Characterization of surjective isometries: the real case
Let $(Ω,μ)$ and $(Λ,λ)$ be complete atomless localizable semifinite measure spaces. Suppose that $E(Ω,μ)$ and $F(Λ,λ)$ 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 $U:E(Ω,μ)\longrightarrow F(Λ,λ)$ has the form $Uf=wΦ(f)$, where $w$ has full support and $Φ$ is induced by a complete measure-class Boolean isomorphism. Both factors are uniquely determined by $U$. Let $(\mathcal{M},τ)$ and $(\mathcal{N},ν)$ be atomless semifinite von Neumann algebras, and let $E(\mathcal{M},τ)$ and $F(\mathcal{N},ν)$ be symmetric operator spaces satisfying the same assumptions on their norms. Every surjective real-linear isometry $V:E(\mathcal{M},τ)_{\mathrm{sa}}\longrightarrow F(\mathcal{N},ν)_{\mathrm{sa}}$ 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.