1 Ω Z Z x liminf fn dµ = f dµ. n k k ≤ !1 Ω k k Ω k k Let f : Ω X be integrable. We define the Bochner integral ! Z Z f dµ := lim fn dµ, n Ω !1 Ω where ( fn) is a sequence of step functions Ω X such that fn f and ! k k ≤ k k fn f pointwise µ-almost everywhere. The definition of the Bochner integral ! for integrable functions is independent of the choice of the sequence ( fn) of step functions, by Lemma 1.7. Moreover, if f is a step function, then this definition of the Bochner integral and the previous definition coincide.
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  • The Plancherel Formula for Parabolic Subgroups

    The Plancherel Formula for Parabolic Subgroups

    ISRAEL JOURNAL OF MATHEMATICS, Vol. 28. Nos, 1-2, 1977 THE PLANCHEREL FORMULA FOR PARABOLIC SUBGROUPS BY FREDERICK W. KEENE, RONALD L. LIPSMAN* AND JOSEPH A. WOLF* ABSTRACT We prove an explicit Plancherel Formula for the parabolic subgroups of the simple Lie groups of real rank one. The key point of the formula is that the operator which compensates lack of unimodularity is given, not as a family of implicitly defined operators on the representation spaces, but rather as an explicit pseudo-differential operator on the group itself. That operator is a fractional power of the Laplacian of the center of the unipotent radical, and the proof of our formula is based on the study of its analytic properties and its interaction with the group operations. I. Introduction The Plancherel Theorem for non-unimodular groups has been developed and studied rather intensively during the past five years (see [7], [11], [8], [4]). Furthermore there has been significant progress in the computation of the ingredients of the theorem (see [5], [2])--at least in the case of solvable groups. In this paper we shall give a completely explicit description of these ingredients for an interesting family of non-solvable groups. The groups we consider are the parabolic subgroups MAN of the real rank 1 simple Lie groups. As an intermediate step we also obtain the Plancherel formula for the exponential solvable groups AN (see also [6]). In that case our results are more extensive than those of [5] since in addition to the "infinitesimal" unbounded operators we also obtain explicitly the "global" unbounded operator on L2(AN).