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Homework Statement
Prove that the closure of a proper ideal in a unital Banach algebra is a proper ideal.
Homework Equations
The hint is to use the result of the previous exercise: If I is an ideal in a unital normed algebra A, and I≠{0}, we have
I=A [itex]\Leftrightarrow[/itex] I contains 1 [itex]\Leftrightarrow[/itex] I contains an invertible element
The Attempt at a Solution
x is in the closure of I if and only if there's a sequence (xn) in I such that xn→x.
It's easy to show that the closure of I is closed under linear combinations:
[tex]\|(ax_n+by_n)-(ax+by)\|\leq |a|\|x_n-x\|+|b|\|y_n-y\|<\varepsilon[/tex]
(I don't feel like typing every word of the argument. I'm assuming that it's obvious what I have in mind.).
It's also easy to show that if i is in the closure of I and x is in A, xi is in the closure of I:
[tex]\|xi_n-xi\|\leq \|x\|\|i_n-i\|<\varepsilon[/tex]
These results imply that the closure is an ideal. What I don't see is how to prove that it's a proper ideal, i.e. that it's neither {0} nor A. The most straightforward approach seems to try to prove that 1 can't be a member, but I don't see how to do that. I'm probably missing something really obvious as usual.