TITLE: Vector valued disc "algebra"
QUESTION [5 upvotes]: I am interested in a vector-valued form of the disc "algebra" (which in this setting is not in general an algebra, hence the scare quotes).  Let $E$ be a Banach space, and let $A(\mathbb D,E)$ be the space of bounded continuous functions $f:\overline{\mathbb D}\rightarrow E$ which are holomorphic on $\mathbb D$.

Has this space been systematically studied?

I am particularity interested in different continuity assumptions.

If $f$ is only weakly-continouous, i.e. $\mu\circ f\in A(\mathbb D)$ for each $\mu\in E^*$, is there a counter-example showing that $f$ need not be norm continuous?

If $E = F^*$ is a dual space, we can ask the same question with $\mu\in F$, i.e. consider weak$^*$-continuity.
There are of course different notions of Banach-space valued Holomorphic functions (weakly or weak$^*$-holomorphic) but it is well-known that these all agree and imply norm analytic.  Thus it is continuity at the boundary which is of interest.  Probably it is just my lack of knowledge about the classical disc algebra which is a problem, and so I'm hoping a few references might help me?

REPLY [2 votes]: I believe the following provides a counter-example for "weak implies norm continuity" in the case $E=c_0$.
Given any $F:\overline{\mathbb D} \rightarrow c_0$ let $F_n:\overline{\mathbb D} \rightarrow \mathbb C$ be the $n$th coordinate.  If $F$ is weakly continuous on $\overline{\mathbb D}$ and analytic on $\mathbb D$, then each $F_n$ is a member of $A(\mathbb D)$ and $\|F_n\|_\infty\leq K$ for some $K$ independent of $n$.  Conversely, if these two conditions hold on the $F_n$ then for any $a=(a_n)\in\ell^1 = c_0^*$ we have that
$$ \newcommand{\ip}[2]{\langle{#1},{#2}\rangle}\ip{a}{F(z)} = \sum_n a_n F_n(z) $$
and so $z\mapsto \ip{a}{F(z)}$ is in $A(\mathbb D)$ as it is the absolutely convergent sum of members of $A(\mathbb D)$.  By properties of functions in Hardy spaces (see e.g. page 50 of these notes) we know that
$$ F_n(\xi) = \int_{\mathbb T} \frac{z F_n(z)}{z-\xi} \ dz \qquad (\xi\in\mathbb D). $$
We now seek to construct $F$ by constructing a suitable sequence $(F_n)$.  That $F(z)\in c_0$ for all $z$ implies that $F_n(z)\rightarrow 0$ pointwise.  Under the assumption that $\|F_n\|_\infty\leq K$ for all $n$, for fixed $\xi\in\mathbb D$ we can apply the dominated convergence theorem to the integral above to see that $F_n(z)\rightarrow 0$ pointwise for $z\in\mathbb T$ implies that $F_n(\xi)\rightarrow 0$.
We shall use Outer functions (page 35 in the notes linked above).  Choose a positive function $h$ on $\mathbb T$ with $\log h$ being integrable; then we can form an outer function $F$ with $|F|$ having nontangential limit $h$ at the boundary, almost everywhere.  It is not clear to me that just because $h$ is continuous it will follow that $F$ is in $A(\mathbb D)$ instead of just a Hardy space; so we work a bit harder.
Firstly, choose $t\mapsto h_n(e^{it})$ to be the piecewise linear function with $h_n(1) = h_n(e^{i2\pi/n})=1/n$ and $h_n(e^{i\pi/n})=1$.  Let $F_n$ be the Outer function with $|F_n|$ having nontangential limit $h_n$ on the boundary, almost everywhere.  We can find $r_n<1$ such that if we define $G_n(z) = F_n(r_nz)$ for $z\in\overline{\mathbb D}$ then there will be $a_n, b_n\in\mathbb T$ with $|a_n-1|<\frac1n, |b_n-e^{i\pi/n}|<\frac1n, ||G_n(a_n)|-\frac1n|<\frac1n$ and $||G_n(b_n)|-1|<\frac1n$.  Certainly $G_n\in A(\mathbb D)$ with $\|G_n\|\leq 1$.
Finally, form $G\in A(\mathbb D, c_0)$ using the sequence $(G_n)$, and observe that
$$ \| G(b_n) - G(a_n) \|_{\infty} \geq | G_n(b_n) - G_n(a_n) | \geq 1 - 3/n $$
so as $a_n \rightarrow 1$ and $b_n\rightarrow 1$, we conclude that $G$ cannot be norm continuous.
Alternative construction (from a hint from Yemon Choi):  Simply directly define
$$ F_n(z) = \exp\big(k_n(e^{-i\pi/n}z-1)\big). $$
where $(k_n)$ is a rapidly increasing sequence.
Then obviously $F_n$ is in the disc algebra.  We have that
$$ |F_n(e^{it})| = \exp\big( k_n(\cos(t-\pi/n)-1) \big). $$
For $t\not=0$ this obviously converges to $0$; we chose $(k_n)$ increasing fast enough so that $\exp( k_n(\cos(-\pi/n)-1) )\rightarrow 0$.  Thus $F_n(z)\rightarrow 0$ pointwise on $\mathbb T$, and so on $\mathbb D$, as before.  The remainder of the argument also runs the same way, as $F_n(1)\rightarrow 0$ while $F_n(e^{i\pi/n}) = 1$.