New insights into linear maps which are anti-derivable at zero

Let $A$ be a Banach algebra admitting a bounded approximate unit and satisfying property $\mathbb{B}$. Suppose $T: A \rightarrow X$ is a continuous linear map, where $X$ is an essential Banach $A$-bimodule. We prove that the following statements are equivalent: $(i)$ $T$ is anti-derivable at zero (i.e., $a b =0$ in $A$ $\Rightarrow T(b)\cdot a + b\cdot T(a) =0$); $(ii)$ There exist an element $ξ\in X^{}$ and a linear map (actually a bounded Jordan derivation) $d: A\to X$ satisfying $ξ\cdot a = a \cdot ξ\in X$, $T(a) = d(a) +ξ\cdot a$, and $d(b)\cdot a + b\cdot d(a)= - 2 ξ\cdot (b a),$ for all $a,b\in A$ with $a b =0$. Assuming that $A$ is a C$^*$-algebra we show that a bounded linear mapping $T: A\to X$ is anti-derivable at zero if, and only if, there exist an element $η\in X^{}$ and an anti-derivation $d: A \rightarrow X$ satisfying $η\cdot a = a \cdot η\in X$, $η\cdot [a,b] = 0$ {\rm(}i.e., $L_η: A \to A$, $L_η (a) = η\cdot a$ vanishes on commutators{\rm)}, and $T(a) = d(a) +η\cdot a$, for all $a,b \in A$. The results are also applied for some special operator algebras.

💡 Research Summary

The paper investigates continuous linear maps that satisfy a weak product‑preserving condition called “anti‑derivable at zero”. Let A be a Banach algebra that possesses a bounded approximate identity and satisfies property B (any bilinear map vanishing on zero products also satisfies φ(ab,c)=φ(a,bc)). Let X be an essential Banach A‑bimodule. The main theorem (Theorem 2.1) states that for a continuous linear map T:A→X the following are equivalent: (i) T is anti‑derivable at zero, i.e., ab=0 ⇒ T(b)·a + b·T(a)=0; (ii) there exist an element ξ in the bidual X** and a bounded Jordan derivation d:A→X such that ξ·a = a·ξ for all a∈A, T(a)=d(a)+ξ·a, and for all a,b with ab=0 we have d(b)·a + b·d(a)=−2 ξ·(ba). Moreover, if every bounded Jordan derivation from A to X is actually a derivation (as is the case for C*‑algebras by Johnson’s theorem), then the equivalent condition can be strengthened to (iii): there exist ξ∈X** and a derivation d:A→X satisfying the same centrality of ξ and the identity d(