Linear maps between C*-algebras that are *-homomorphisms at a fixed point

Abstract

Let $A$ and $B$ be C$^*$-algebras. A linear map $T:A\to B$ is said to be a $^*$-homomorphism at an element $z\in A$ if $a b^*=z$ in $A$ implies $T (a b^*) =T (a) T (b)^* =T(z)$, and $ c^* d=z$ in $A$ gives $T (c^* d) =T (c)^* T (d) =T(z).$ Assuming that $A$ is unital, we prove that every linear map $T: A\to B$ which is a $^*$-homomorphism at the unit of $A$ is a Jordan $^*$-homomorphism. If $A$ is simple and infinite, then we establish that a linear map $T: A\to B$ is a $^*$-homomorphism if and only if $T$ is a $^*$-homomorphism at the unit of $A$. For a general unital C$^*$-algebra $A$ and a linear map $T:A\to B$, we prove that $T$ is a $^*$-homomorphism if, and only if, $T$ is a $^*$-homomorphism at $0$ and at $1$. Actually if $p$ is a non-zero projection in $A$, and $T$ is a $^*$-homomorphism at $p$ and at $1-p$, then we prove that $T$ is a Jordan $^*$-homomorphism. We also study bounded linear maps that are $^*$-homomorphisms at a unitary element in $A$.