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We give a complete description of the structure of surjective isometries between the unitary groups of unital C*-algebras. While any surjective isometry between the unitary groups of von Neumann algebras can be extended to a real-linear Jordan *-isomorphism between the relevant von Neumann algebras, this is not the case for general unital C*-algebras. We show that the unitary groups of two C*-algebras are isomorphic as metric groups if and only if the C*-algebras are isomorphic in the sense that each of them can be decomposed as the direct sum of two C*-algebras with the first parts being linear *-algebra isomorphic and the second parts being conjugate-linear *-algebra isomorphic. We emphasize that in this paper by an isometry we merely mean a distance preserving transformation; we do not assume that it respects any algebraic operation.
Osamu Hatori. "Isometries of the unitary groups in C*-algebras." Studia Mathematica 221.1 (2014): 61-86. <http://eudml.org/doc/285618>.
@article{OsamuHatori2014, abstract = {We give a complete description of the structure of surjective isometries between the unitary groups of unital C*-algebras. While any surjective isometry between the unitary groups of von Neumann algebras can be extended to a real-linear Jordan *-isomorphism between the relevant von Neumann algebras, this is not the case for general unital C*-algebras. We show that the unitary groups of two C*-algebras are isomorphic as metric groups if and only if the C*-algebras are isomorphic in the sense that each of them can be decomposed as the direct sum of two C*-algebras with the first parts being linear *-algebra isomorphic and the second parts being conjugate-linear *-algebra isomorphic. We emphasize that in this paper by an isometry we merely mean a distance preserving transformation; we do not assume that it respects any algebraic operation.}, author = {Osamu Hatori}, journal = {Studia Mathematica}, keywords = {isometry; Jordan *-isomorphism; unitary group; -algebra; von Neumann algebra}, language = {eng}, number = {1}, pages = {61-86}, title = {Isometries of the unitary groups in C*-algebras}, url = {http://eudml.org/doc/285618}, volume = {221}, year = {2014}, }
TY - JOUR AU - Osamu Hatori TI - Isometries of the unitary groups in C*-algebras JO - Studia Mathematica PY - 2014 VL - 221 IS - 1 SP - 61 EP - 86 AB - We give a complete description of the structure of surjective isometries between the unitary groups of unital C*-algebras. While any surjective isometry between the unitary groups of von Neumann algebras can be extended to a real-linear Jordan *-isomorphism between the relevant von Neumann algebras, this is not the case for general unital C*-algebras. We show that the unitary groups of two C*-algebras are isomorphic as metric groups if and only if the C*-algebras are isomorphic in the sense that each of them can be decomposed as the direct sum of two C*-algebras with the first parts being linear *-algebra isomorphic and the second parts being conjugate-linear *-algebra isomorphic. We emphasize that in this paper by an isometry we merely mean a distance preserving transformation; we do not assume that it respects any algebraic operation. LA - eng KW - isometry; Jordan *-isomorphism; unitary group; -algebra; von Neumann algebra UR - http://eudml.org/doc/285618 ER -