やがてめぐになる

さんすうとえいごのおべんきょうブログ。ほかにもかきたいことをかきます。

Is the conjugate of a dense and continuous embedding a dense and continuous one as well?

I make a note about the title because it took a hard time finding a proof.
This topic relates to an abstract Wiener space in the Malliavin calculus.

Proposition. Let  B be a real separable Banach space and  H be a real separable Hilbert space embedded densely and continuously to  B. Then,  B^* is densely and continuously embedded to  H^*.

Proof. Let  \iota be a continuous linear injection from  H to  B such that  \iota(H) is dense in  B. In order to see that  \iota^*(B^*) is dense in  H^*, it suffices to show that for  h\in H^{**},  h|_{\iota^*(B^*)}=0 implies  h=0, where  \iota^* represents the conjugate of  \iota.
In what follows,  \langle\cdot,\cdot\rangle_{H} represents the inner product of a (general) Hilbert space  H and the natural bilinear form between a (general) Banach space  B and its conjugate  B^* is denoted by  {}_B\langle \cdot,\cdot \rangle_{B^*}. For simplicity, the conjugate space  H^* is identified with  H by the Riesz representation theorem, and corresponding elements are expressed as the same symbols.
Suppose  h\in H^{**} and  {}_{H^*}\langle\iota^*\ell,h\rangle_{H^{**}}=0 for any  \ell\in B^*. Since  {}_{H^*}\langle\iota^*\ell,h\rangle_{H^{**}}=\langle h, \iota^*\ell \rangle_{H^*}={}_{B}\langle \iota h, \ell \rangle_{B^*}, the assumption  {}_{H^*}\langle\iota^*\ell,h\rangle_{H^{**}}=0 implies
\begin{align*}
& {}_{B}\langle \iota h, \ell \rangle_{B^*} = 0. \tag{1}
\end{align*}
Now we set the continuous linear functional  \tilde{\ell}_h defined on  \iota(H) as follows:
\begin{align*}
\iota(H)\ni\iota g \mapsto \langle g, h \rangle_{H^*}\in\mathbb{R}.
\end{align*}
Since  \iota(H) is dense in  B, for any  x\in B, there is a sequence  (g_n)_n in  H such that  \iota g_n \to x as  n\to \infty in  B. Thus, we get  \ell_h\in B^*, by defining, for  x\in B,  {}_B\langle x,\ell_h\rangle_{B^*}:=\lim_{n\to\infty}\tilde{\ell}_h(\iota g_n). We note that the definition of  \ell_h is independent of the manner to choose the sequence  (g_n)_n. Then, putting  \ell=\ell_h in (1) yields  h=0.  \Box