Topological characterisation of weakly compact 
operators 

Antonio M. Peralta a, 1, Ignacio Villanueva b,2, 

J.D. Maitland Wright C,*, Kari Ylinen d 

a Departamento de Análisis Matemático, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain 
b Departamento de Análisis Matemático, Facultad de Matemáticas, Universidad Complutense de Madrid, 

28040 Madrid, Spain 

Abstraet 

e Mathematical Sciences, University of Aberdeen, Aberdeen AB 24 3 U E, Scotland, U K 
d Department of Mathematics, University ofTurku, F1N-200l4 Turku, Finland 

Let X be a Banach space, Then there is a locally convex topology for X, the "Right topology," such that 
a linear map T, from X into a Banach space Y, is weakly compact, precisely when T is a continuous map 
from X, equipped with the "Right" topology, into Y equipped with the norm topology, When T is only 
sequentially continuous with respect to the Right topology, it is said to be pseudo weakly compacto This 
notion is related to Pelczynski's Property (V). 

Keywords: Weakly compact operators; Right topology; Mackey topology 

* Corresponding author. 
E-mail addresses:aperalta@ugr.es(A.M.Peralta).ignaciov@mat.ucm.es(lVillanueva).j.d.m.wright@abdn.ac.uk 

(J.D.M. Wright), ylinen@utu.fi (K. Ylinen). 
1 Partially supported by I+D MCYT project no. MTM 2005-02541 , and Junta de Andalucía grant FQM 0199. 
2 Partially supporled by I+D MCYT project no. MTM2004-01308. 



969 

1. Introduction 

One of the fundamental theorems of c1assical measure theory is the Riesz Representation The­
orem, first obtained for scalar and then for vector measures. Let T be a compact Hausdorff space; 
e (T) fue space of complex valued continuous functions on T; B(T) fue space of bounded Baire 
measurable functions on T, Y a Banach space and T : e (T) -+ Y a bounded linear operator. The 
generalised Riesz Representation Property holds for T if fuere exists an operator Too : B (T) -+ Y 
which extends T and is such fuat, when Un) (n = 1,2, ... ) is a bounded sequence in B(T) which 
converges pointwise to J, fuen 11 TOO Un) - TooU) 11 -+ O. When Y is finite dimensionalfuen fuis 
is true for any T, but for a general Banach space, Y, fue appropriate condition needed is fuat T 
be a weakly compact operator. 

So, when generalising vector measure theory to non-commutative C* -algebras, a natural set­
ting was to consider a weakly compact operator T mapping a C* -algebra A into a Banach 
space Y. From considerations of non-commutative measure theory we have been led to the obser­
vation that weakly compact operators from a fixed Banach space X to an arbitrary Banach space 
y can be characterised by a continuity property. More precisely, fuere is a "Right topology" 
for X, such that, a linear map froID X into Y is weakly compact precisely when it is a continu­
ous map from X, equipped wifu fue Right topology, into Y, equipped wifu fue norm topology. 
There are several different ways of proving the results below. We have strived for maximal c1arity 
rather than maximal generality or maximal conciseness; but the arguments are brief. We present 
proofs which use only well-known c1assical results in Functional Analysis; more sophisticated 
approaches are possible. In a sequel to this note we will give a number of applications, generali­
sations and further developments, particularly for the multi-linear situation. 

When T is only sequentially continuous wifu respect to fue Right topology, it is said to be 
pseudo weakly compacto When a Banach space X has fue property fuat every pseudo weakly 
compact operator from X to another Banach space is weakly compact, then X is said to be 
sequentially Right. It is proved fuat every Banach space possessing Pe!czynski's Property (V) 
must be sequentially Righ!. 

2. Continuity from the Right topology to the norm topology 

In this note let X and Y be Banach spaces. Whenever Z is a Banach space, let its c10sed unit 
ball be denoted by 21. Let us recall [4,6]fuat fue Mackey topology for fue dual pair (X**, X*) 
is the topology of uniform convergence on sets K e X*, where K is absolutely convex and 
(J (X*, X**) compacto That is, where K is a weakly compact, absolutely convex subset of fue 
Banach space X*. We denote fuis topology by r(X**, X*); it is fue finest locally convex topology 
for fue dual pair (X**, X*). We identify X wifu its canonical embedding in X** and call fue 
relative topology induced on X by r (X**, X*), fue "Right topology" for X. See also [5], where 
fuis topology is discussed and called fue "quasi-Mackey topology." We shall call a linear map 
between Banach spaces an operator precisely when it is bounded. 

