Casson

Casson is a surname. In mathematics, the name is associated with concepts in low-dimensional topology named after the mathematician Andrew J. Casson. The best known is the Casson invariant, an integer-valued invariant of oriented integral homology 3-spheres.

The Casson invariant is defined by counting, with signs, irreducible SU(2) representations of the fundamental group of the manifold. It provides information about the topology of 3-manifolds. The invariant was extended to rational homology spheres by Walker and Lescop, giving rise to the Casson–Walker–Lescop invariant.

The Casson handle is a topological construction in 4-manifold topology. It replaces a standard 2-handle by an infinite sequence of linked kinky handles arranged in a tree-like fashion to encode smoothing data. Casson handles contributed to the study of smooth structures on 4-manifolds and are featured in Freedman’s work on topological 4-manifolds.

The surname is borne by various individuals in science, arts, and public life. The mathematical invariants and constructions named Casson are among the best-known uses of the term.