Let ^{X, E) be the space of continuous functions from the completely regular Hausdorff space X into the Hausdorff locally convex space E, endowed with the compact-open topology. Our aim is to characterize the ^(X, E) spaces which have the following property: weak-star and weak sequential convergences coincide in the equicontinuous subsets of ^(X, E)'. These spaces are here called Grothendieck spaces. It is shown that in the equicontinuous subsets of E' the σ(E', E)-and β(E', ^-sequential convergences coincide, if ^(X, E) is a Grothendieck space and X contains an infinite compact subset. Conversely, if X is a G-space and E is a strict inductive limit of Frechet-Montel spaces ^(X, E) is a Grothendieck space. Therefore, it is proved that if £ is a separable Frechet space, then E is a Montel space if and only if there is an infinite compact Hausdorff X such that , E) is a Grothendieck space.1. Introduction. In this paper X will always denote a completely regular Hausdorff topological space, E a Hausdorff locally convex space, and &( X, E) the space of continuous functions from Xinto E, endowed with the compact-open topology. When E is the scalar field of reals or complex numbers, we write ^(X) instead ^( X, E).It is well known that Ή{X, E) is a Montel space whenever ^(X) and E so are, hence, if and only if X is discrete and E is a Montel space (see [5]
, [16]).We study what happens when X has the following weaker property: the compact subsets of X are G-spaces (see below for definitions).We obtain in Theorem 4.4 that if £ is a Frechet-Montel space and X has that property, then ^( X, E) is a Grothendieck locally convex space. The key in the proof is the following fact: every countable equicontinuous subset of ^(X, E)' lies, via a Radon-Nikodym theorem, in a suitable L\τ 9 Eβ). As a consequence of a theorem of Mύjica [10], the same result is true when E is a strict inductive limit of Frechet-Montel spaces.In §3 we study the converse of 4.4. In Corollary 3.3 it is proved that if X contains an infinite compact subset, E is a Frechet separable space and #( X, E) is a Grothendieck space, then E is a Montel space. This property characterizes the Montel spaces among the Frechet separable spaces.