The problem of countably quasi-barrelledness of weighted spaces of continuous functions, of which there are no results in the general setting of weighted spaces, is tackled in this paper. This leads to the study of quasi-barrelledness of weighted spaces in which, unlike that of Ernst and Schnettler (1986), though with a similar approach, we drop the assumption that the weighted space has a fundamental sequence of bounded sets. The study of countably quasi-barrelledness of weighted spaces naturally leads to definite results on the weighted (DF)-spaces for those weighted spaces with a fundamental sequence of bounded sets.2000 Mathematics Subject Classification: 46A08, 46E30.
Introduction and notations.The countably barrelledness, countably quasi-barrelledness, barrelledness, and quasi-barrelleness of the space C(X) of continuous functions on a completely regular Hausdorff space X equipped with the compact open topology (c-op), including when it is a (DF)-space and (gDF)-space, is well known (cf. [7,13]). In the more general setting of weighted spaces, Ernst and Schnettler [3] studied the (gDF) and the quasi-barrelledness of weighted spaces by constructing a Nachbin family on X which is based on the assumption that the weighted space has a fundamental sequence of bounded sets. A weighted space need not have a fundamental system of bounded sets. There are classical examples of such spaces, for example, let X be a noncompact locally compact and σ -compact space, and let C(X) be the space of all continuous real-valued functions on X equipped with the compact-open topology. C(X) is a weighted space [14], metrizable and not normable [11, Observation 10.1.25] and hence it does not have a fundamental sequence of bounded sets [6]. See also [10, page 9] for another example. Following the same approach, the countably barrelledness and barrelledness of weighted spaces was studied in [10] without assuming that the weighted space has a fundamental sequence of bounded sets. This paper is a follow up of [10] although we have made it independent of it. We characterise countably quasi-barrelled (Section 3) and quasi-barrelled (Section 4) weighted spaces by a constructed Nachbin family on X without assuming that the spaces have a fundamental sequence of bounded sets. This approach makes the study of (DF)-weighted spaces easy as it can be seen in Section 5. In Section 6, we show that every countably quasi-barrelled weighted space satisfies the countable neighborhood property (cnp).As an application, we recover the known results for the countably quasi-barrelledness and quasi-barrelledness of C(X) equipped with compact open topology (Corollary 3.10) and at the same time get new results for the space of bounded continuous