Given any smooth 4-manifold bounding a Seifert manifold, the Seifert action on its boundary can be used to define their boundary Dehn twists. If the given 4-manifold is simply-connected, this Dehn twist is always topologically isotopic to the identity, but usually not smoothly isotopic, making it a very nice potential example of exotic diffeomorphisms. In this talk, we prove that for any Brieskorn homology sphere bounding a positive-definite 4-manifold, their boundary Dehn twists are always infinite-order exotic. This is a joint work with JungHwan Park and Masaki Taniguchi.

This talk is based on joint work with Sungkyung Kang and JungHwan Park. We show that the (2n,1)-cable of the figure-eight knot has infinite order in the smooth concordance group, for any n≥1. The proof relies on the real κ-invariant, which satisfies a real version of the 10/8-inequality, in combination with techniques involving higher-order branched covers of knots and surfaces. Together with earlier work by Hom, Kang, Park, and Stoffregen, this result implies that any nontrivial cable of the figure-eight knot has infinite order in the smooth concordance group.