Characterization of surjective isometries: the real case

Abstract

Let $(\Omega,\mu)$ and $(\Lambda,\lambda)$ be complete atomless localizable semifinite measure spaces. Suppose that $E(\Omega,\mu)$ and $F(\Lambda,\lambda)$ are real rearrangement-invariant Banach function spaces with order-continuous norms, in the Banach-lattice sense, and that neither norm is proportional to the $L_2$-norm. Every surjective real-linear isometry $U:E(\Omega,\mu)\longrightarrow F(\Lambda,\lambda)$ has the form $Uf=w\Phi(f)$, where $w$ has full support and $\Phi$ is induced by a complete measure-class Boolean isomorphism. Both factors are uniquely determined by $U$. Let $(\mathcal{M},\tau)$ and $(\mathcal{N},\nu)$ be atomless semifinite von Neumann algebras, and let $E(\mathcal{M},\tau)$ and $F(\mathcal{N},\nu)$ be symmetric operator spaces satisfying the same assumptions on their norms. Every surjective real-linear isometry $V:E(\mathcal{M},\tau)_{\mathrm{sa}}\longrightarrow F(\mathcal{N},\nu)_{\mathrm{sa}}$ has the form $V(x)=hJ(x)$, where $J:\mathcal{M}\longrightarrow\mathcal{N}$ is a normal surjective Jordan $*$-isomorphism and $h\in LS(\mathcal{Z}(\mathcal{N}))_{\mathrm{sa}}$ is central with full support; again, the two factors are unique. We also identify the bounded skew-Hermitian operators on the real self-adjoint part and derive commutative and noncommutative isometric forms of Mityagin's question.