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Uniqueness of algebra norm on quotients of the algebra of bounded operators on a Banach space. (English) Zbl 07895009

Summary: We show that the quotient algebra \(\mathcal{B}(X) / \mathcal{I}\) has a unique algebra norm for every closed ideal \(\mathcal{I}\) of the Banach algebra \(\mathcal{B}(X)\) of bounded operators on \(X\), where \(X\) denotes any of the following Banach spaces:
\(( \bigoplus_{n \in \mathbb{N}} \ell_2^n )_{c_0}\) or its dual space \(( \bigoplus_{n \in \mathbb{N}} \ell_2^n )_{\ell_1} \),
\(( \bigoplus_{n \in \mathbb{N}} \ell_2^n )_{c_0} \oplus c_0(\Gamma)\) or its dual space \(( \bigoplus_{n \in \mathbb{N}} \ell_2^n )_{\ell_1} \oplus \ell_1(\Gamma)\) for an uncountable cardinal number \(\Gamma \),
\(C_0( K_{\mathcal{A}})\), the Banach space of continuous functions vanishing at infinity on the locally compact Mrówka space \(K_{\mathcal{A}}\) induced by an uncountable, almost disjoint family \(\mathcal{A}\) of infinite subsets of \(\mathbb{N} \), constructed such that \(C_0( K_{\mathcal{A}})\) admits “few operators”.
Equivalently, this result states that every homomorphism from \(\mathcal{B}(X)\) into a Banach algebra is continuous and has closed range. The key step in our proof is to show that the identity operator on a suitably chosen Banach space factors through every operator in \(\mathcal{B}(X) \setminus \mathcal{I}\) with control over the norms of the operators used in the factorization. These quantitative factorization results may be of independent interest.

MSC:

46H10 Ideals and subalgebras
46H40 Automatic continuity
47L10 Algebras of operators on Banach spaces and other topological linear spaces
46B26 Nonseparable Banach spaces
46B45 Banach sequence spaces
46E15 Banach spaces of continuous, differentiable or analytic functions
47B01 Operators on Banach spaces
47L20 Operator ideals

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