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Let $\mathcal{A}$ be a unital $C^{*}$-algebra and $S(\mathcal{A})$ denote the states space of $\mathcal{A}$.

For $a\in \mathcal{A}$ , define $W(a) =\{\phi(a):\phi\in S(\mathcal{A})\}$

It's known that $W(a)$ is a non-empty convex compact set in the complex plane $\mathbb{C}$.

Let $I$ is an essential ideal of $\mathcal{A}$.

Conjecture: If $a\in I$ and $W(a) \subset \overline{\mathbb{D}}$ and $W(a) \cap \mathbb{T}$ is infinite, then $W(a) =\overline{\mathbb{D}}$

Redefine $W(a) =\{\phi(a):\phi\in \mathcal{A}^{*} : \phi(1)=1=\|\phi\|\}$

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    $\begingroup$ I’m voting to close this question because it seems to be a moving target. I recommend taking some time to formulate a definite question and test it on some obvious examples before posting. $\endgroup$
    – Yemon Choi
    Commented Oct 25 at 23:13
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    $\begingroup$ For instance, in Edit 2, isn't the set of such phi just the set of states? In which case, your "redefinition" of W(a) is just the same as the original definition. $\endgroup$
    – Yemon Choi
    Commented Oct 25 at 23:15
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    $\begingroup$ Note that my example can be adapted to show $W(a)$, as originally defined, can be any compact convex set, so your edit 1 or any similar edit is completely hopeless. As for edit 2, as written, $W(a)$ is literally $\{1\}$, so I have no idea what you’re asking about. $\endgroup$
    – David Gao
    Commented Oct 26 at 0:12
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    $\begingroup$ @YemonChoi I concur. In fact, as written $W(a)$ is even worse, since it’s written as $\phi(a) = 1$ instead of $\phi(1) = 1$. $\endgroup$
    – David Gao
    Commented Oct 26 at 0:13

1 Answer 1

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No, this is not true. For example, let $\mathcal{A} = \ell^\infty \oplus \ell^\infty$ and $I = \ell^\infty \oplus c_0$. Let $\{\alpha_n\}_n$ be a dense sequence in the right half of the closed unit disk in the complex plane (i.e., the closure of $\{\alpha_n\}_n$ is $\{x \in \overline{\mathbb{D}}: \text{Re}(x) \geq 0\}$). Let $a = (\alpha_1, \alpha_2, \cdots) \oplus 0 \in I$. It is then not hard to check that $W(a)$ is $\{x \in \overline{\mathbb{D}}: \text{Re}(x) \geq 0\}$, contradicting your conjecture.

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