For any uniformly closed subalgebra A of C(K) for a compact Hausdorff space K without isolated points and $x_0\in A$, we show that every complete norm on A which makes continuous the multiplication by $x_0$ is equivalent to $\parallel ·{\parallel }_{\infty }$ provided that $x_0^{-1}\left(\lambda \right)$ has no interior points whenever λ lies in ℂ. Actually, these assertions are equivalent if A = C(K).

A new, unified presentation of the ideal norms of factorization of operators through Banach lattices and related ideal norms is given.

It is shown that if α,ζ are ordinals such that 1 ≤ ζ < α < ζω, then there is an operator from $C\left({\omega }^{{\omega }^{\alpha }}\right)$ onto itself such that if Y is a subspace of $C\left({\omega }^{{\omega }^{\alpha }}\right)$ which is isomorphic to $C\left({\omega }^{{\omega }^{\alpha }}\right)$, then the operator is not an isomorphism on Y. This contrasts with a result of J. Bourgain that implies that there are uncountably many ordinals α for which for any operator from $C\left({\omega }^{{\omega }^{\alpha }}\right)$ onto itself there is a subspace of $C\left({\omega }^{{\omega }^{\alpha }}\right)$ which is isomorphic to $C\left({\omega }^{{\omega }^{\alpha }}\right)$ on which the operator is an isomorphism.

Description of multiplication operators generated by a sequence and composition operators induced by a partition on Lorentz sequence spaces $l(p,q)$, $1<p\le \infty$, $1\le q\le \infty$ is presented.

Let S¹ be the stopping time space and ℬ₁(S¹) be the Baire-1 elements of the second dual of S¹. To each element x** in ℬ₁(S¹) we associate a positive Borel measure ${\mu }_{x**}$ on the Cantor set. We use the measures ${\mu }_{x**}:x**\in ℬ₁\left(S¹\right)$ to characterize the operators T: X → S¹, defined on a space X with an unconditional basis, which preserve a copy of S¹. In particular, if X = S¹, we show that T preserves a copy of S¹ if and only if ${\mu }_{T**(x**)}:x**\in ℬ₁\left(S¹\right)$ is non-separable as a subset of $ℳ\left(2^{\mathbb{N}}\right)$.

The aim of this paper is to prove that derivations of a C*-algebra A can be characterized in the space of all linear continuous operators T : A → A by the conditions T(1) = 0, T(L∩R) ⊂ L + R for any closed left ideal L and right ideal R. As a corollary we get an extension of the result of Kadison [5] on local derivations in W*-algebras. Stronger results of this kind are proved under some additional conditions on the cohomologies of A.

For p ≥ 1, a set K in a Banach space X is said to be relatively p-compact if there exists a p-summable sequence (xₙ) in X with $K\subseteq \sum ₙ\alpha ₙxₙ:\left(\alpha ₙ\right)\in B_{{\ell }_{p^{\text{'}}}}$. We prove that an operator T: X → Y is p-compact (i.e., T maps bounded sets to relatively p-compact sets) iff T* is quasi p-nuclear. Further, we characterize p-summing operators as those operators whose adjoints map relatively compact sets to relatively p-compact sets.