In this article, we give a representation of bounded complex linear operators that preserve idempotent elements on the Fourier algebra of a locally compact group. When such an operator is, moreover, positive or contractive, we show that the operator is induced by either a continuous group homomorphism or a continuous group antihomomorphism. If the groups are totally disconnected, bounded homomorphisms on the Fourier algebra can be realized by the idempotent preserving operators.

This paper considers Banach algebras with properties 𝔸 or 𝔹, introduced recently by Alaminos et al. The class of Banach algebras satisfying either of these two properties is quite large; in particular, it includes C *-algebras and group algebras on locally compact groups. Our first main result states that a continuous orthogonally additive n-homogeneous polynomial on a commutative Banach algebra with property 𝔸 and having a bounded approximate identity is of a standard form. The other main results describe Banach algebras A with property 𝔹 and having a bounded approximate identity that admit non-zero continuous symmetric orthosymmetric n-linear maps from An into ℂ.

We investigate the global versions of the Kleinecke–Shirokov theorem for skew derivations in Banach algebras. Centralizing skew derivations on Banach algebras are also studied.

We introduce generalized triple homomorphisms between Jordan–Banach triple systems as a concept that extends the notion of generalized homomorphisms between Banach algebras given by K. Jarosz and B. E. Johnson in 1985 and 1987, respectively. We prove that every generalized triple homomorphism between $\text{J}{{\text{B}}^{*}}$-triples is automatically continuous. When particularized to ${{C}^{*}}$-algebras, we rediscover one of the main theorems established by Johnson. We will also consider generalized triple derivations from a Jordan–Banach triple $E$ into a Jordan–Banach triple $E$-module, proving that every generalized triple derivation from a $\text{J}{{\text{B}}^{*}}$-triple $E$ into itself or into ${{E}^{*}}$is automatically continuous.

For ${{C}^{*}}$-algebras $\mathcal{A}$ of real rank zero, we describe linear maps $\phi $ on $\mathcal{A}$ that are surjective up to ideals $\mathcal{J}$, and $\text{ }\pi \text{ (}A\text{)}$ is invertible in $\mathcal{A}/\mathcal{J}$ if and only if $\text{ }\pi \text{ (}\phi (A))$ is invertible in $\mathcal{A}/\mathcal{J}$, where $A\,\in \,\mathcal{A}$ and $\text{ }\pi \text{ }\text{:}\mathcal{A}\to \mathcal{A}\text{/}\mathcal{J}$ is the quotient map. We also consider similar linear maps preserving zero products on the Calkin algebra.

Let $T$ be a sectorial operator. It is known that the existence of a bounded (suitably scaled) ${{H}^{\infty }}$ calculus for $T$, on every sector containing the positive half-line, is equivalent to the existence of a bounded functional calculus on the Besov algebra $\Lambda _{\infty ,1}^{\alpha }({{\mathbb{R}}^{+}})$. Such an algebra includes functions defined byMikhlin-type conditions and so the Besov calculus can be seen as a result on multipliers for $T$. In this paper, we use fractional derivation to analyse in detail the relationship between $\Lambda _{\infty ,1}^{\alpha }$ and Banach algebras of Mikhlin-type. As a result, we obtain a new version of the quoted equivalence.

The norm problem is considered for elementary operators of the form $U_{a,b}\,{:}\,{\cal A}\,{\longrightarrow}\,{\cal A},\;x\longmapsto axb\,{+}\,bxa (a,\,b\,{\in}\,{\cal A})$ in the special case when ${\cal A}$ is a subalgebra of the algebra of bounded operators on a Banach space. In particular, the lower estimate $\|U_{a,b}\|\geq\|a\|\|b\|$ is established when the Banach space is a Hilbert space and ${\cal A}$ is the algebra of all bounded linear operators.

Let $(M_n)$ be a sequence of positive numbers satisfying $M_0\,{=}\,1$ and \[\displaystyle \frac{M_{n+m}}{M_n M_m}\,{\geq}\,{{n+m}\choose{m}} \] for all non-negative integers $m$, $n$. Let \[D([0,1], M)=\left\{f\,{\in}\,C^{\infty}([0,1]):\|f\|_{D}=\sum_{n=0}^{\infty} \frac{\|f^{(n)}\|_{\infty}}{M_n}\,{<}\,\infty\right\}.\] With pointwise addition and multiplication, $D([0,1],M)$ is a unital commutative semisimple Banach algebra. If $\lim_{n\to\infty} (n!/M_n)^{1/n}\,{=}\,0,$ then the maximal ideal space of the algebra is $[0,1]$, and every non-zero endomorphism $T$ has the form $Tf(x)\,{=}\,f(\phi(x))$ for some selfmap $\phi$ of the unit interval. The authors have previously shown for a wide class of $\phi$ mapping the unit interval to itself that if $\|\phi'\|_\infty\,{<}\,1$, then $\phi$ induces a compact endomorphism. The paper investigates the extent to which this condition is necessary, and the spectra of all compact endomorphisms of $D([0,1],M)$ are determined. Some of the authors' earlier results on general endomorphisms of $D([0,1],M)$ are simplified and strengthened.