Distributions: Topology and Sequential Compactness.
May 1, 2014
Contents
1 Introduction 2
2 General definitions, and methods for Topological Vector Spaces. 4
3 The topology of D(Ω) 9 3.1 The spaces DK ...... 10 3.2 LF-spaces ...... 12 3.3 D(Ω) is an LF-space ...... 14 3.4 Semi-norms defining the topology on D(Ω) ...... 15
4 Properties of the Space D(Ω) 15 4.1 Completeness of D(Ω) ...... 15 4.2 Non-metrizability of D(Ω) ...... 18 4.3 D(Ω) is a Montel Spaces ...... 19 4.4 Separability ...... 19
5 The space D0(Ω) 20 5.1 Positive distributions ...... 21 5.2 Topologies on D0(Ω) ...... 21 5.3 Non-metrizability ...... 22 5.4 These Spaces are Hausdorff ...... 24 5.5 The Banach-Steinhaus Theorem & Barrelled Spaces ...... 24 5.6 The Banach-Alaoglu-Bourbaki Theorem ...... 25 5.7 Sequential Compactness ...... 27 5.8 More Montel Spaces & Equivalence of the Topologies on Bounded sets ...... 28 5.9 The Weak topology on D(Ω) ...... 29 5.10 Reflexitivity ...... 30 5.11 Completeness ...... 31 5.12 The space D as a subset of D0 and Separability...... 32
6 Multiplying Distributions 33 6.1 Multiplication between subspaces of functions and D0 ...... 33 6.2 The Impossibility of Multiplying two Distributions in General ...... 34 6.3 Division by Analytic Functions in the Case of One Real Variable...... 35 6.3.1 Division by x ...... 36 6.3.2 Division by Polynomials...... 36
1 6.3.3 Division by Analytic Functions with Zeroes of Finite Order...... 36 6.4 Applications of Multiplication...... 38
7 Rapidly Decreasing Functions. 38 7.1 Relation between S and D(Sn)...... 40 7.2 The Fourier transform on S ...... 40
8 Tempered Distributions 41 8.1 Relation between S 0 and D0(Sn)...... 42 8.2 The Fourier Transform on S ’...... 42
9 The space E0. 43
10 Comparison and Relations between the spaces D, S and E and their duals. 43
11 Tensor Product of two Distributions 44 11.1 The Tensor Product of two Vector Spaces ...... 44 11.2 The Tensor Product of Function Spaces ...... 44 11.3 Tensor Products of Distributions ...... 45 0 11.3.1 The Space L(D(Ω1), D (Ω2)) ...... 48 11.4 Topological Tensor Products ...... 48 11.4.1 The π-topology ...... 48 11.4.2 The -topology ...... 49
12 Kernels and the kernels theorem. 49 12.1 Regular Kernels ...... 52
13 Nuclear Locally Convex Spaces 55 13.1 Nuclear Mappings ...... 55 13.2 Nuclear Spaces ...... 56 13.3 Nuclear Spaces and the Kernels Theorem...... 57
A Appendix: Completions 57
1 Introduction
This essay largely concerns the topological properties of the space of distributions as developed by Laurent Schwartz between 1945 and 1950. We begin this introduction with some discussion of the history and uses of distributions and will then move on to an outline of the main topics of the essay. Much of the historical details are taken from [6]. Examples of generalized functions pre-date the rigorous development of distributions. A particularly prevalent example is Heaviside’s operational calculus in [12] where he, unrigorously, uses algebraic manipulation of differential operators to treat many problems, particularly in electrodynamics. Schwartz was unaware of Heaviside’s calculus when he initially created the theory of distributions. He was told of it by electrical engineers who were able to put the theory of distributions to great use. In [6] L¨utzensays that the theory of distributions was well received by physicists as it “allowed them to use improper functions in good conscience”. Though it was in fact Sobolev who first defined distributions as the continuous linear forms on a space of test functions. Schwartz did not know of this work when he began to work on the
2 generalized solutions to partial differential equations. He originally worked by introducing the convolution operators T : D → E which satisfy
T (φ ∗ ψ) = T (φ) ∗ ψ.
Here, for example δ is the identity operator. This theory along with many theorems was made by Schwartz in October 1944. It was originally very fruitful and Schwartz was able to prove the analogies of many of the theorems which hold in the current theory of distributions. However, it became more challenging when dealing with Fourier transforms. Consequently, in 1945 Schwartz switched to studying D0. L¨utzenreports in [6] that Schwartz, himself, felt that there were two factors which made D0 a good place to work on his theories of generalized solutions. Firstly, that functional analysis was at that time making great developments and Schwartz’s own private work on the dual spaces of Fr´echet spaces allowed him to manipulate these dual spaces without having a ‘concrete representation’. Secondly, he already knew that δ as a measure could already be represented as a functional. Distribution spaces have many applications, particularly in PDE, which are largely not addressed in this essay, in particular the work done by Lars H¨ormander. Furthermore, the space of tempered distributions allows for many generalizations and extensions of theories using Fourier analysis. The distributions generalize the way in which locally summable functions and measures can act as the kernels of integral operators on an appropriate space of test functions. This, after appropriate checks and with restrictions, allows us to extend the weak formulation of PDEs outside function spaces. This essay begins by introducing the space of test functions and studying its topology, especially results about compactness and metrizability. This space is the smooth functions of compact support. The topology on this space was developed by Schwartz in a method which was extended by Schwartz along with Dieudonn´eto a general class of topological vector space called LF-spaces. We work by first introducing the LF-space in general and seeing how D can be viewed as a particular case. The Theory of Distributions is a theory of duality. We define the space of distributions to be the analytic dual of the test functions. We can also define many operations on the space of distributions; multiplication, differentiation, Fourier transform, in terms of the adjoint map of the maps on the space of test functions where they are defined in the classical sense. We can, therefore, discover many topological properties of the space of distributions by looking at theorems about dual topologies. For example, the Banach-Alaoglu-Bourbaki Theorem which shows us that weak-* bounded, weak-* closed sets in the dual of a Topological Vector Space are weak-* compact. Again in exploring the dual topologies we will focus on compactness and metrizablity results. In fact for both the distributions and the test functions we can show that, although neither space is metrizable, the closed, bounded sets are compact and metrizable (so also sequentially compact). This will lead to the equivalence of the weak-* and strong dual topology on bounded sets and the reflexivity of all spaces. We then explore the multiplication and division of distributions. Multiplication, can be defined by duality between the space E of smooth functions and the distributions. We will explore the continuity properties of this mapping and show that it cannot be extended to a multiplication between two distribution spaces, though here we only look at the case of one variable. We then go on to look at the case where the smooth function we multiply is fixed and examine the invertibility of this operation. This shows another example of how distributions differ from functions. Division by an analytic function is possible in general even when the function has zeros (though these have to be of finite order), the result of division is, however, not uniquely defined. We finish the section on multiplication by very briefly showing how these
3 results make clear and rigorous how to pose linear PDE with smooth coefficients in the space of distributions. We will go on to look at some other spaces of distributions which are found by enlarging the space of test functions. These are the space S 0 of tempered distributions which is defined as the dual of S the space of rapidly decreasing function. This space is particularly useful for Fourier analysis, which we will look at extremely briefly. We also look at the space E0 which are the distributions of compact support. The topology of all these spaces are very similar and we will not reprove all the results that we gave for the space D0. The next section looks at tensor products of distributions which allows us to develop the theory of kernel operators and examines the way in which an element of D0(Ω × Ω) can act as a mapping from D(Ω) to D0(Ω). We also look at the topology on tensor products as developed by Grothendiek in an example of how the theory of distributions fed back into functional analysis. We will demonstrate Schwartz’s Kernels Theorem which gives that all linear mappings form D(Ω) to D0(Ω) can be given in this way. This was proved by Schwartz for example in [11] or [10] using the theory of nuclear locally convex spaces which was again work by Grothendieck when he was Schwartz’s student. This theory is very beautiful but in this section we show how the algebraic, not the topological isomorphism, can be found from a much simpler proof using Fourier transforms. We then look at fundamental kernels and parametrices for differential operators and we relate their regularity properties to the hypo-ellipticity of the operator. We finish by briefly giving some notions of Nuclear spaces including the ‘idea’ of Schwartz’s proof of the kernels theorem and of a very elegant similar proof given in [4]. Nuclear spaces also aided Schwartz in creating the theory of vector valued distributions in [10].
Acknowledgements
I would like to thank Prof. Cl´ement Mouhot for supervising this essay, and also Tim Talbot for helping me understand some of the material on topological vector spaces viewed through their neighbourhoods and particularly for pointing out that not all neighbourhoods are open which is a piece of information without which nothing makes any sense! I also used to a great extent my notes from the part III courses ‘Functional Analysis’ taught by Dr Andras Zsak, and ‘Distribution Theory and Applications’ taught by Dr Anthony Ashton.
2 General definitions, and methods for Topological Vector Spaces.
As the theory of distributions concerns topological vector spaces, there are several definitions and Theorems which we will use repeatedly throughout so I will list them here. Definition. A Topological Vector Space (TVS) is a set V related to a field F which carries the normal algebraic structure of a vector space with a topology which has the following properties. (1) (x, y) 7→ x + y : V × V → V is continuous. (2) (λ, x) 7→ λx : F × V is continuous. where both V × V and F × V carry the normal product topology. Because of these properties it is possible to recover the whole of the topology of a vector space V by looking at the neighbourhood base around 0. In particular addition and multiplication are separately continuous since λ 7→ (λ, x)
4 for x fixed is continuous and x 7→ (x, y) is continuous for y fixed. Definition. A neighbourhood of x is a set N such that there exists U ∈ τ with
x ∈ U ⊂ N
Definition. A local neighbourhood base of 0 is a collection of sets B s.t. N is a neighbourhood of 0 iff ∃B ∈ B with B ⊂ N. Suppose we are given a collection of sets B in a vector space V then for each B ∈ B construct the set B0 = {x ∈ B|∃A ∈ B, x + A ⊂ B} we hope these will be the open neighbourhoods of 0. Let B0 be the collection of such B0 for each B ∈ B then the set: A = {x + B0|x ∈ V,B0 ∈ B0} we hope will form a base for a topological vector space topology on V provided that the sets in B satisfy some conditions to make them consistent with the axioms of a topological vector space. We will find this conditions but first need to make some definitions. Definition. A set, N, in V is called balanced if for all |λ| ≤ 1 we have that λN ⊂ N. Definition. A set A in V is called absorbing if for every φ there exists a scalar λ such that φ ∈ λA. To find what conditions we need the set B to satisfy we will look at the neighbourhoods of 0 in some Topological Vector Space V . Firstly it is obvious that they all must contain 0. We know that addition is continuous from V × V to V . So if N is an open neighbourhood of 0 in V then there must be some open neighbourhoods of 0 M,L such that M + L ⊂ N. (Since the topology on V ×V has a base of the form M ×L for M,L open in V .) If we take M 0 = M ∩L then M 0 + M 0 ⊂ N. Since every neighbourhood of 0 in V contains an open neighbourhood this means we have the following property:
(A) If N is a neighbourhood of 0 there will exist a neighbourhood of 0, M, such that M + M ⊂ N.
