# Ideals in Commutative Banach Algebras

Suppose that $A$ is a natural Banach function algebra on $K$, a compact Hausdorff space. So $A$ is realised as an algebra of continuous functions on $K$, is a Banach algebra for some norm (necessarily dominating the supremum norm) and each character on $A$ is given by evaluation at a point of $K$.

If $F\subseteq K$ is closed, then $$I(F)=\{f\in A : f(k)=0 \ (k\in F) \}$$ is a closed ideal in $A$. If e.g. $A=C(K)$ then every closed ideal is of this form.

What's a simple example of an $A$ where not every closed ideal is of this form?

If I look in Bonsall+Duncan, I find that the Disc Algebra is an example. But quite a bit of theory is needed to show this. I'd like an easy example which I can explain to students. For bonus marks:

Can we find an $A$ which is conjugate closed?

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how can we prove that I(F) is closed? – Alka Goyal Feb 17 '14 at 14:22

## 1 Answer

$C^1([0,1])$ - take the ideal of functions which vanish at $p$ and whose derivatives vanish at $p$.

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D'oh! That was easy... Thank you! – Matthew Daws Mar 16 '12 at 16:27
No worries, I just happen to have got stuck on / noticed this some years ago while messing around with hulls and kernels – user16299 Mar 16 '12 at 16:50