The Bornological Space Associated with a Locally Convex Space | 12

ABSTRACT

We start with the following: 12.1 Definition.

Let (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1914.tif"/> ) be a LCS and U https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1915.tif"/> a local base for P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1916.tif"/> . A subset V of X is said to be bornivorous if it absorbs any P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1917.tif"/> –bounded subset of X. Suppose now that U x = { V ⊂ X:V = ΓV and bornivorous } . https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1918.tif"/>

Then U https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1919.tif"/> ^x determines a unique locally convex topology, denoted by P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1920.tif"/> ^x, with U https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1921.tif"/> ^x as a local base. P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1922.tif"/> ^x is referred to as the bornological topology associated with P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1923.tif"/> , and (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1924.tif"/> ^x) is called the bornological space associated with (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1925.tif"/> ). A LCS (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1926.tif"/> ) is called a bornological space if P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1927.tif"/> = P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1928.tif"/> ^x; in this case, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1929.tif"/> is called a bornological topology.

Remark:

It is clear that P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1930.tif"/> ≤ P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1931.tif"/> ^x (hence P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1932.tif"/> ^x is always Hausdorff), and that P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1933.tif"/> ^x is the finest locally convex topology with the same bounded sets as P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1934.tif"/> .

As the family B https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1935.tif"/> _von of all closed, bounded disks in (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1936.tif"/> ) is a fundamental system of bounded sets in X, it follows that a subset V of X is bornivorous if and only if it absorbs any member in B https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1937.tif"/> _von.

12.2 Proposition

Every metrizable LCS (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1938.tif"/> ) is bornological.

Proof.

Let { V_n : n ≥ 1 } be a local base for P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1939.tif"/> such that V n + 1 + V n + 1 ⊂ V n https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1940.tif"/> , and let W be an absolutely convex, bornivorous subset of X. If W is not a P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1941.tif"/> –neighboourhood of 0, then V n ⊆ 2 n W https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1942.tif"/> (for all n ≥ 1). For any n ≥ 1, let x_n ∊ V_n\2ⁿW. Then {x_n} is a null sequence, hence P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1943.tif"/> –bounded and thus absorbed by W, which contracdicts x n ∉ 2 n W ( for all n ≥ 1 ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1944.tif"/> .

12.3 <target id="page_176" target-type="page">176</target>Lemma

Let (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1945.tif"/> ) be a LCS. Then the following statements are equivalent:

(X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1946.tif"/> ) is bornological.

Every seminorm on X, which is bounded on bounded subsets of X, is P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1947.tif"/> –continuous.

For any LCS Y, any linear map T : X → Y which sends bounded subsets of X into bounded subset of Y (such an T is said to be locally bounded) is continuous.

Proof.

The equivalence of (a) and (b) is trivial.

(a) ⇒ (c) Let U be an absolutely convex 0–neighbourhood in Y. Then T − 1 ( U ) = ΓT − 1 ( U ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1948.tif"/> which is bornivorous [since T is locally bounded], hence a P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1949.tif"/> –neighbourhood of 0, thus T is continuous.

(c) ⇒ (a): Let P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1950.tif"/> _x be the bornological topology associated with P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1951.tif"/> . Then the identity map I_X : (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1952.tif"/> ) → (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1953.tif"/> ^x) is locally bounded, hence continuous, thus P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1954.tif"/> ^x ≤ P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1955.tif"/> . As P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1956.tif"/> ≤ P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1957.tif"/> ^x is always true, we conclude that (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1958.tif"/> ) is bornological.

12.4 Proposition

Every separated quotient space of a bornological space is bornological, and the locally convex direct sum of a family of bornological spaces is bornological.

Proof.

(a) Let (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1959.tif"/> ) be a bornological space, let M be a closed vector subspace of

X and Q_M : X → X/M the quotient map. If U is absolutely convex and bornivorous in (X/M, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1960.tif"/> ), then so is Q M − 1 ( U ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1961.tif"/> , hence Q M − 1 ( U ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1962.tif"/> is a P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1963.tif"/> –neighbourhood of 0, thus U is a P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1964.tif"/> –neighbourhood of 0; consequently, X/M is bornological.

(b) The proof is similar to that of (a), hence will be omitted.

Remark

If the inductive topology T https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1965.tif"/> _ind w.r.t. to a family of bornological spaces (under continuous linear maps) is Hausdorff, then T https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1966.tif"/> _ind is bornological.

12.a

Bornological spaces :

The product space of a family of bornological spaces is, in general, not bornological, and any subspace of a bornological space is, in general, 177not bornological.

Dieudonné has shown that any finite–dimensional subspace of a bornological space is bornological, but it is not true for countable–codimensional subspaces as shown by Valdivia [1972]. Moreover, Valdivia [1971] has shown that any countable–codimensional subspace of a sequentially complete bornological space is bornological.

12.5 Proposition

Let (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1967.tif"/> ) be a LCS and P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1968.tif"/> ^x the bornological topology associated with P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1969.tif"/> . Then there exists a family { ( E α , ‖ ⋅ ‖ α ) : α ∈ Λ } https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1970.tif"/> of normed spaces and a family { T α : α ∈ Λ } https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1971.tif"/> of linear maps T α : E α → X https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1972.tif"/> with X = ∪ α ∈ Λ T α ( E α ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1973.tif"/> such that P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1974.tif"/> ^x is the inductive topology w.r.t. { ( E α , ‖ ⋅ ‖ α , T α ) : α ∈ Λ } https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1975.tif"/> .

Proof.

Let B https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1976.tif"/> _von be the family of all closed, absolutely convex bounded subsets of X. For any B ∊ B https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1977.tif"/> _von, let J_B : X(B)→ X be the canonical embedding. Then J_B : (X(B),γ_B) → (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1978.tif"/> ) is continuous [by (12.2) and (12.3)] and X = ∪ B ∈ B von X ( B ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1979.tif"/> [since B https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1980.tif"/> _von is fundamental]. On the other hand, an absolutely convex subset of X is a P X https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1981.tif"/> –neighbourhood of 0 if and only if it absorbs any member in B https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1982.tif"/> _von, thus P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1983.tif"/> ^x is the finest locally convex topology on X such that all J_B : (X(B),γ_B) → (X, P X https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1984.tif"/> ) are continuous; in other words P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1985.tif"/> ^x is the inductive topology w.r.t. {(X(B),γ_B,J_B) : B ∊ B https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1986.tif"/> _von}.

The preceding result, together with (9.b), enables us to make clear the essential difference between the bornological structure and the topological structure of a vector space; the former is a collection of internal pieces each of which is a normed space, while the latter is a collection of external hulls each of which is a normed space.

12.b Continuity of linear map on bornological spaces :

Let (X, P https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203749807/1e575708-9d79-4f43-a374-4d458ec8fff6/content/eq1987.tif"/> ) be a bornological space, let Y be a LCS and T ∊ L^*(X,Y). Then the following statements are equivalent.

(i) T is continuous.

(ii) {Tx_n} is a null sequence in Y whenever {x_n} is a null sequence in X.

178(iii) T is locally bounded (i.e., T sends any bounded subset of X into a bounded subset of Y).