Since, multiplication is continuous given an open neighbourhood of 0, N, we have t > 0 and M a neighbourhood of 0 such that if |λ| < t and x ∈ M we have λx ∈ N. Therefore [ M 0 = λM ⊂ N |λ| (B) If N is a neighbourhood of 0 then there will exist M ⊂ N such that M is balanced. We also know that multiplication is continuous V → V when λ is fixed (and supposed not to be 0) so if N is an open neighbourhood of 0 there exists M, an open neighbourhood of 0, 1 such that λ M ⊂ N. Hence M ⊂ λN so we have the following property: 5 (C) N is a neighbourhood of 0 and λ 6= 0 means that λN is a neighbourhood of 0. We also know that multiplication is continuous if x ∈ V is fixed. So given N an open 1 neighbourhood of 0 there exists t > 0 such that |λ| < t means that λx ∈ N hence x ∈ λ N. So we have property (D) Every neighbourhood of 0 is absorbing. Therefore, B is going to be a base of neighbourhoods for we need that collectively the sets that can be generated by arbitrary unions and finite intersections of elements of B will have the properties above. Suppose that B satisfies the following properties: (A’) If N ∈ B then there exists M ∈ B such that M + M ⊂ N (B’) If N ∈ B there exists B a balanced set such that there is M ∈ B with M ⊂ B ⊂ N (C’) If λ is a scalar, N ∈ B then there exists M ∈ B such that M ⊂ λN (D’) If N ∈ B and x ∈ V then there exists λ such that x ∈ λN (E’) If N,N 0 ∈ B then there exists M ∈ B such that M ⊂ N ∩ N 0 We now claim that A as defined above will define a vector space topology on V . First we show that it does indeed define a topology. y ∈ (x + A) ∩ (x0 + A0) we have 0 ∈ (x − y + A) ∩ (x0 − y + A0) and since 0 ∈ x − y + A we know that y − x ∈ A so by the definition of B0 there exists BB0 such that y − x + B ∈ A and similarly we can find B0 such that y − x0 + B0 ∈ A0 so if we take C = B ∩ B0 then there is C0 ∈ B0 such that C0 ⊂ C then y + C0 ⊂ (x + A) ∩ (x0 + A0) which shows that A forms a base for the topology. We would like it to be a topological vector space topology so we need that addition is continuous. This means that if we are given x, y, z ∈ V with y + z = x. 6 and also we are given A ∈ B0 we would like to find B,C ∈ B0 such that (y + B) + (z + C) ⊂ x + A we know that we have D ∈ B0 such that D + D ⊂ A then we can take B = C = D to get (y + B) + (z + C) = x + D + D ⊂ x + A We also need to show that multiplication is continuous. This means that if we are given x, y ∈ V and λ ∈ F such that λ.y = x and we are also given A ∈ B0 then we would like to find > 0 and B ∈ B0 such that if |t| < then (λ + t).(y + B) ⊂ x + A this will be ⇔ λy + ty + (λ + t)B ⊂ x + A ⇔ ty + (λ + t)B ⊂ A we know that we can find C a balanced open neighbourhood of 0 such that C + C ⊂ A then since C is absorbing we can choose such that ty ∈ C, ∀ |t| < and since C is balanced and λC will be an open neighbourhood of 0 for all non-zero λ we can find B such that (λ + t)B ⊂ C ∀ |t| < by letting B be a shrunken C. Consequently we have (λ + t)(y + B) = x + ty + (λ + t)B ⊂ x + C + C ⊂ x + A Proposition. If a vector space V carries two topological vector space topologies τ, τ 0 which prescribe the same neighbourhoods of 0 then τ = τ 0. This shows the topology constructed above is the unique one admitting this neighbourhood base so we have an identification between topologies for TVS and neighbourhood bases. Proof. In this case let B be the collection of neighbourhoods of 0 in both τ and τ 0. . Further, suppose U is an open set in τ then if x ∈ U, ∃V ∈ B0 with x + V ⊂ U then since V is a neighbourhood of 0 in τ 0 there will be a V 0 ∈ τ 0 with 0 ∈ V 0,V 0 ⊂ V then x + V 0 ⊂ U so U is open in τ 0 and this argument is symmetric so τ = τ 0. 7 Further, most of the spaces I will use will be locally convex and Hausdorff. (Because of the definition of a TVS any T1 space will be Hausdorff so the Hausdorff condition is necessary that the topology ’sees’ the difference between any two points.) In particular to show that a space is Hausdorff it is sufficient to show that given any x ∈ V there is some N a neighbourhood of 0 with x∈ / N. Definition. A locally convex space is a topological vector space which has a base of convex, balanced neighbourhoods of 0. To each convex, balanced set in the neighbourhood we can associate a semi-norm µB(x) = inf{λ > 0|x ∈ λB} This is called the Minkowski functional of the set B. Therefore, we can equivalently define a locally convex topology by a collection of semi-norms {pi|i ∈ I} where I is some indexing set. This has a base of local neighbourhoods of the form. V = {x|pi1 (x) < 1, ..., pik (x) < k} where k ∈ N, i1, ..., ik ∈ I, 1, ...k ∈ R. We can see here that we automatically have continuity of addition for any convex, balanced 1 1 possible base as if N is a convex neighbourhood base then 2 N + 2 N ⊂ N. We also have a few helpful little things like for N any convex, balanced neighbourhood of 0 we will have: 1 1 1 1 N − N = N + N ⊂ N, 2 2 2 2 1 1 1 N + N + N ⊂ N, 3 3 3 etc. Consequently, to check a convex, Balanced, collection will form a neighbourhood base we only need that λN is a neighbourhood and that we have the intersection condition. Therefore the topology we described above generated by semi-norms is always a TVS topology. And the topology is defined by any collection of semi-norms whose semi-balls for a sub-local-base. We can also see that a locally convex space is Hausdorff iff \ ker(pi) = {0} i∈I Definition. A Fr´echetspace is a locally convex space which is metrizable with a complete, translation-invariant metric. It will also be useful to have definitions of bounded sets and Cauchy sequences in non-metric spaces. Definition. A subset B of a topological vector space is said to be bounded if for every N a neighbourhood of 0, there exists a scalar λ s.t. B ⊂ λN. So we can see that since every neighbourhood is absorbing that {x} is a bounded set for x ∈ V . Proposition. In a locally convex space defined by a collection of semi-norms pi, i ∈ I a set B is bounded iff pi(B) is bounded for each i. 8 Proof. Let Ni = {x|pi(x) ≤ 1} and let B be bounded. Then there exists λ with λNi ⊃ B so if x ∈ B then pi(x) ≤ λ. Conversely, suppose that B is a set with x ∈ B ⇒ pi(x) ≤ λi. Then let N be an arbitrary neighbourhood of 0 since the semi-norms define the topology there is a set 0 N = {x|pi1 (x) ≤ 1, ..., pin (x) ≤ k} 0 0 with N ⊂ N then let µ = max(λi/i, ..., λn/n). Then µN ⊃ B so µN ⊃ B. Definition. A sequence (xj) in a topological vector space is Cauchy if for every N a neighbour- hood of 0, there exists K ∈ N s.t. if j, k ≥ K, xj − xk ∈ N. Proposition. In a locally convex space all Cauchy sequences are bounded. Proof. Given a Cauchy sequence {φj} and N a neighbourhood of 0 we wish to show that there exists a µ s.t. φj ∈ µN∀j. As {φj} is Cauchy there exists J s.t. for all j ≥ J φj − φJ ∈ N ⇒ φj ∈ φJ + N there exists a λ s.t. φJ ∈ λN and since N is convex λN + N ⊂ (λ + 1)N. λ 1 ( This is because λx + y = (λ + 1)( λ+1 x + λ+1 y). φj ∈ λN + N ⊂ (λ + 1)N 0 0 0 and there exists λ s.t. k ≤ J − 1 ⇒ φk ∈ λ N so set µ = max(λ, λ ). The Hanh-Banach theorem will also be used frequently so we recall it here. The Hahn-Banach Theorem. If X is a real or complex vector space, p : X → R a positively homogeneous, subadditive funtional. Let Y be a closed subspace of X and f : Y → R a linear map such that |f(y)| ≤ p(y) for all y ∈ Y . Then f extends to f˜ a linear map on X and furthermore |f˜(x)| ≤ p(x), ∀x ∈ X . We usually take p to be a semi-norm defining a locally convex space topology. We also claim that a linear functional u : V → F is continuous with respect to an lcs topology defined by pi, i ∈ I iff there exists a finite collection i1, ..., in and a positive real, C, such that |u(x)| ≤ C max pi (x). k k 3 The topology of D(Ω) We will construct a topology on D(Ω) the space of continuous functions of compact support n inside Ω ⊂ R via a limiting process on spaces of the form DK which are defined below. This topology is called an LF-space topology. Also following Schwartz in [3] we will give an explicit collections of semi-norms which define the same topology. 9 3.1 The spaces DK If K is a compact subset of Ω the we can define a linear subspace of D(Ω), DK to be the set ∞ of functions whose support is contained inside K. This is not the same as the space Cc (K) = C∞(K) as the functions must be able to be smoothly extended to the whole of Ω. Then we have [ [ D(Ω) = DK = DKn K n n where ∪nKn = Ω. For example in Ω = R we can take Kn = {x | |x| ≤ n}. We can define a Fr´echet space topology on each DK via the semi-norms α pn,K (φ) = Σ|α|≤n sup |∂ φ(x)| x∈K We now show that this does indeed define a Fr´echet space topology: (1) DK is metrizable: Since the semi-norms make DK into a locally convex, Hausdorff space and there are countably many of them we have the standard construction of a metric. −n pn(ψ − φ) d(φ, ψ) = Σn2 1 + pn(ψ − φ) (noticing that p1 ≤ p2 ≤ p3 ≤ ....) Proposition. This is a metric defining the topology on DK Proof. d(φ, ψ) = 0 iff pn(φ − ψ) = 0, ∀n which is iff φ = ψ. We know wish to show the triangle inequality. This will follow from: p (a + b) p (a) p (b) n ≤ n + n 1 + pn(a + b) 1 + pn(a) 1 + pn(b) by summing both sides. This in turn follows from pn(a + b) ≤ pn(a) + pn(b) 1 1 ⇒ ≤ pn(a) + pn(b) pn(a + b) 1 1 ⇒ 1 + ≤ 1 + pn(a) + pn(b) pn(a + b) 1 1 ⇒ ≤ 1 + 1 1 + 1 pn(a+b) pn(a)+pn(b) p (a + b) p (a) p (b) ⇒ n ≤ n + n pn(a + b) + 1 1 + pn(a) + pn(b) 1 + pn(a) + pn(b) p (a + b) p (a) p (b) ⇒ n ≤ n + n 1 + pn(a + b) 1 + pn(a) 1 + pn(b) So this defines a metric. 