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The question in this post is the question below from an article by Rodriguez & Rueda Zoca [1]. enter image description here


Below is a complimentary salad/side dish that accompanies the main course.

Let $B^2(X,Y)$ denote the space of all $\mathbb{C}$-valued bounded bilinear forms. We identify $B^2(X,Y)$ with $(X\mathbin{\hat{\otimes}_\pi}Y)^*$ as it's customary, and use the notation $$\beta(x\otimes y) = \beta(x,y)\hspace{6mm} x\in X,\ y\in Y$$ for $\beta\in (X\mathbin{\hat{\otimes}_\pi}Y)^*$. Let $B^2_f(X,Y)$ denote the set of the bilinear maps of finite rank, and $B^2_{wsc}(X,Y)$ denote those that are weakly sequentially continuous.

Let $A\subset X\mathbin{\hat{\otimes}_\pi}Y$ be the set of weak limit points of the sequences of the form $(x_n\otimes y_n)$, where both $(x_n)$ and $(y_n)$ are weakly null. Let $A^\perp$ be the set of all $\beta\in (X\mathbin{\hat{\otimes}_\pi}Y)^*$ such that $\beta(A)=\{0\}$. Clearly $A^\perp$ is weak$^*$-closed and $$B^2_f(X,Y) \subset B^2_{wsc}(X,Y) \subseteq A^\perp$$ Thus, an equivalent formulation of Question 3.9 is

$A^\perp = B^2(X,Y)$?

As exemplary partial answers with an extra condition: if $X$ or $Y$ has the Dunford-Pettis property (DPP), then $B^2_{wsc}(X,Y) = B^2(X,Y)$ (the latter condition is weaker than DPP), so yes. Clearly it is enough to merely assume that $B^2_{wsc}(X,Y)$ is weak$^*$ dense in $B^2(X,Y)$.

If $X$ or $Y$ has the approximation property (AP), then $B^2_f(X,Y)$ is weak$^*$ dense in $B^2(X,Y)$, so the answer is yes.

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  • $\begingroup$ It is true if the map from the projective tensor product is replaced by the injective tensor product (e.g., reduce to the case where the sequences are Schauder bases). Thus it is true if the map from the projective tensor product to the injective tensor product is one-to-one. So for a counterexample you need spaces that fail the approximation property. $\endgroup$ Commented Feb 12 at 20:44
  • $\begingroup$ Professor Johnson, thank you, this is absolutely true. $\endgroup$
    – Onur Oktay
    Commented Feb 18 at 10:16

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