discrete geometry, Graph theory, simplicial complex Codimension 2 surfaces in 4 manifolds By oliverknill November 19, 2023 November 19, 2023 4-manifolds, Codimension 2 surface, Morse-Sard What is {f=0,g=0} for two functions f,g on a graph G. If G is a 4-manifold, these are 2-manifolds or empty.