Theorem 1. Let T: X --+ Y be a weakly compact operator. Then T** is a continuous map 
Jmm X**, equipped with the r(X**, X*) topology, into Y, equipped with the norm topology. 

Proof. Let K be fue weak closure of T*[Y¡*] in X*. By Ganlmacher's fueorem [3], T*: Y* -+ X* 
is weakly compac!. So K is an absolutely convex, weakly compact se!. That is, K is (J (X*, X**) 
compact. 



970 

Lel (x,J be a nel in X** which converges lo x in lhe Mackey lopology, r(X**, X*). Then 
(x) - x) -+ O, uniformly on K, and hence 

Bul 

sup { 1 ((x). - x), 1» 1: 1> E K} ;? sup{ 1 ( (x). - x), T* ¡ff) 1: ¡ff E yn 
= sup{ I(T**(x). - x), ¡ff)I: ¡ff E y:} = 11 T**(x). - x) 11· 

Therefore 11 T** (x). - x) 11 -+ O. D 

Lemma 2. Let (y») be a Cauchy net in a Banach space Y. Ihen there exists z E Y such that 
11 y). - zll -+ O. 

Proof. See [3, p. 28]. D 

Lernma 3. Let T : X --+ Y be a linear map whose restriction faX 1 is a continuous map from XI, 
equipped with the topology induced by the Right topology on X, into Y, equipped with its norm 
topology. Ihen T is a bounded linear operator. 

Proof. Lel (xn ) be a sequence in X which converges in norm lo O. Lel K be any absolulely 
convex, weakly compacl subsel of X*. Then K is bounded in norm. So (xn) -+ O uniformly 
on K. Thus (xn) -+ O in lhe Righttopology. Since Ilxn 11 -+ O, Ilxn 11 oc:; 1 for a11 sufficienUy large n. 
So 11 T X n 11 -+ O. Thus T is a bounded linear operalor. D 

Theorem 4. Let T: X --+ Y be a linear map whose restriction fa Xl is a continuous map from Xl, 
equipped with the topology induced by the Right topology on X, into Y, equipped with its norm 
topology. Ihen T is weakly compacto 

Proof. By Lemma 3, T is a bounded linear operalor from X inlo Y. As usual, we identify X wilh 
ils canonical image in X**. Then, by Goldsline's lheorem, see [3, p. 424], X¡ is a (X**, X*) 
dense in X~*. Since r(X**, X*) is a 10ca11y convex lopology for lhe dual pair (X**, X*), lhe 
closure oflhe convex sel X¡ in lhe r(X**, X*)-lopology is also X~*, see [3, Cora11ary 14, p. 418]. 
So, given any z E X~*, there exists a net (x)J in Xl which converges to z in the T (X**, X*) 
lopology. So lhis nel also converges lo z in lhe a (X**, X*) lopology. 

Fix ¡ff in Y*, lhen T*¡ff E X*. So (x), T*¡ff) -+ (z, T*¡ff), and lhus (T**x)., ¡ff) -+ (T**z, ¡ff). 
Hence (Tx)., ¡ff) -+ (T**z, ¡ff). 

Fix & > O. Then, because of lhe continuily hypolhesis imposed on T, lhe sel T-¡ {y: 
Ilyll <s) nx¡ is oflheform V nx¡, where V is openin lhe r(X**,X*) lopology. Solhere 
exisls U such lhal U is an open absolulely convex neighbourhood of O in r (X**, X*) and 
U e V. Then ~U is an open absolulely convex neighbourhood of O, in lhe r(X**, X*) lopol­

ogy. So there exists AO such that A ) AO implies XA - Z E ~ U. Thus, for fL ) AO and A ) AO, 

x). - xl'(x). - z) + (z - xl') E U. So ~(x). - xl') E un Xl. Hence 11 T (x). - xl') 11 oc:; 2&. It fo11ows 
fram lhis lhal (T x») is a Cauchy nel (wilh respectto lhe norm) in Y. So il converges in norm lo an 
elemenl of Y, say W. Bul il fo11ows fram lhe preceding paragraph lhal w = T**z. So T**z E Y. 
Hence T** maps X** into Y, which implies that T is weakly compact. D 



971 

Corollary 5. Let T: X -+ Y be a linear map. Ihen the following conditions are equivalent: 

(i) T is continuous from X, equipped with the Right topology, into Y, equipped with the norm 
topology. 

