Linear isometries between real JB*-triples and C*-algebras

Abstract

Let $T:A\to B$ be a (not necessarily surjective) linear isometry between two real JB${}^{\ast }$-triples. Then for each $a\in A$ there exists a tripotent $u_a$ in the bidual, $B^{\prime \prime },$ of $B$ such that \begin{enumerate}[$(a)$] \item $\{u_a,T(\{f,g,h\}),u_a\}=\{u_a,\{T(f),T(g),T(h)\},u_a\}$, for all $f,g,h$ in the real JB${}^{\ast }$-subtriple, $A_a,$ generated by $a$; \item The mapping $\{u_a,T(\cdot ),u_a\}:A_a\to B^{\prime \prime }$ is a linear isometry. \end{enumerate} Furthermore, when $B$ is a real C${}^{\ast }$-algebra, the projection $p=p_a=u_a^{\ast }{u}_a$ satisfies that $T(\cdot )p:A_a\to B^{\prime \prime }$ is an isometric triple homomorphism. When $A$ and $B$ are real C${}^{\ast }$-algebras and $A$ is abelian of real type, then there exists a partial isometry $u\in B^{\prime \prime }$ such that the mapping $T(\cdot )u^{\ast }u:A\to B^{\prime \prime }$ is an isometric triple homomorphism. These results generalise, to the real setting, some previous contributions due to C.-H. Chu and N.-C. Wong, and C.-H. Chu and M. Mackey in 2004 and 2005. We give an example of a non-surjective real linear isometry which cannot be complexified to a complex isometry, showing that the results in the real setting can not be derived by a mere complexification argument.

to appear in Quart. J. Math