On the local structure of rank-one convex hulls

In this note we prove that if K is a compact set of m x n matrices containing an isolated point X with no rank-one connection into the convex hull of K \ {X}, then the rank-one convex hull separates as K rc = (K \ {X}) rc ∪{X}. This is an extension of a result of P. Pedregal, which holds for 2 × 2 matrices.