(ii) T is continuous fmm Xl, equipped with the relative topology induced by the Right topology, 
into Y, equipped with the norm topology. 

(iii) T is weakly compacto 
(iv) T is a bounded linear operator and T**: X** --+ Y** is continuous from X**, equipped 

with the r (X**, X*) topology, into Y** equipped with the norm topology. 

Proof. (i) implies (ii) trivially and (ii) implies (iii) by Theorem 4. Suppose lhat (iii) holds. Then 
T maps Xl into a weakly compact subset of Y. But weakly compact subsets of a Banach space 
are norm bounded. So T is a bounded linear operator. Hence T** is well defined. By well-known 
praperties of weakly compact operators, T** maps X** into Y. Then (iv) follows by applying 
Theorem 1. Clearly (iv) implies (i). D 

Corollary 6. A Banach space X is reflexive if, and only if, the Right topology for X coincides 
with the norm topology. 

Proof. The space is reflexive precisely when lhe identity map on X is weakly compact (see [3, 
Theorem VA.7]). By Corallary 5 lhe identity map is weakly compact, if, and only if, lhe norm 
topology is weaker lhan lhe Right topology. Since lhe Right topology is always weaker lhan lhe 
norm topology, lhe result follows. D 

3. Pseudo weakly compact operators 

By the Eberlem Smulian theorem we know that weak compactness is, in sorne sense, a sequen­
tial property. We have already seen in the previous section that a (bounded) linear operator, T, 
between two Banach spaces, X and Y, is weakly compact if and only if it is continuous from X, 
equipped wilh lhe Right topology, into Y, equipped wilh lhe norm topology. Clearly such an op­
erator T is sequentially continuous fram X, equipped wilh lhe Right topology, into Y, equipped 
with the norm topology. It is natural to ask if the converse is true. 

Definition 7. Let X and Y be Banach spaces. Let T: X -+ Y be a linear map such lhat, when 
X n -+ O in lhe Right topology lhen 11 T X n 11 -+ O. Then we call T pseudo weakly compac!. 

By (lhe praof ol) Lemma 3, a pseudo weakly compact linear map belween Banach spaces is 
always a bounded operator. 

The following example shows that, in general, not every pseudo weakly compact operator is 
weakly compac!. 

Example 8. Let T be lhe identity map fram el onto el. Since el is not reflexive, its unit ball is 
not weakly compact, see [3, Theorem VA.7]. So T is not a weakly compact operator. 

On lhe olher hand, when X n -+ O, in lhe Righttopology lhen X n -+ O, in lhe r ((e 1 )** , (e 1 )*)­
topology. So X n -+ O, in lhe a((ll)**, (el)*)-topology. Hence X n -+ O, in lhe weak topology 
of el. But, by [3, Iv'S.14], lhis implies lhat X n -+ O, in lhe norm topology, lhat is, IITxnl1 -+ O. 
So T is pseudo weakly compac!. 



972 

When X is a C* -algebra, then its second dual, X**, can be identified with the von Neumann 
envelope of X, when X is represented on its universal representation (Hilbert) space [7]. Then 
if follows fram a fundamental result of Akemanu, see Theorem I1I.5.7 in [7], lhat when lhe (J­

strong* topology of X** is restricted to the unit ball of X, it coincides with the restriction of the 
Right topology to Xl (see [7] for lhe basic definitions). In [8] lhe last two aulhors of lhe present 
note introduced the notion of quasi completely continuous linear operators from a C* -algebra 
into a Banach space. It tums out that an operator from a C* -algebra into a Banach space is quasi 
completely continuous if, and only if, it is pseudo weakly compact. This is because, as remarked 
above, lhe Right topology coincides, on lhe norm closed unít ball, wilh lhe restriction of lhe 
(J-strong* topology of X**. Therefore, fram [8, Proposition 2.2], for a linear operator T fram a 
C* -algebra into a Banach space the following are equivalent: T is weakly compact; T is quasi 
completely continuous; T is pseudo weakly compact. 

