For (algebraic) tensor products, it is well-known that the functor $A\otimes_R \cdot:Mod_R\rightarrow Mod_R$ is only (left-) exact when $A$ is a flat $R$-module. In particular, all vector spaces are flat. What happens in the continuous (archimedean) setting?:

Let $B$ be a separable infinite-dimensional Banach space and suppose that $ f:E\rightarrow F, $ is a continuous linear injective map from a separable nuclear space $E$ to a separable Banach space $F$, both infinite-dimensional (if it matters). Let $\otimes_{\epsilon}$ denote the injective tensor product of LCS and let $\hat{\otimes}_{\epsilon}$ denote its completion.

Is the map $ 1_{B}\hat{\otimes}_{\epsilon} f: B\hat{\otimes}_{\epsilon} E \rightarrow B\hat{\otimes}_{\epsilon} F, $ a continuous linear 1-1 map also?

Related: This post is related to this unanswered post.

  • $\begingroup$ Do you mean just the injective tensor product or the completed injective tensor product? $\endgroup$ Commented Jun 30, 2020 at 9:59
  • $\begingroup$ @JochenWengenroth (I made the correction) but indeed I'm interested in the completed injective tensor product. $\endgroup$
    – ABIM
    Commented Jun 30, 2020 at 10:05

1 Answer 1


This is always true (without nuclearity): If $T_j:E_j\to F_j$ are continuous linear maps between Hausdorff locally convex spaces and $E_2$ is complete then $$ T_1\hat\otimes_\varepsilon T_2: E_1 \hat\otimes_\varepsilon E_2 \to F_1\hat\otimes_\varepsilon F_2$$ is injective if so are $T_1$ and $T_2$. This is 16.2.2 in Jarchows's book Locally Convex Spaces.

  • $\begingroup$ Thank you very much Jochen. Out of curiousity I'm assuming the result fails if the completion is not taken? $\endgroup$
    – ABIM
    Commented Jun 30, 2020 at 14:58
  • 2
    $\begingroup$ No, without the completions it is much easier. $\endgroup$ Commented Jun 30, 2020 at 15:10
  • $\begingroup$ I just wanted to mention that I really enjoyed your book "Derived Functors in Functional Analysis" :) I had read it during my PhD some years ago $\endgroup$
    – ABIM
    Commented Jun 30, 2020 at 18:50
  • $\begingroup$ Thank you...... $\endgroup$ Commented Jun 30, 2020 at 20:54

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