10 Now we want to show that this metric defines an equivalent topology. We do this by showing both topologies have the same neighbourhoods of 0. Look at the set {φ|d(0, φ) ≤ }. Then there exists an N such that −n Σn≥N+12 ≤ /2. Then there exists λn, n = 1, ..., N such that −n pn(φ) pn(φ) ≤ λn ⇒ 2 ≤ /2N 1 + pn(φ) then the set {φ|pn(φ) ≤ λn, n = 1, ..., N} is contained in the ball so the epsilon ball is a neighbourhood of 0 in the semi-norm topology. Now since pn(φ) ≤ ⇒ pn−1(φ) ≤ it is sufficient to look at the sets {φ|pn(φ) ≤ λ} then there exists such that −n pn(φ) 2 ≤ ⇒ pn(φ) ≤ λ 1 + pn(φ) so the ball in the metric topology is contained in this set. So this set is a neighbourhood of 0 in the metric topology. There are two interesting points which can be taken from this proof. The first is that the balls are not bounded in the sense of the locally convex space topology as none of the basic open neighbourhoods are bounded for the lcs topology. It might at first seem that since B 1 n form a neighbourhood base of 0 that the −balls would be bounded via B = nB 1 but this n metric is not positively homogeneous i.e. d(0, tφ) 6= t.d(0, φ) so tB 6= Bt. in general. This shows that a metrizable space is not necessarily locally bounded. Secondly, this construction works in any locally convex space which can be defined by a non- decreasing, countable sequence of semi-norms. It is also the case that any locally convex space that can be defined by a countable sequence of semi-norms can be defined by a non-decreasing sequence of semi-norms (by listing these q1, q2, q3, ... then looking at pn(x) = maxk≤n(qn(x))). This shows that any first countable, locally convex space is metrizable. (2) DK is complete with this metric: It can be seen that if a sequence φj is d-Cauchy then for each n, pn(φj −φk) → 0 as j, k → ∞. α α Therefore, ∂ φj is a Cauchy sequence with the uniform norm for each α. So ∂ φj → φα 11 α uniformly and φα = ∂ φ0 and φ0 ∈ DK since the boundary values of every partial derivative will be 0. This is the same as saying φj → φ0 in the DK topology. α Proof that φα = ∂ φ0: We do this iteratively by showing it for first partial derivatives. |φ0(x + h1) − φ0(x) − φ(1,0,...,0)(x)h1| ≤ |φ0(x + h1) − φj(x + h1)| + |φ0(x) − φj(x)| +|φj(x + h1) − φj(x) − ∂x1 φj(x)h1| + |h1||∂x1 φj(x) − φ(1,0,...,0)(x)| We may as well assume that |h1| ≤ 1 so we can choose j such that φj is uniformly less than away from φ0 and ∂x1 φj is uniformly less than away from φ(1,0,...,0) so that we have: φ0(x + h1) − φ0(x) − φ(1,0,...,0)(x)h1| ≤ 3 + |φj(x + h1) − φj(x) − ∂x1 φj(x)h1| and for this j we can choose h1 so that RHS is ≤ 4. 3.2 LF-spaces In order to introduce the topology on D(Ω) it is first helpful to introduce in general a new kind of TVS topology. This will be a generalization of the kind of topology we will place on D(Ω). Definition. An LF-space is also called the strict inductive limit of Fr´echetspaces. If we have a sequence F1 ⊂ F2 ⊂ F3... of nested vector spaces each with a Fr´echetspace topology. We also ask that the subspace topology S induced by Fn+1 on Fn is the original topology on Fn. Then if we let F = n Fn we can define the LF-space topology on F via a local base of neighbourhoods of zero. B = {V |0 ∈ V,V is convex, balanced ,V ∩ Fn is a neighborhood of 0 in Fn∀n} This topology is locally convex since the base we have just constructed is of convex, balanced sets. Fn is called a sequence of definition for F . Proposition. The subspace topology induced by F on Fn is the original topology on Fn. Proof. Here we want to show this by proving that the basic neighbourhoods in one topology are neighbourhoods in the other. If V is a neighbourhood of 0 in F then V contains a convex, balanced neighbourhood of 0 W ∈ B as B is a neighbourhood base of 0 in F . W ∩ Fn is a neighbourhood of 0 in Fn by the definition of B. So V ∩ Fn ⊃ W ∩ Fn is a neighbourhood of 0 in the original topology. Now we want to prove that if W is a neighbourhood of 0 in the original topology on Fn then there exists a V which is a neighbourhood of 0 in F s.t. V ∩ Fn = W . It is sufficient to do this for convex neighbourhoods only. As Fn+1 induces the original topology on Fn there is a neighbourhood Wn+1 of 0 in Fn+1 s.t. Wn+1 ∩ Fn = Wn. We will show below that Wn+1 can also be taken to be convex. Iteratively we have Wn+k+1 a convex neighbourhood of 0 in Fn+k+1 s.t. Wn+k+1 ∩ Fn+k = Wn+k. Let [ V = Wn n this has the required properties, and it is a neighbourhood of 0 as it is convex and its intersection with each Fn is a neighbourhood of 0 in Fn. 12 Proposition. An LF-space is Hausdorff. Proof. This follows [4] book fairly closely. This is proved similarly to the second part of the above proof. We want to show that if x ∈ F, x 6= 0 then there exists some En with x ∈ En. Since En is Hausdorff there exists a convex neighbourhood Nn of 0 in En such that x∈ / Nn. We know there is a neighbourhood of 0 Wn+1 in En+1 such that Wn+1 ∩ En = Nn. We would like to reduce Wn+1 to a convex Nn+1 such that x∈ / Nn+1 and Nn+1 ∩ En = Nn. S Then if we take N = n Nn we have that N is convex so by the definition of an LF-space topology N is a neighbourhood of 0 in F and x∈ / N so F is Hausdorff. Now we need to show that if Nn is a convex neighbourhood of 0 in En not containing x then there exists a convex neighbourhood of 0 Nn+1 in En+1 not containing x such that Nn+1 ∩ En = Nn. This will complete both the above proofs. Since, En+1 induces the topology on En there exists a neighbourhood M of 0 in En+1 such that M ∩ En = Nn then M contains some convex neighbourhood C with x∈ / C since En+1 is Hausdorff and locally convex. Then look at the convex hull of C and Nn this contains C so is a neighbourhood in En+1 and contains Nn so its restriction to En contains Nn. Suppose it contains x then there exists y ∈ C and z ∈ Nn such that x = ty + (1 − t)z which means that ty = x − (1 − t)z ∈ En so y ∈ C ∩ En ⊂ Nn which means that x ∈ Nn as Nn is convex. This gives a contradiction so x∈ / conv(Nn ∪ C) and letting Nn+1 = conv(Nn ∪ C) will give all the required properties. 13 3.3 D(Ω) is an LF-space S If K1 ⊂ K2 ⊂ K3... is a nested sequence of compact sets s.t. n Kn = Ω then D(Ω) can be made into an LF-space with sequence of definition DKn , n = 1, 2, 3, .... It can often make more sense to choose Kn = ωn where ωn is a relatively compact, open set. This is because if K is an arbitrary compact set then for any x ∈ K \ int(K) we will have φ(x) = 0 for every φ ∈ DK . Firstly, we notice that the topologies we have defined on each DK space are compatible with each other. That is if K ⊂ K0 then the subspace topology induced on DK by DK0 is the initial topology on DK . This is because pn,K0 |K = pn,K so the semi-norms defining both topologies are the same. Proposition. This topology is independent of the choice of Kn. 0 0 0 0 Proof. Suppose K1 ⊂ K2 ⊂ K3 ⊂ ... and K1 ⊂ K2 ⊂ K3 ⊂ ... are two such sequences. Let τ, τ be the corresponding topologies. Suppose N is a convex neighbourhood of 0 in τ then we want 0 to look at N ∩ D 0 . There exists an m such that K ⊃ K so Kn m n N ∩ D 0 = (N ∩ D ) ∩ D 0 Kn Km Kn and we already know that N ∩ DKm is a neighbourhood of 0 in DKm . We know that DKm induces the right subspace topology on D 0 so N ∩ D 0 is a neighbourhood of 0 in D 0 so Kn Kn Kn N is a neighbourhood of 0 in D(Ω). This argument is symmetric which is sufficient for our conclusion. From above it is clear that this will induce the right topology on DK either by choosing it to be part of the sequence of definition of noting that in any such sequence there must be an n s.t. Kn ⊃ K and DKn will induce the right subspace topology on DK . In this case we can explicitly find neighbourhoods of 0 n D(Ω) which match the basic neigh- 0 bourhoods of 0 in DK . If N = {φ|pn(φ) ≤ C} then N = {φ| ||φ||n ≤ C} is a neighbourhood of 0 α 0 in D(Ω) s.t. N ∩ DK = N. Where ||φ||n = Σ|α|≤n supx∈Ω |∂ φ(x)|. There is an important consequence of the definition of this topology for working out whether a linear functional on D(Ω) is a distribution. Proposition. If E is a locally convex space and u : D(Ω) → E is a linear function. Then u is continuous iff u|DK is continuous for every K. Proof. Suppose U is a convex, balanced neighbourhood of 0 in E then u−1(U) is a convex, balanced set in D(Ω) and −1 −1 u (U) ∩ DK = (u|DK ) (U) so if U is a convex, balanced neighbourhood of 0 u−1(U) is convex so is open iff u|−1 (U) is open DK for every K which gives the result. 14 3.4 Semi-norms defining the topology on D(Ω) The topology on D(Ω) is locally convex so we can define it by a collection of semi-norms. We start by looking at a neighborhood base which is given explicitely in Schwartz’s book. If ∅ = U1 ⊂ U¯1 ⊂ U2 ⊂ U¯2 ⊂ U3 ⊂ U¯3 ⊂ ... S is a sequence of relatively compact open sets s.t. n Un = Ω, and m1 < m2 < m3 < ... is a strictly increasing sequences of integers, 1 ≥ 2 ≥ 3 ≥ ... a sequence of real numbers decreasing to 0. Then α V (U, m, ) = {φ | Σ|α|≤mi sup |∂ φ(x)| ≤ i for each n = 1, 2, 3, ...