The following definition now makes sense. 

Definition 9. A Banach space X is said to be sequentially Right if every pseudo weakly compact 
operator on X is weakly compact; in other words, if each operator on X which is sequentially 
continuous wilh respect to lhe Right topology is also continuous wilh respect to lhe Right topol­
ogy. 

We have already seen in lhe above paragraph lhat every C* -algebra is sequentially Righ!. 
Clearly every reflexive Bauach space is also sequentially Righ!. We also know lhat every Bauach 
space can be embedded isometrically in the commutative C* -algebra of weak* continuous func­
tions on its dual ball. So it follows fram Example 8, lhat lhe praperty of being sequentially Right 
is not inherited by subspaces. But we do have lhe following resul!. 

Proposition 10. Every closed complemented subspace of a sequentially Right Banach space is, 
itself, sequentially Right. 

Proof. Let X be a sequentially Right Bauach space and let Z be a closed subspace of X forwhich 
lhere exists a bounded prajection P fram X onto Z. Let Y be a Bauach space aud T: Z -+ y 
a pseudo weakly compact operator. We show lhat T P : X -+ Y is pseudo weakly compac!. Let 
(xn ) be a sequence in X converging to O wilh respect to lhe Right topology of X. Let A be au 
absolutely convex (J (Z*, Z**)-compact subset of Z*. Writing W = p- l ({O)), we cau identify X 
(via a topological isomorphism) wilh lhe el-direct sum W E!l Z, aud lhen X* is identified wilh lhe 
eco -direct sum W* EB Z* and X** with the ¡l-direct sum W** EB Z**. Then the sequence (P xn ) 

in Z converges to O wilh respect to lhe Right topology of Z, because any absolutely convex 
a (Z*, Z**)-compact set A e Z* can be interpreted as a a (X*, X**)-compact subset of X* = 

W* E!l Z*, aud we have {Pxn , 10) = {xn, 10), uniformly on 10 EA. Hence limn~co liT PXn II = O. 
Thus T P is pseudo weakly compact on X, aud so it is weakly compact by assumption. But lhe 
restriction of T P to Z is T which is lherefore weakly compacto D 

Corollary 11. Every closed complemented subspace of a C* -algebra is sequentially Right. 

Lemma 12. Let T be a linear operator between two Banach spaces X and Y. Then T is Right­
Right continuous if, and only if, it is bounded. 



973 

Proof. Suppose first lhat T: X -+ Y is a bouuded linear operator. Let (x,J be a Right-null net 
in X. We shall see lhat T(x,J is Right-null in Y. Indeed, let K be an absolutely convex weakly 
compact subset of Y*, that is, a a (Y*, Y**)-compact subset in Y*. Since T* : Y* --+ X* is weakly 
continuous, it follows lhat T*(K) is an absolutely convex a (X*, X**)-compact subset in X*. Let 
K¡ = T*(K). Since (x,J -+ O in lhe Right-topology of X, we have 

0= lim sup I ¡ff(x,J I = lim supIT*(1))(x,JI = lim supl1>(T(x,J)I, 
J...-+co 1/rEKl A--+oofEK A--+oofEK 

which shows lhat (T (x,J) -+ O, uniformly on K, and hence (T (x,J) -+ O, in lhe Right-topalogy 
of Y. This proves lhat T is Right-Right continuous. 

To prove the converse, let liS first assume that every Right-null sequence in Y is norm-null. 
We shall prove lhat, in lhis case, every Right-Right continuous operator T: X -+ Y is bounded. 
Let (xn ) be a norm-null sequence in X. Clearly (xn ) is Right-null in X and, by lhe hypolhesis 
on T, we have T (xn) -+ O in lhe Right topology of Y, lherefore, by lhe assumptions on Y, 
11 T (xn ) 11 -+ O. 

