We shall now complete the circle by discussing a few useful examples of the situation described in the opening paragraph of this paper. Recall that this situation is as follows: Given a topological vector space E of real continuous functions oil the locally compact space X, required to identify the topological dual E' of E.
The number of possible examples in plainly unlimited. We shall merely discuss a few of the most frequently occurring instances, the method being adequately indicated in this fashion.
1 ~ When X
is
compact and E is the Banach space of all real continuous functions on X, taken with the uniform norm:Iltll
= sup zEA."It( )1, (14.1)
the solution of the problem is immediate by very definition of a Radon m e a s u r e ; s e e [B], pp. 41-48. A slight extension of this result arises when X is locally compact and E the space of all real continuous functions on X which tend to zero at infinity: here too the solution is immediate ([B], Exercice 9, p. 67). In either case, the dual E' may be identified with the set ~j~l (X) o] all bounded Radon measures on X (the condition o]
boundedness being void i] X is compact) in such a manner that to any u E E' corresponds a unique /~ E ~ 1 (X) such that
u (1) = Sx 1 (x) (x) (14.2)
lot all l E E . See als0 [2], p. 39; and [10].
Two consequences are perhaps worth noting. To begin with, the useful criterion given by Banach ([1], Th~or&me 8, p. 224) for the weak convergence of sequences of
functions in E is now just a combination of the Banach-Steinhaus theorem and the Lebesgue theorem on the termwise integration of sequences of summable functions.
Secondly, regarding E is an algebra under pointwise mutiplication, use of the notion of support of a R a d o n measure leads to a very direct proof of a well known theorem oil the structure of closed ideals ill E, namely: Let ~ be any closed ideal in ~, and let E be the set o] points x o] X at which at least one o/the members o] ~ is non-zero;
then ~ contains those and only those ]unctions o] E which vanish on C E (at least). The proof of this requires only the observation that, if IX is a Radon measure and / a continuous function, and if the measure defined symbolically by d v = / d i x is zero, then any point x at which ](x)~O belongs to the complement of the support of IX.
2 ~ When X is non-compact, a number of important exanlples are covered by the following set-up. L e t ~ be a set of subset~ A of X ; without loss of generality one may assume that the sets A are closed and t h a t ~ is an increasing directed set.
In all examples met in practice, the union of the sets A E ~ is X itself. L e t then E be the space of all real continuous functions / on X which satisfy the condition that, for each A E ~ and each ~ > 0 , there is a compact set K ~ X such that
I ] ( x ) l < e for x E A - K . (14.3) On E one takes the topology of the ~-convergence, that is the topology defined 1)y the seminorms
Na (l) = sup I/(x)l
xe.-t
with A E 6 . E is then a separated locally convex space. I t is not difficult to show t h a t : the dual ~' may be identi/ied with the set o] bounded Radon measure.% each o/which has its support contained in some set A E ~ , the identi]ication between an element u E E' and the corresponding measure t x being obtained by (14.2). Too see this, observe first that the restriction to (~ of any given u E E' is continuous on ~, so t h a t there is a Radon measure IX such t h a t (14.2) is valid for all ] E ~ . On the other hand, the continuity of u (on E) implies the existence of a set A E ~ and a finite, positive number M such t h a t
l u
(/)l-<
M . N a (t)for all / E E. If in this relation we restrict / to ~, it is easily concluded t h a t IX has its support contained in A and is in addition bounded (having a norm in ~ 1 (X) at most M). This being so, (14.2) extends to all / E ~ by virtue of the condition (14.3).
The two most important examples of the present situation arise when ~ comprises all the finite subsets of X or all the compact subsets of X; in either case, E embraces
A Theory of Radon Measures on Locally Compact Spaces. 163 all continuous functions on X, the condition (14.3) being void. The second instance here has been discussed by Hewitt ([11], Theorem 21).
I t is perhaps interesting to observe t h a t if | has a countable base (i. e., if there exists a sequence (An) of sets e | such t h a t every A E ~ is contained in some An), the space E, which is in any case complete, is metrisablc. As a consequence, the subsets of E' which are weakly bounded have the property t h a t their members are supported by a
/ixed
set A fi ~ and arc further of uniformly bounded total variation.The method is limited in two main directions, each of which m a y be instanced by an example.
3 ~ Suppose t h a t X is non-compact and t h a t E is the space of all bounded, real continuous functions on X taken with the norm (14.1). The equation (14.2) defines a member u of E' if and only if the Radon measure # is bounded. On the other hand, as is well known, not every element u of the dual can be represented in the form (14.2). I t is shown in [12] that, in order to obtain the general member of E', the right member of (14.2) must be supplemented by a linear combination of two generalised limits at infinity on X. Alternatively, a representation (14.2) m a y be effccted for all u e E ' by replacing X by its ~cch compactification. I t is of course true t h a t a resprescntation of the form (14.2) can be accorded to every u e E', but only on condition that u bc allowed to be a general bounded, finitely-additive set-function.
4 ~ If X is compact and if E comprises discontinuous functions, it is again true in general that not every u can be represented in the form (14.2) with # a Radon measure on X. An example is provided by taking for E the vector space generated by all bounded, real, semicontinuous functions on X, the norm being (14.1) once more.
I t is quite easy to show t h a t a general u e E' has a decomposition u = u' i ~ u " in which u' has a representation (14.2) whilst u " is orthogonal to all continuous functions (with- out necessarily being zero on E itself).
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