} x/∈Ui letting m, range over all possible such sequences gives a neighbourhood base of 0. Proposition. These two defintions give the same topology on D(Ω). Proof. Here we use the fact that if the neighborhood base of 0 in each topology are neighbour- hoods in the other topology then the two topologies are the same. Let Kn = U¯n. α V (U, m, ) ∩ DK = {φ | Σ|α|≤mi sup |∂ φ(x)| ≤ i, i = 1, 2, ..., n} x/∈Ui so this is a neighbourhood of 0 in DKn as it contains the set {φ|pmn (φ) ≤ n} ∩ {φ|pmn−1 (φ) ≤ n−1} ∩ ... ∩ {φ|pm0 (φ) ≤ 0}. Now suppose that V ∈ B then for each Kn,V ∩ DKn is a neighbourhood of 0 in DKn so contains a set of the form {φ|pmn (φ) ≤ n} as the semi-norms are non-decreasing. Therefore V (U, m, ) ⊂ V so these two topologies coincide. This topology can then be defined by the semi-norms α N(m, )(φ) = sup( sup |∂ φ(x)|/n). n |α|≤mn,x/∈Un 4 Properties of the Space D(Ω) Here we will explore some results relating to compactness, completeness, separability and metriz- ability of the space D(Ω). These will have several implications fro the topological properties of the dual. 4.1 Completeness of D(Ω) We will see later that D(Ω) is not metrizable. However, we can still define a notion of topological completeness. Since D(Ω) is not first countable it is not sufficient just to look at sequences. This also gives an opportunity to introduce filters which generalize the notion of sequences to non- metric spaces and will characterize continuity and completeness in this spaces in the same way sequences do for metric spaces. Definition. A filter on a set E is a family F of subsets of E. (1) ∅ ∈ F (2) A, B ∈ F ⇒ A ∩ B ∈ F (3) A ∈ F,B ⊃ A ⇒ B ∈ F. 15 Definition. A filter F converges to a point x if for every neighbourhood N of x, ∃A ∈ F s.t. A ⊂ N i.e. N ⊂ F. Lemma. In a Hausdorff TVS limits are unique. Proof. Let F be a filter and suppose F → x and F → y. Then since the space is Hausdorff there exists U, V neighbourhoods of x and y respectively such that U ∩ V is disjoint. However since F → x, U ∈ F and since F → y, V ∈ F so U ∩ V ∈ F which is a contradiction. We can relate this to the concept of sequences. To every sequence x1, x2, x3, ... ⊂ E the associated filter F is the subsets of E which contain all but finitely many elements of the sequence. A sequence converges iff the associated filter converges. Definition. A Cauchy filter is a filter F s.t. if N is a neighbourhood of 0 there exists A ∈ F s.t. A − A ⊂ N. A TVS is said to be complete if every Cauchy filter converges. A metrizable TVS is topologically complete then it is complete in the old sense. Definition. B is a base for a filter F if ∀A, B ∈ B there is some C in B such that C ⊂ A ∩ B and then F is the collection of sets N such that there is some B ∈ B with B ⊂ N. Theorem. Every LF-space, F , is complete. Proof. This follows [4] reasonably closely. Let F be a Cauchy filter and U be the filter of all neighbourhoods of 0. Let F 0 be the smallest filter all the sets of the form U + V,U ∈ U,V ∈ F. This is a base for a filter since (U + V ) ∩ (U 0 + V 0) ⊃ (U ∩ U 0) + (V ∩ V 0) . Suppose F 0 converges to x then we claim that F converges to x. This is because if F 0 converges to x, then for every N a neighbourhood of x there exists B ∈ F 0 with B ⊂ N then we have some U a neighbourhood of 0 and V ∈ F with U + V ⊂ B. So we have V ⊂ B which means that V ⊂ N so F converges to x. Also, F 0 is a Cauchy filter. If N is a neighbourhood of 0 then there exists M a balanced neighbourhood of 0 such that +M +M +M ⊂ N. Then there exists A ∈ F such that A−A ⊂ M then we will have that (A + M) − (A + M) = A − A + M − M = (A − A) + M + M ⊂ N 0 0 Then let Fn = {U ∈ En | U = V ∩ En, for some V ∈ F }. We claim that if for every V ∈ F that V ∩ En is non-empty then Fn is a Cauchy filter on En. This is because Fn will satisfy all the axioms of a filter except, possibly, that it may contain ∅, this shows it is a filter. If N is a neighbourhood of 0 in En then it contains a convex, balanced neighbourhood M. Then there 0 0 0 exists a neighbourhood of 0 M in E such that M ∩ En = M. Then since F is a Cauchy filter there will be some A ∈ F 0 such that A − A ⊂ M 0 then A ∩ En − A ∩ En ⊂ M which shows that Fn is a Cauchy filter. Next we show that there is some n such that Fn does not contain ∅. Suppose that for every 0 n there is a set Un + Vn ∈ F where Un is a convex neighbourhood of 0 and V ∈ F such that (Un + Vn) ∩ En = ∅ 16 then let [ [ U = Un,V = Vn n n . Since Un is open for each n this means U will be open so U + V ∈ F 0 and [ U + V = (x + U) x∈V so is open. Since the subspace topology induced by E on EN is the original topology then (U + V ) ∩ EN is open in EN . However, [ [ (U + V ) ∩ E1 = (Un + Vn) ∩ E1 ⊂ (Un + Vn) ∩ En = ∅ n n which is a contradiction to (U + V ) ∩ E1 being open. Consequently, there exists an N such that FN is a Cauchy filter. We want to show that FN converges in En. Since En is metrizable we have a countable neighbourhood base of 0 in 1 terms of the balls of radius around 0. Call these B 1 . Since FN is a Cauchy filter there exists n n An ∈ FN such that An − An ⊂ B 1 . Then for each n we can choose n \ xn ∈ Ak 1≤k≤n so that xn will be a Cauchy sequence. Since En is a Fr´echet space it is complete so xn converges to some element x. 1 We then claim that FN converges to x. Let Mn be the ball of radius n around x. Then 1 there exists some K so that m ≥ K means that d(xm, x) ≤ 2n and let m be possibly larger so 0 that Am − Am ⊂ B1/2n then xm ∈ Am so Am ⊂ Mn. It follows from this that F converges to x since for each M a neighbourhood of x in E there is some Mn ⊂ M ∩ EN 0 and we can find an A ∈ FN with A ⊂ Mn and we claim that we can choose C ∈ F such that C ⊂ M and C ∩ EN = A. 0 0 We find this C by iterating upwards. So we find a A ∈ EN+1 and A ∩ EN = A and 0 A ⊂ M ∩ EN+1 by noticing that FN+1 also doesn’t contain the empty set. Then we can move upward and take the union over all these sets to get C. Since, at the beginning we noticed that if F 0 converges to x then so does F we have shown that F converges to x and therefore that E is complete. We also proved here that any metric space in which every Cauchy sequence converges will also have that any Cauchy filter converges. In the specific case of D(Ω) we also have a quicker way of seeing that all Cauchy sequences converge. We recall that in a locally convex space Cauchy sequences are bounded. 17 Proposition. In D(Ω) if B is a bounded set then there exists a K with B ⊂ DK . Proof. This follows [2]. Suppose E ⊂ D(Ω) is not in any DK . Then give our sequence of compacts Kn increasing to Ω there must be an xn, φn s.t. xn ∈ Kn \ Kn−1 and φn ∈ E with φn(xn) 6= 0. Then let 1 N = {φ| |φ(x )| < |φ (x )|} n n n n since only finitely many of the xn are in each Kn N is in B. However φn ∈/ nN for each n so there is not λ with E ⊂ λN. Proposition. In D(Ω) every Cauchy sequence converges. Proof. This is a result of the previous proposition, that Cauchy sequences are bounded and the fact that DK is complete. So since any Cauchy sequence is bounded it will be contained inside some DK and therefore will converge as Cauchy sequences converge in DK . 4.2 Non-metrizability of D(Ω) For this section it is useful to have the following definition. Definition. A topological space X is a Baire space if for every countable sequence Fn of closed S sets with n Fn = X there exists N with intFN 6= ∅. With this definition the Baire Category theorem states that a complete, metric space is a Baire space. Proposition. D(Ω) is not a Baire space. 0 Proof. DK is nowhere dense since if V ⊂ DK and K ∩ K then V ∩ DK0 = ∅ so V cannot be a neighbourhood of 0. DK is a linear subspace of D(Ω) has empty interior. As for suitably chosen S sequence Kn, D(Ω) = n DKn then D(Ω) is not a Baire space. It is a consequence of this that D(Ω) is not completely metrizable. Since D(Ω) is topologically complete if it were metrizable it would be completely metrizable this shows that D(Ω) is not metrizable. Furthermore, as D(Ω) is locally convex, if it were first countable then it would be metrizable. Therefore, the space D(Ω) is not first countable. Similarly, we can work from the fact that any topological vector space has a base of neigh- bourhoods which are balanced. Proposition. If X is a Topological Vector Space with a balanced, countable neighbourhood base Vn in which every Cauchy sequence converges then X is a Baire space. T Proof. We prove that if Un is a countable sequence of open dense sets then Un in X. Let T n W be any open set in X we will prove that ( n Vn) ∩ W 6= ∅. Since V1 is open and dense there exists x1 ∈ W ∩ V1 and W1 and open neighbourhood of 0 such that x1 + W1 ⊂ V1 ∩ W and W1 ⊂ V1. Then (x1 + W1) ∩ U2 has non-empty interior so there exists x2 and W2 ⊂ V2 such that x2 + W2 ⊂ (x1 + W1) ∩ U2. Itteratively, we can produce the sequence x1, x2, x3, ... and W1,W2,W3, ... such that n ! \ xn + Wn ⊂ Un ∩ W k=1 We then claim that x1, x2, x3, ... is a Cauchy sequence. Then let m ≥ n xm ∈ xn + Wn ⇒ xm − xn ∈ Wn 18 so this is a Cauchy sequence. Therefore there is some x with xn → x so x ∈ xn + Wn for each T n therefore x ∈ ( n Un) ∩ W which gives the result. 