We notice that in every refiexive Banach space every Right-null sequence is norm-null, this 
is because lhe closed unit ball of its dual space is absolutely convex and weakly compacto Sup­
pose lhat T : X -+ Y is a linear operator which is Right-Right continuous. From lhe first par! 
of lhe proof, for each 1> in Y*, we have 1> Right-Right continuous. Therefore, 1> o T : X -+ ]K 

is a Right-Right continuous operator and hence, from the comments preceding this paragraph, 
1> o T is oounded. This proves lhat T is weakly continuous and so, by lhe Uniform Boundedness 
Theorem, bounded. D 

Let X be a Banach space. A series Ln Xn in X is called weakly unconditionally Cauchy 
(w.u.C.) if lhere exists C > O such lhat for any finite subset F e PI and Sn = ±1, we have 
11 LnEF SnXn 11 oc:; C. From [2, Exercise 2, p. 113] we know lhat when Ln Xn is a w.u.c. se­
ries in X, and (en) is the canonical basis of ca, then there exists a unique bounded linear operator 
U: Co --+ X, mapping each en to X n. Since (en) is bounded and a-strong*-null in (the commuta­
tive C*-algebra) co, and lhe a-strong* and lhe Right topalogies coincide on lhe c10sed unit ball 
of co, it follows lhat (en) tends to O in lhe Right topology of Co. Now Lemma 12 tells us lhat 
(U (en» = (xn) is Right-null. We have proved: 

Lemma 13. Let X be a Banach space and Ln Xn a w.u.C. series in X. Ihen (xn) is a Right-null 
sequence in X. 

Let X and Y be Banach spaces and T: X -+ Y a linear mapping. We say lhat T is uncondi­
tionally converging if, for every w.u.c. series Ln Xn in X, the series Ln T (xn) is unconditionally 
convergent. 

Proposition 14. Every pseudo weakly compact operator between two Banach spaces is uncon­
ditionally converging. 

Proof. Let T: X -+ Y be a pseudo weakly compact operator. Let us assume lhat T is not 
unconditionally converging, lhen, by [2, Exercise 8, p. 54], T fixes a copy of Co. Thus lhere 
exist bounded linear operators U: co --+ X and V: Y --+ co, such that VT U is an isomorphism. 
Let (en) denote the canonical basis of ca. From Lemma 13 and the comments preceding it, 
U (en) = Xn is a (bounded) Right-null sequence in X and T (xn) does not tend to O in lhe norm 



974 

topology of Y. However fuis contradicts T being pseudo weakly compact, fuerefore, T must be 
unconditionally converging. D 

Let us recal! fuat a Banach space X is said to have Pe!czynski's Property (V) if, for every 
Banach space Y, every unconditionally converging operator is weakly compact. We c1early have: 

Corollary 15, Every Banach space satisfying property (V) is sequentially Right. 

Since every JB*-triple satisfies property (V) (see [1]), we obtain: 

Corollary 16, Every JB*-triple is sequentially Right. 

Acknowledgment 

Peralta and Ylinen thank Wright for making them wannIy welcome in their separate visits to Aberdeen in the process 
of writing this papero 

References 

[1] Ch.-H. Chu, P. Mellan, JB *-triples have Pelczynski's Property V, Manuscripta Math. 93 (3) (1997) 337-347. 
[2] J. Diestel, Sequences and Series in Banach Spaces, Grad. Texts in Math., vol. 92, Springer, New York, 1984. 
[3] N. Dunford, J.T. Schwartz, Linear Operators (vol. I), Interscience, New York, 1967. 
[4] G. Kothe, Topological VectorSpaces, Springer, 1969. 
[5] J. Qiu, Local completeness and dual local quasi-completeness, Proc. Amer. Math. Soco 129 (2000) 1419-1425. 
[6] A.P. Robertson, W.J. Robertson, Topological Vector Spaces, Cambridge University Press, 1973. 
[7] M. Takesaki, 1beory of Operator Algebras 1, Springer, New York, 1979. 
[8] J.D.M. Wright, K. Ylinen, Multilinear maps on products of operator algebras, J. Math. Anal. Appl. 292 (2004) 558-

570. 


	2007Topologicalcharacterization_Página_1
	2007Topologicalcharacterization_Página_2
	2007Topologicalcharacterization_Página_3
	2007Topologicalcharacterization_Página_4
	2007Topologicalcharacterization_Página_5
	2007Topologicalcharacterization_Página_6
	2007Topologicalcharacterization_Página_7