## Geometry and Topology

•  Hannah Schwartz, Princeton
•  The failure of the 4D light bulb theorem with dual spheres of non-zero square
•  04/20/2021
•  2:50 PM - 3:40 PM
•  Online (virtual meeting) (Virtual Meeting Link)
•  Honghao Gao (gaohongh@msu.edu)

Examples of surfaces embedded in a 4-manifold that are homotopic but not isotopic are neither rare nor surprising. It is then quite amazing that, in settings such as the recent 4D light bulb theorems of both Gabai and Schneiderman-Teichner, the existence of an embedded sphere of square zero intersecting a surface transversally in a single point has the power to "upgrade" a homotopy of that surface into a smooth isotopy. We will discuss the limitations of this phenonemon, using contractible 4-manifolds called corks to produce homotopic spheres in a 4-manifold with a common dual of non-zero square that are not smoothly isotopic.

## Contact

Department of Mathematics
Michigan State University