4.3 D(Ω) is a Montel Spaces Definition. A Montel space is a topological vector space which is locally convex, Hausdorff and has the Heine-Borel property. (That every closed bounded set is compact.) We wish to show that D(Ω) is a Montel space. Since we know already that every bounded set in D(Ω) is contained inside some DK then if we can show that each DK is a Montel space we will have the result. In order to do this we first recall the Arz`ela-AscoliTheorem. Arz`ela-AscoliTheorem. If K is a compact, Hausdorff topological space. Let C(K) be the Banach space of continuous functions K → R with the supremum norm. If E is a uniformly bounded, equicontinuous set in C(K) then it is relatively compact. Proposition. DK is a Montel space. Proof. As DK is metrizable we have compactness iff sequential compactness (for its subsets). Consequently, it is sufficient to prove that a bounded sequence in DK has a convergent subse- quence. Given this let {φj} be a bounded sequence in DK then φj is uniformly bounded in each α of its partial derivatives which shows that {φj} is equicontinuous. So for each α, {∂ φj} is a uniformly bounded, equicontinuous set and therefore has a convergent subsequence. If we order our set of multi-indexes α(1), α(2), α(3), ... and the α(1)th partial derivatives of this subsequence is a uniformly bounded, equicontinuous set so has a subsequence which converges (2) to some continuous function φα(1) , then we look at the α partial derivatives of this further subsequence and etc. α Then we can form a cantor diagonal sequence φjk s.t. ∂ φjk → φα where φα is a continuous α function which is 0 on the boundary of K. It follows that ∂ φ0 = φα so φ0 is in DK and φjk → φ0 uniformly in each of its derivatives. This also shows that D(Ω) is sequentially compact since any bounded sequence will be in some DK which is sequentially compact. It is interesting to notice that the only Banach spaces which are Montel spaces are finite dimensional which is further proof that the topology on DK is not normable. This is because if a vector space has a bounded, compact neighbourhood of 0 then it must be finite dimensional. 4.4 Separability We are going to show that D(Ω) is separable. First we need a Lemma. Lemma. If X is a separable metric space and Y is a subspace then Y is separable. Proof. Since X is separable it has countable dense subset {xn} and so there exists a subset {xnj } such that {xnj + B1} covers Y and all the xnj are a distance less than 1 away from Y . Then we can choose a ynj ∈ Y ∩ (xnj + B1). Then if we repeat this procedure but replacing 1 1 1 with 2 , 3 , ... we will produce a countable dense sequence in Y . Proposition. The space D(Ω) is separable. 19 ∞ Proof. First we prove that the space C (K) (with the same semi-norms as for DK ) is separable ∞ and since DK is a subspace of this and C (K) is a Fr´echet space this will show that DK is separable. Let T be the map T (φ) = ∂x1 ∂x2 ...∂xn φ. Since K is bounded if ||ψ − T (φ)||∞ ≤ then we can integrate ψ to a function Ψ such that if max{α1, ..., αn} ≤ 1 then |∂α(Ψ − φ)(x)| ≤ .diam(K). Fix some φ in C∞(K) we want to show it can be approximated by polynomials. By the n above argument and if we can approximate T φ by a polynomial we can choose pn such that for all α with max{α1, ..., αn} ≤ n then 1 |∂α(p − φ)(x)| ≤ . n n ∞ Then the sequence pn will tend to φ in the topology on C (K). Since the polynomials with rational coefficients are dense in the polynomials and since the Stone-Weierstrass theorem gives that any continuous function can be uniformly approximated by polynomials we have that DK is separable. Since D(Ω) is the union of countably many spaces of the form DK we have that D(Ω) is separable. 5 The space D0(Ω) We define this as the analytic dual of D(Ω) the space of continuous linear forms on D(Ω) In this section we will describe two topologies on the space D0(Ω) and discuss the properties of these topologies including issues around compactness, metrizability, separability and completeness. This is the space of distributions. As a result of our criterion for a function from an LF-space to annother LCS to be continuous we have the following criteria for a linear form u to be a distribution. Proposition. A linear functional u on D(Ω) is a distribution iff α (1) For each K there exists a C and an N s.t. |u(φ)| ≤ CΣ|α|≤N supx∈K |∂ φ(x)| (2) For each sequence φj → 0, u(φj) → 0. Proof. These are both results of the fact that u is continuous iff its restriction to each DK is continuous. As D(Ω) is not metric (2) is not just a result of translation being continuous however since every convergent sequence is in some DK it does hold. The space of distributions does indeed generalise the notion of function. We can identify the 1 0 L functions with a subset of D (Ω) via f ↔ uf where Z uf (φ) = f(x)φ(x)dx. Ω Similarly, measures can be identified with distributions via µ ↔ uµ where Z uµ(φ) = φ(x)µ(dx). Ω 20 5.1 Positive distributions Before exploring the topology we look at an interessting way in which D0 is not like a function space. A distribution u, is positive if for every φ ≥ 0, u(φ) ≥ 0. A difference between function spaces and spaces of distributions is that distributions cannot in general be written as the difference of two positive distributions so it is not possible to build up any theory by looking first at positive distributions (for instance like is done for Lebesgue integration). This is because of the following result. First we need the Riesz-Representation Theorem. Riesz Representation Theorem. If K is a compact, Hausdorff topological space then the dual to the Banach space (C(K), || ||∞) is the space of Borel measures on K with the total variation norm. Proposition. Every positive distribution can be identified with a Borel Measure. Proof. Suppose u is a positive measure. d For K ⊂ R a compact let ||φ||∞,K = supK |φ|. Suppose φ ∈ D then fix K compact, ρK ∈ D s.t. ρ = 1 on K. Then ||φ||∞,K ρ − φ ≥ 0 ⇒ u(||φ||∞,K ρ − φ) ≥ 0 ⇒ ||φ||∞,K u(ρ) ≥ u(φ) so u is continuous with respect to the uniform norm on K and so can be extended continu- ously to a linear form on C(K). So it is a consequence of this and Riesz representation that u|DK is a measure on K call it µ|K . 0 Its clear that if K ⊂ K then µK |K0 = µK0 . Given φ ∈ D∃K s.t. φ ∈ DK then let µ(φ) = µK (φ) this is independent of the choice of K, and so as we can approximate the indicator functions of compact sets in Rd by functions in D, µ defines a regular Borel measure (not necessarilly finite) on Rd. 5.2 Topologies on D0(Ω) We will introduce two topologies on D0(Ω). Defintion. The weak-* topology on D0(Ω) is the topology of point-wise convergence it is the weakest topology which makes all the evaluation maps continuous. It is a locally convex topology defined by the semi-norms pφ(u) = |u(φ)| φ ∈ D(Ω) Definition. The strong dual topology is the topology of uniform convergence on the bounded subsets of D(Ω). It is a locally convex topology defined by the semi-norms. pB(u) = sup |u(φ)| φ∈B where B ranges over the bounded subsets of D(Ω). 21 5.3 Non-metrizability As we have already seen the strong topology is metrizable iff it is first countable. So we would like to investigate whether the strong dual topology can be defined by a countable collection of semi-norms. If A and B are bounded sets in D(Ω) and A ⊂ B then pA ≤ pB so pA is continuous with respect to a topology which is generated by a set of semi-norms which include pB. Since every bounded subset of D(Ω) is contained in a bounded set of the form B = {φ | pn(φ) ≤ Mn}. We can take our sequence of semi-norms to be associated with bounded sets of the above form. These sets are closed, balanced and convex. Furthermore if we have a countable collection of bounded sets B1,B2,B3, ... we can add countably more sets so that this sequence is closed under finite unions and multiplication by an integer. This will mean that if T : D0 → R is continuous with respect to the topology then |T (u)| ≤ pB(u) for some B. This means that φˆ is continuous with respect to the strong topology since {φ} is bounded. Claim: If we have a sequence as defined above an element of D then φˆ is continuous with re- ˆ ˆ spect to the semi-norms pB1 , pB2 , pB3 , ... only if φ ∈ Bi for some i. We know that φ is continuous with respect to the topology induced by the semi-norms if there exists an i with |u(φ)| ≤ pBi (u) 0 for every u ∈ D (Ω). If φ∈ / Bi then there exists n such that pn(φ) > supψ∈B pn(ψ). So there exists an x0 such that α |Σα≤n∂ φ(x0)| > sup pn(ψ) ψ∈B so the element u with α u(φ) = Σα≤n∂ φ(x0) ˆ has that u(φ) > pBi (φ). Therefore φ is continuous only if there exists an i with φ ∈ Bi. Since {φ} is a bounded set this means that the strong dual topology is defined by the semi- norms pB1 , pB2 , pB3 , ... only if for every φ there is some Bi with φ ∈ Bi. We show below that this is not possible. Proposition. Let K have non-empty interior. If B1,B2, ... is a collection of bounded sets in S some DK then there exists φ ∈ DK which is not it n Bn Proof. By possibly making the Bn bigger we may as well take them to be sets