O. IntroductionIf E is a nuclear space, Grothendieck [1] proved that for every 0-neighborhood U in E, there is an absolutely convex 0-neighborhood V in E, Vc U such that •v is norm-isomorphic to a subspace of l ~ (whatever pE[1, oo)). As usual, Ev is the completion of the linear space I~)=E/A n-iV normed with the gauge of V.n If E is the nuclear space s of rapidly decreasing sequences and F is any infinitedimensional Banach space with Schauder basis, then for every 0-neighborhood U in E there exists an absolutely convex 0-neighborhood Vc U in E such that /~v is norm-isomorphic to F (see [6]). This result of Saxon was improved later on by Valdivia [8] proving its validity when E is an arbitrary nuclear space and F any infinite-dimensional separable Banach space.In the present paper we bring up these results into the context of 2-nuclearity. Namely, we will prove that for certain sequence spaces 4, a Mackey space a can be found satisfying the following condition: If F is any infinite-dimensional Banach space with Schauder basis, for every 0-neighborhood U in o-there is an absolutely convex 0-neighborhood V in o-such that ffv is norm-isomorphic to F. As a consequence we prove an embedding theorem of k-nuclear spaces into some product of any given infinite-dimensional Banach space with Schauder basis. This research, supported by the University of Extremadura, was carried out during the author's visit to the University of Kaiserslautern (F.R.G.).
Definitions and previous resultsThe linear sequence spaces 2 we will deal to are assumed throughout to be normal, additive, decreasing rearrangement invariant and such that 2ci q for some q>0 (see [4] for definitions). The diametral dimension of a Hausdorff locally convex space E, denoted by A (E), is the collection of all sequences (6n) of nonnegative numbers such that for every 0-neighborhood U in E there is a 0-neighborhood V in E, VcU such that On(V, U)<=M6n for every nEN and for some M_->0. Here 6n(V, U) is the n-th Kolmogoroff diameter of V with respect to U. This notion of diametral dimension, different of that of Terzioglu [7], fits better in our context of 2-nuclearity which is defined as follows: A Hausdorff locally convex space E is said to be 2-nuclear if for each p>0 the following condition holds: For every 0-neighborhood U in E there is a 0-neighborhood V in E, Vc U such that (~,(V, U))E2P (a sequence (~n) is in 2p iff (~) is in 4).Following [4] we denote by 4 + the subset of all sequences (~,) in 2 such that 1"