An obstruction to the smoothability of singular nonpositively curved metrics on 4-manifolds by patterns of incompressible tori


Abstract in English

We give new examples of closed smooth 4-manifolds which support singular metrics of nonpositive curvature, but no smooth ones, thereby answering affirmatively a question of Gromov. The obstruction comes from patterns of incompressible 2-tori sufficiently complicated to force branching of geodesics for nonpositively curved metrics.

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