of the form Bj = {φ | pn(φ) ≤ Mn,j} Then let aj = Mj,j +1. So if φ is an element of DK with pj(φ) ≥ aj then φ∈ / Bj for any j so it is not in the union. So it remains to construct such a function. Since K has non-empty interior we can find U1,U2,U3, ... a countable sequence of open balls inside K and there exists a sequence Vn 1 of open balls such that for each n we have diam(Vn) = 2 diam(Un) and we have φn ∈ DK such that φn is identically 1 on Vn and identically 0 outside Un. Then we can produce the function 22 φn(x) sin(anx1) and patch these together to get a function with the required properties. This is because for x ∈ int(Vn) we have ∂α(φ (x) sin(a x )) = ∂α1 sin(a x ) n n 1 x1 n 1 so as without loss of generality we can make an ≥ 1 and large enough so that the sine function does a full wiggle inside Vn then n pn(φn) ≥ an ≥ an. Since for any φ we have that {φ} is a bounded set and that if B is bounded in D(Ω) then B ∩ DK will be bounded the above proposition shows that the strong dual topology is not first countable. Now we will show that the weak-* topology is not first countable. This is equivalent to saying that it cannot be defined by a countable set of semi-norms of the form pφ for φ ∈ D. For this we need a lemma. ˜ ˜ T Lemma. If f, f1, ..., fn are linear functionals on a vector space V . and ker((f)) ⊃ k≤n ker(fk) ˜ then f ∈ span(f1, ..., fn). n ˜ Proof. Let T : V → scalars be the map x 7→ (f1(x), ..., fn(x)). Then ker(T ) ⊂ ker(f) so there ˜ exist a map g such that f = g ◦ T and g will be of the form g(y) = a1y1 + ... + anyn which means ˜ that f(x) = a1f1(x) + ... + anfn(x). 0 Then suppose that the weak-* topology on D is defined by the semi-norms pφ1 , pφ2 , pφ3 , ... then find some other φ we know that φˆ is weak-* continuous. Which means the set V = {u | |u(φ)| < } contains a set of the form W = {u | |u(φ1)| < 1, ..., |u(φn)| < n} T ˆ which means that U = k≤n ker(φk) ⊂ V and u ∈ U ⇒ λu ∈ U which means that λu(φ) < for each λ so u(φ) = 0 for each u ∈ U therefore ker(φˆ) ⊃ U which by the lemma above means ˆ ˆ ˆ that φ ∈ span(φ1, ..., φn) which means that φ ∈ span(φ1, ..., φn) which in turn implies that D(Ω) is countable dimensional. This is a contradiction because if D(Ω) is countable dimensional then so is DK which would mean that DK was the union of countably many finite dimesional subspace. span{e1} ⊂ span{e1, e2} ⊂ span{e1, e2, e3} ⊂ ... These subspaces are closed (as they are finite dimensional) and nowhere dense since any open neighbourhood of 0 is absorbing. So the weak-* topology is not metrizable. 23 5.4 These Spaces are Hausdorff Now we wish to show that both these topologies are Hausdorff.Since every weak neighbourhood of 0 is a strong neighbourhood of 0 it is sufficient to show that for every u 6= 0 ∈ D0(Ω) there + is some φ ∈ D(Ω) such that u(φ) > 0 then fix 0 < α < u(φ) and the sets Uα = {v | v(φ) > α} − and Uα = {v | v(φ) < α} are open neighbourhoods of u and 0 respectively which are disjoint. 0 Therefore D (Ω1) is Hausdorff for both the strong and weak-* topologies. If u and v are not equal then we can find N,M open neighbourhoods of 0 such that N ∩ (u − v + M) = ∅ then u + M and v + N are disjoint open sets which separate u and v. 5.5 The Banach-Steinhaus Theorem & Barrelled Spaces The Banach-Steinhaus theorem says that if a collection of linear functional on a Banach space X are pointwise (weak-*) bounded then they are uniformly bounded (bounded in the strong dual topology). We wish to extend this result to the spaces D, D0. First we will need the notion of a barrel and a barrelled space. Definition. A set B in a TVS. E is a barrel if it is closed, convex, balanced and absorbing. (Absorbing means if x ∈ E∃λ s.t. x ∈ λB.) Definition. A Barrelled space is a TVS in which every barrel is a neighbourhood of 0. A Fr´echet space, F , is barrelled as a consequence of the Baire Category theorem. If B is a barrel then F = S nB so by BCT nB has non-empty interior for some n so B has non- n∈N empty interior. Since B is convex and balanced if N ⊂ B is open then conv(N ∪ (−N)) is a neighbourhood of 0 contained in B. An LF-space, F , with sequence of definition Fn is barrelled since if B is a barrel in F , then B ∩ Fn is a barrel so a neighbourhood of 0 in Fn so since B is convex it is a neighbourhood of 0 in F . Theorem. If a set B is weak-* bounded in D0(Ω) then it is strongly bounded. Proof. First, we show that if B is weak-* bounded then it is equicontinuous at 0. Let > 0. T −1 Then let W = u∈B u (B). We want to show that W is a neighbourhood of 0 in D(Ω). Clearly, W is convex, balanced and closed. We wish to show it is absorbing. Fix φ ∈ D(Ω). Then we have a constant C with: |u(φ)| ≤ C ∀ u ∈ B Since B is bounded so contained in some DK for some K and so for φ ∈ B we will have α |u(φ)| ≤ CΣ|α|≤n|∂ φ| which must be bounded on B so there is a λ s.t. |u(φ)| ≤ λ ∀ u ∈ B and since u is linear −1 −1 u (λB) = λu (B) ⊃ λW so φ ∈ λW and since D(Ω) is barrelled W is a neighbourhood of 0. Now we show that since B s equicontinuous it is strongly bounded. Let A be a bounded set in D(Ω). Then take , W as before. W is a neighbourhood of 0 in D(Ω) so there exists µ s.t. A ⊂ µW so u(A) ⊂ µB for each u ∈ B so B is strongly bounded. 24 It is a consequence of this that if ui is a sequence with ui(φ) → vφ 6= ±∞. Then the linear map u defined by u(φ) = vφ is continuous. This is because the weak-* closure of the sequence is equicontinuous as was shown in the above proof so u will be continuous. Since the strong topology is stronger this shows that the bounded sets are the same for both topologies. 5.6 The Banach-Alaoglu-Bourbaki Theorem In normed spaces the Banach-Alaoglu theorem says that the unit ball of the dual is weak-* compact. We hope to extend this to a form which will be useful in the space of distributions. First we state Tychonoff’s theorem which we will use. Tychonoff’s Theorem. If Xi, i ∈ I is a collection of compact spaces with some indexing set I Q then i∈I Xi is compact with the product topology. We introduce the notion of a polar. Definition. Suppose E is a topological vector space and E0 is its analytic dual. Then if A ⊂ E the polar of A is A˚ = { u ∈ E0 | sup |u(φ)| ≤ 1} φ∈A Using this we have Banach-Alaoglu-Bourbaki. If E is a topological vector space and E0 its dual and if U is a neighbourhood of 0 in E we have that U˚ is weak-* compact. Proof. Fix x ∈ E then there is λ such that x ∈ λU or equivalently 1 x ∈ U λ consequently if f ∈ U˚ we will have 1 |f x | ≤ 1 λ so that |f(x)| ≤ |λ| We notice also that U˚ is weak-* closed since if u∈ / U˚ then there exists a φ ∈ U such that u(φ) > 1 so the set [ V = { u | |u(φ)| > 1} φ∈U is weak-* open since it is the union of weak-* open sets. This set is also the complement of U˚ which means that U˚ is weak-* closed. It is also balanced since if u ∈ U˚ and |λ| ≤ 1 then we have |λu(φ)| = |λ||u(φ)| ≤ |λ| ≤ 1 for each φ ∈ U which means that λu ∈ U.˚ The polar is convex since if u and v are in U˚ and 0 ≤ t ≤ 1 and φ ∈ U we will have |tu(φ) + (1 − t)v(φ)| ≤ t|u(φ)| + (1 − t)|v(φ)| ≤ t + (1 − t) = 1 25 which means that tu + (1 − t)v ∈ U.˚ Let Mφ = supu∈B |u(φ)|. Then let Fφ = [−Mφ,Mφ]. Then let Y F = Fφ φ∈D(Ω) with the product topology. Let πφ be the co-ordinate projections. F is compact by Tychonoff’s theorem. Now we construct a map θ : B → F . θ(u) = (u(φ))φ∈D(Ω) Now we need to prove that θ is a homeomorphism onto its image and that its image is closed in F . −1 ˆ θ ◦ φ = πφ on the image of θ so θ−1 is weak-* continuous. ˆ θ ◦ πφ = φ so θ is continuous. The image of θ is the subspace of F where fλφ+µψ = λfφ + µfpsi \ Im(θ) = ker(πλφ+µψ − λπφ − µπψ) φ,ψ,λ,µ which is closed. Theorem. Any weak-* bounded, weak-* closed set in the dual space of a barrelled TVS is weak-* compact. Proof. We use the theorem above and show that any weak-* bounded, weak-* closed set is contained in the polar of a neighbourhood of 0 in the space E. Suppose B is such a weak-* bounded, weak-* closed set then let B˜ be formed by B˜ = { x ∈ E | |u(φ)| ≤ 1, ∀ u ∈ B} This space is weakly closed in E so is closed in E. It is also balanced and convex by similar arguments as we used to show U˚ is balanced and convex. Also if we fix x ∈ E then since B is weak-* bounded there is some λ such that sup |u(x)| ≤ λ u∈B which means that 1 x ∈ B˜ λ so that B˜ is absorbing and therefore a barrel. Since our TVS is barrelled this means B˜ is a neighbourhood of 0 and B is contained in the polar of B˜. 26 5.7 Sequential Compactness 0 Proposition. A weak-* bounded sequence un in D (Ω) has a convergent subsequence. For this we need some Lemmas. Lemma 1. Let K be the closure of a simply connected domain. If u ∈ D0 with u(φ) ≤ α 1 CΣα≤N supK |∂ φ| for each φ ∈ DK then there exists a C function f and a multi-index α such that for every φ ∈ DK |α| α u(φ) = (−1) uf (∂ φ) and α is of the form α1 = α2 = α2 = ... = αn = N + 3 Proof. This follows [2]. We look at the map T = ∂x1 ∂x2 ...∂xn and want to show that it is 0 invertible on DK . Lets look at the map S = ∂x1 ∂x2 ...