Proposition 13.8.2.label Let $E$ be a locally convex space over $K \in \RC$. If there exists a fundamental system of convex and circled neighbourhoods $\fB \subset \cn_{E}(0)$ such that for each $V \in \fB$, $\wh E_{V}$ has the approximation property, then $E$ has the approximation property.

Proof. Let $V \in \fB$, $\pi_{V}: E \to \wh E_{V}$ be the canonical projection, and $A \subset E$ be precompact, then $\pi_{V}(A)$ is precompact as well. Since $\wh E_{V}$ has the approximation property, there exists $T \in E_{V}^{*} \otimes \wh E_{V}$ such that $Tx - x \in \pi_{V}(V)$ for all $x \in \pi_{V}(A)$. As $E_{V}$ is dense in $\wh E_{V}$, there exists $S \in E_{V}^{*} \otimes E_{V}$ such that $Sx - Tx \in \pi_{V}(V)$ for all $x \in \pi_{V}(A)$. In which case, $Sx - x \in 2\pi_{V}(V)$ for all $x \in \pi_{V}(A)$, and $S \circ \pi_{V}(x) - \pi_{V}(x) \in 2\pi_{V}(V)$.

Write $S = \sum_{j = 1}^{n} \phi_{j} \otimes y_{j}$. For each $1 \le j \le n$, choose any representative $x_{j} \in \pi_{V}^{-1}(y_{j})$, then for any $x \in A$,

\[\pi_{V} \braks{x - \sum_{j = 1}^n x_j\dpn{x, \phi_j \circ \pi_V}{E}}= S \circ \pi_{V}(x) - \pi_{V}(x) \in -2\pi_{V}(V) = 2\pi_{V}(V)\]

Finally, since $\ker(\pi_{V}) = \bigcap_{\lambda > 0}\lambda V \subset V$, $x - \sum_{j = 1}^{n} x_{j}\dpn{x, \phi_j \circ \pi_V}{E}\in -3V = 3V$. Therefore if $R = \sum_{j = 1}^{n} (\phi_{j} \circ \pi_{V}) \otimes x_{j} \in E^{*} \otimes E$, then $Rx - x \in 3V$.$\square$

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