A new Poincaré type rigidity phenomenon with applications

Abstract: We discover a new Poincar\'e type phenomenon by establishing an optimal rigidity theorem for local CR mappings between circle bundles that are defined in a canonical way over (possibly reducible) bounded symmetric domains. We prove such a local CR map, if nonconstant, must extend to a rational biholomorphism between the corresponding disk bundles. The result includes as a special case the classical Poincar\'e--Tanaka--Alexander theorem. Among other applications, we show, for two irreducible bounded symmetric domains with rank at least two, a local CR diffeomorphism between (open connected pieces of) their anti-canonical circle bundles extends to a norm-preserving holomorphic isomorphism between their anti-canonical bundles. The statement fails in the rank one case. As another application, we construct, for any $n \geq 2,$ a countably infinite family of compact locally homogeneous strongly pseudoconvex CR hypersurfaces (in complex manifolds) of real dimension $2n+1$ with transverse symmetry such that they are all obstruction flat and Bergman logarithmically flat. Moreover, their local CR structures are mutually inequivalent. Such a family cannot exist in dimension three by known results: A Bergman logarithmically flat CR hypersurface must be spherical, and so is a compact obstruction flat CR hypersurface with transverse symmetry.