∂xn on DK . Then for φ ∈ DK since φ is 0 on the boundary of K we have: Z φ(y) = S(φ)(x)dx {xi≤yi,i=1,...,n} This shows that S is injective which implies that Sn is injective. It also shows that |φ(y)| is bounded by the L1 norm of Sφ. The mean value theorem gives that |φ| ≤ diam(K) max ||(∂x φ)||∞ i i which implies that N 0 N+1 ||φ||N ≤ C||S φ||∞ ≤ C ||S φ||L1 So we have 00 N+1 |u(φ)| ≤ C ||φ||N ≤ C3||S φ||L1 since SN+1 is injective we can define a map u0 on its image so that u = u0 ◦ SN+1 therefore 0 N+1 |u (ψ)| ≤ C3||ψ||L1 ψ ∈ Im(S ) 00 Using the Hahn-Banach theorem we can extend this to u a functional on DK such that for all φ ∈ DK we have that 00 |u (φ)| ≤ C3||φ||L1 So u00 ∈ (L1(K))0 which can be identified with L∞(K). This gives us a function g ∈ L∞(K) which we can then integrate twice normally in all the derivatives to produce the required function 2 f. Since K is bounded we will have ||f||∞ ≤ (diam(K)) ||g||∞. This also shows that the supremum bound of f is bounded in terms of C00 and properties of K. 0 Lemma 2. If uj is a pointwise bounded sequence in D (Ω) then there is an N such that for α each j there exists a Cj such that uj(φ) ≤ CjΣα≤N supK |∂ φ|. Proof. Suppose that this is false. Then we can extract a subsequence such that α un(φn) > nΣ|α|≤n sup |∂ φn| K and possibly by rescaling we can let pn(φn) = 1. This will give that φn is a bounded sequence in 0 DK . However the Banach-Steinhaus theorem says that a weak-* bounded set in D is bounded on a bounded set in D and {un} is not bounded on {φn} which is a contradiction. 27 0 Proof. Proof of the proposition: Suppose uj is a bounded sequence in D (Ω) and K some compact |α| α subset of Ω then by lemma 1 there exists fj for each j such that uj(φ) = (−1) ufj (∂ φ) and by lemma 2 this α is the same for each j. Now we want to show that fj is a uniformly bounded, equicontinuous set when restricted to K. We have: Z f(x)(−1)|α|∂αφ(x)dx = u(φ) K for every φ ∈ DK We would like to show that fj are uniformly bounded and equicontinuous by showing that fj and their order one partial derivatives are uniformly bounded. The Banach-Steinhaus theorem gives that uj will be bounded uniformly on each bounded subset of DK which shows that the Cj in lemma 2 can all be taken to be the same. We saw in the proof of lemma 1 that the supremum bound of f and its derivatives only depends on this C and properties of K which shows that the fj are uniformly bounded and equicontinuous since they are also uniformly bounded in each of the order one partial derivatives. Therefore we can apply Arz`ela-Ascolito the sequence fj to find a sub-sequence fjk which converges uniformly to a function f. So the sequence ujk |DK converges to the linear form on DK α α N+3 induced by (−1) ∂ f since uj = uf ◦ T . We can do this for each DKn for some sequence of compacts Kn increasing to Ω, by extracting successive subsequences. We can then form a Cantor diagonal sequence and since every φ is contained in some DKn this will converge in the weak-* sense. This shows that all Cauchy sequences converge in D0 since we know that Cauchy sequences are bounded and so by the above proposition they will have a convergent subsequence. It is also the case that if a subsequence of a Cauchy sequence converges to a point u then the whole sequence must, therefore in D0 with the weak-* topology all Cauchy sequences converge. 5.8 More Montel Spaces & Equivalence of the Topologies on Bounded sets We recall that the bounded sets are the same for both topologies. In this section we will prove that D0(Ω) has the property that the topologies restricted to bounded sets are the same and are metrizable when restricted to these sets. We also show that the bounded sets are compact. First we prove a lemma. Lemma. If E is an equicontinuous set in D0(Ω). Then the topology of bounded convergence and the weak-* topology are the same on E. Proof. Fix A closed and bounded in D(Ω) we have already proved that A is weak-* compact thanks to Banach-Alaoglu-Bourbaki. Fix > 0 we wish to find a neighbourhood of 0 in E s.t. supφ∈A |u(φ)| ≤ for all u ∈ E. This will show that PA|E is weak-* continuous. As E is equicontinuous there is a weak-* neighbourhood of 0 U ∈ D s.t. |u(φ)| ≤ for all u ∈ E, φ ∈ U. Then since A is compact we have φ1, φ2, φ3, ..., φn ∈ A s.t. φ1+U, φ2+U, ..., φn+U cover A. For each i = 1, ..., n we have Wi weak-* neighbourhood of 0 s.t. u ∈ Wi ⇒ |u(φi)| ≤ 28 T let W = Wi this is also a weak-* neighbourhood of 0. Then for u ∈ W sup |u(φ)| = max sup |u(φi + ψ)| φ∈K i≤n ψ∈U ≤ max |u(φi)| + sup |u(ψ)| ≤ 2 i≤n ψ∈U Then, since every bounded set is contained in a closed, bounded set pB is continuous for every B bounded. This shows the strong dual topology is weaker that the weak-* topology on E. As {φ} is bounded for each φ we already know that the weak-* topology is weaker than the strong dual topology so they must be the same. From this we can show that D0(Ω) is a Montel space. Proposition. D0(Ω) is a Montel space with the strong dual topology. Proof. We have already that D0 is locally convex and Hausdorff. We would like to exploit the fact that we know that weak-* bounded, weak-* closed sets are weak-* compact after Banach- Alaoglu-Bourbaki. As the strong dual topology is stronger than the weak-* topology we have that B strongly bounded ⇒ B¯w∗ is weakly compact. Also from the proof of Banach-Steinhaus we have B strongly bounded ⇒ B weak-* bounded ⇒ B equicontinuous. So we know that B¯w∗ is equicontinuous so by the above lemma on B¯w∗ the strong and weak-* topologies coincide. So B¯w∗ is strongly compact and B is a strongly closed subset of B¯w∗ so B is strongly compact. It is in fact a general truth that the duals of Montel Spaces are Montel Spaces. Proposition. Restricted to equicontinuous sets, the topology of convergence on M a dense subset of D is equal to the strong dual and weak-* topologies. This along with separability of D shows that these topologies restricted to equicontinuous sets are metrizable. Proof. In the proof of the Lemma we can see that if M is a dense set in D(Ω) then we can choose the φ1, ..., φn to be in M. Therefore, the restriction to any bounded set of the topology induced by the strong or the weak topology is the same as the topology which is induced by the evaluation maps of a dense set in D(Ω). Since we know that D(Ω) is separable with countable dense set M we can look at the weakest topology on D0(Ω) which will make the evaluation maps of all the elements of M continuous. This has a countable base neighbourhood base so is metrizable so any of these topologies restricted to a bounded set are metrizable. This gives annother proof that bounded sets in D0 are sequentially compact since sequential compactness is equivalent to compactness in a metrizable space. 5.9 The Weak topology on D(Ω) We work toward showing that the space D is reflexive. We do this by analogy with the theorem for normed spaces which says that a Banach space which is reflexive if and only if its unit ball is weakly compact. Consequently we need to define the weak topology on D. 29 Definition. The Weak topology is the weakest topology on the space D(Ω) which makes all the 0 elements of D (Ω) continuous. It is a locally convex topology defined by the semi-norms pu where pu(φ) = |u(φ)| where u ranges over the distributions. As with normed spaces u is continuous with respect to the weak topology on D(Ω) iff u ∈ D0. We already know that u ∈ D0 implies that u is continuous. Suppose u is weakly continuous then V = {φ | |u(φ)| < } is weakly open contains a set of the form {φ | |ui(φ)| < 1, i = 1, ..., n} 0 for some set of ui ∈ D . Then \ U = ker(un) ⊂ V n if φ ∈ U then λφ ∈ U so |u(λφ)| < for all λ so u(φ) = 0 for φ ∈ U. So ker(u) ⊃ U which 0 means that u ∈ span{u1, ..., un}. Which means that u ∈ D Proposition. Any set B which is closed and bounded in the normal topology on D(Ω) is weakly compact. Proof. If B is closed and bounded in the LF-space topology then we already know it is compact in the LF-space topology. Any weakly open set is also open in the LF-space topology so any weak open cover of B is an open cover of B in the normal topology so has a finite subcover. Hence B is weakly compact. 5.10 Reflexitivity Definitions. The bidual, F”, of a TVS F is the dual of F’ with its strong dual topology. A space is called reflexive if F” is isomorphic to F via the cannonical mapping x → xˆ. By the lemma in the previous section the strong and weak-* topologies coincide on the 00 equicontinuous sets if f ∈ D then f|E is weak* continuous for E some equicontinuous set. For this section, we need to recall a special case of the separation form of the Hahn-Banach Theorem. Hahn-Banach Separation. If X is a locally convex space and A, B are disjoint convex sets s.t. A is compact and B is closed. Then there exists f ∈ X0 s.t. sup f(a) < inf f(b) a∈A b∈B Now, on the way to proving that D is reflexive we show something similar to Goldstine’s theorem for normed spaces which says that the ball of the bidual is the weak-* closure of the image of the ball under the canonical injection of D into D00 via the evaluation map. Proposition. This canonical mapping D → D00 is an injection. Proof. We want that if u(φ) = u(ψ) for every u ∈ D0 then φ = ψ. Suppose φ 6= 0 then we can define a linear map on span(φ) by lettingu ˜(λφ) = λ then the Hahn-Banach theorem allows us to extend this to a linear functional u ∈ D0 so D0 strongly separates the points of D so the canonical mapping is indeed an injection. 