Diffeomorphisms from dancing circles

par Danica Kosanovic (Bern)

vendredi 13 mars 2026, 13:30 → 14:30 Europe/Paris

Fokko du Cloux (Bat. Braconnier)

It is still unknown whether every diffeomorphism of the 4-sphere is isotopic to the identity, i.e. whether the smooth mapping class group is nontrivial. I will explain how all candidate classes constructed recently arise from 1-parameter families of embedded circles, and often reduce to a single class. Analogues of such families have been shown to be nontrivial in some non-simply connected 4-manifolds.