30 Proposition. Let B be the weakly bounded set in D defined by |u(φ)| ≤ Mu 0 where u ranges over D and Mus are arbitrarily chosen positive reals. Then let 00 00 B = {f ∈ D | |f(u)| ≤ Mu} this is a weak-* bounded set in D00. Let K be the weak-* closure of the image of B under the canonical injection into D00. Then K = B00. Proof. K is weak-* closed and bounded so by Banach-Alaoglu-Bourbaki it is compact. Suppose 00 K 6= B then we know that K ⊂ B so pick some f0 ∈ B \ K then by Hahn-Banach separation there exists u ∈ D0 s.t. sup f(u) < f0(u) f∈K as everything is convex and balanced we may as well have these quantities positive. Since 00 f ∈ B , f0(u) ≤ Mu but sup |f(u)| = sup |u(φ)| = Mu f∈K φ∈B 00 which gives Mu < Mu for this u which is a contradiction. Hence, K = B . Corollary. D(Ω) is reflexive. Proof. Let Dˆ be the image of D in D00. The restriction to Dˆ of the weak-* topology on D00 is the weak topology on D. Let B be a closed, bounded set in D and Bˆ its image in D00. Then by ¯ Bˆ is weak-* compact so weak*-closed. By above, Bˆ = Bˆw∗ = B00. So D00 = D. 5.11 Completeness We already know that all Cauchy sequences converge in D0(Ω) but since the space is not metriz- able we would like to show that D0(Ω) is complete. Proposition. The space D0(Ω) is complete for the strong dual topology. Proof. Let F be a Cauchy filter on D0(Ω) and define F (φ) = {A(φ) | A ∈ F } where A(φ) = {u(φ) | u ∈ A} then we claim that F (φ) is a Cauchy filter in R. Indeed given > 0 we know that φˆ is a continuous linear form on D0(Ω) so there is a neighbourhood U of 0 such that u ∈ U means that |u(φ)| < . Then there is some A ∈ F with A−A ⊂ U which means that A(φ)−A(φ) ⊂ (−, ). Since R is complete we know that A(φ) → tφ. We define a function u : D(Ω) → R by u(φ) = tφ. Since it follows from the definition that A(λφ + µψ) = λA(φ) + µA(ψ), u is a linear functional. We now claim that F converges uniformly to u on any bounded set B, of D(Ω). This follows as Bˆ is an equicontinuous set of maps from D0(Ω) to R so their images are uniformly Cauchy. This means that u|B is continuous and bounded. Since u is bounded on all bounded sets and linear u ∈ D0(Ω). Hence F → u. 31 5.12 The space D as a subset of D0 and Separability. We have already seen how the space D embeds into D0 via the same identification as for the 1 0 Lloc functions. Here, we state but do not prove that D is sequentially dense in D . This argument relies on mollifying a distribution u by convolving it with a bump function which will produce a smooth function and then multiplying this function by a smooth function which has large compact support. This result has some implications for the space D0. First, however, we would like to investigate continuity properties of the map I : D → D0. This map cannot be a homeomorphism since the I(D) is dense in D0 yet D is complete so would form a closed subspace of D0 if the subspace topology was equivalent to the initial topology. Proposition. The map I : D(Ω) → D0(Ω) which goes via φ 7→ uφ where Z uφ(ψ) = φ(x)ψ(x)dx Ω is continuous when D0(Ω) carries either the weak-* or strong dual topology. Proof. We prove this by showing that I is sequentially continuous. We have already shown that since D0(Ω) is a locally convex space with either topology that sequential continuity will imply continuity. Consequently, we take some sequence φj → 0 and wish to show that uφj → 0 in either topology. Since the φj converge to zero their supports are all contained in some fixed compact set K. Z |uφj (ψ)| = | φj(x)ψ(x)dx K Z ≤ sup |φj(x)| |ψ(y)|dy x∈K K ≤ sup |φj(x)| sup |ψ(y)| |K| x∈K y∈K = |K|p0,K (φj)p0,K (ψ) which will tend to 0 as j → 0. Since p0,K is bounded on bounded subsets of D(Ω) this means that I is sequentially continuous with respect to both the weak-* and strong dual topology. We know that I−1 cannot be continuous. We now have some consequences of the continuity of I and the density of its image. Proposition. D0(Ω) is sequentially separable with either of the topologies. Proof. This is a consequence of the density of I(D(Ω)) in D0(Ω) and the fact that D(Ω) is separable. Also, since the bounded sets in D(Ω) are metrizable then if N is a countable sequentially dense set in D0(Ω) and B is a bounded set in D0(Ω) then we have N ∩ B ⊃ N ∩ B = B Which means that N = D0(Ω). We also see that if T is a continuous map from D0(Ω) to some other space then T ◦ I will be a continuous map from D(Ω). 32 6 Multiplying Distributions First we introduce the space E(Ω) this is the space of smooth function on Ω. This is a first countable locally convex space defined by the semi-norms α pn,K (φ) = Σ|α|≤n sup |∂ φ(x)| x∈K By choosing a countable sequence of compacts increasing to Ω we can show that this is a Fr´echet space in exactly the same way as for DK . We show that if φj is a Cauchy sequence then it is uniformly Cauchy on each compact in each of its derivatives. Therefore for each compact α K the sequence ∂ φj converges uniformly to a function φα so ordering the multi-indexes and extracting subsequences gives a Cantor diagonal sequence φjk which converges uniformly on K in each partial derivative. Extracting successive sub-sequences for each Kn will give a Cantor diagonal sequence which converges uniformly in each derivative on each compact and therefore in the sense of E. In this section we always consider D0 with the weak-* topology. 6.1 Multiplication between subspaces of functions and D0 It is possible via duality to define multiplication between E and D0. ψu(φ) = u(ψφ) This agrees with our previous notion of multiplying funtions when u can be identified with an element of L1. This is a bilinear map E × D0 → D0 Proposition. Multiplication is a separately continuous mapping and if φn → 0 in E and un → 0 0 0 in D then φnun → 0 in D Proof. First fix φ ∈ E then look at u 7→ φu call this map mφ we want to show this is weak-*- to - ˆ ˆ weak-* continuous. Choose ψ ∈ D then φ.ψ ∈ D so ψ ◦ mφ = ψ.φ. Therefore this is continuous. 0 Now fix u ∈ D and look at φ 7→ φu call this map mu we want to show this is continuous, ˆ it is sufficient to show that if ψ ∈ D then ψ ◦ mu is continuous and since E is metrizable it is sufficient to prove this is sequentially continuous. So take some sequence φj → 0 we want to show that u(ψ.φj) → 0 which true since φj → 0 uniformly in each derivative on each compact so ψ.φj → 0 uniformly in each derivative so u(ψ.φj) → 0 as u is continuous. Now given φj → 0, uj → 0 then for each ψ ∈ D we wish to show that uj(ψ.φj) → 0. The support of ψ is contained in some compact set K and for this K there exists a function ϕ ∈ DK which majorizes ψ.φj for each j (as this seqence tends to 0 uniformly in each derivative). uj(ϕ) → 0 which gives the result. Proposition. The map M :(φ, u) 7→ φ.u is not a continuous mapping from E × D0 to D0 Proof. We argue by contradiction. Suppose the map were continuous then we would have ψ1, ..., ψr, ξ1, ..., ξr ∈ D and K a compact subset of Ω, n an integer such that. max |u(φ.ψi)| ≤ pn,K (φ) max |u(ξj)| ∀φ, u i j 33 S then supp(φ.ψi) ⊂ supp(ψ) so without loss of generality we can take supp(φ) ⊂ supp(ψi) and S i K ⊂ i supp(ψi) so we only need to look at u|DK . Fix some x ∈ K such that ψi(x) 6= 0 for (α) some i, and let u = δx for some |α| > n then we have (α) α (β) (α−β) |δx (φ.ψi)| = |δx(∂ (φ.ψi))| = |Σβ≤αcα,βφ (x)ψi (x)| (α) then maxj |ξ(x)| is just some number and maxi |ψi(x)| 6= 0 so making φ (x) sufficiently large relative to the lower derivatives will give a contradiction. We now introduce the notion of the order of a distribution. We have already seen for each u ∈ D0 to each compact set K ⊂ Ω we can associate an integer N and a constant C s.t. α |u(φ)| ≤ CΣ|α|≤n sup |∂ φ(x)| K for all φ ∈ DK If there is an N which works for every compact set then we call the smallest such N the order of u. Otherwise we say u has infinite order. Proposition. It is possible to define multiplication between the subspace of distributions of order less than or equal to N and the N times continuously differentiable functions in Ω Proof. This is because the distributions of order less than or equal to N are in the analytic N N N dual of the space Cc (Ω) and the multiplication between C (Ω) and D(Ω) is into Cc (Ω) so multiplication can be defined by duality as before. 6.2 The Impossibility of Multiplying two Distributions in General In this section we follow Schwartz proof in [5] closely. For this we will need to have the notion of the derivative of a distribution. Definition. If u in D0 and α is a multi-index we can define ∂αu via the ’integration by parts formula’ ∂αu(φ) = (−1)|α|u(∂αφ) This defines a distribution since its restriction to any DK is continuous and linear as differ- entiation is a continuous, linear map on DK . This definition agrees with the normal derivative on distributions which can be identified with functions with a continuous derivatives up to the |α|th order, thanks to the normal integration by parts formula. 1 Now we want to look at the particular distribution on Ω = R, p.v.( x ) defined by: 1 Z 1 p.v. φ = lim φ(x)dx x →0 |x|> x