Stability under lamination and polycrystalline effective conductivity

We prove the stability under lamination of a set of real, symmetric 3 times × × 3 matrices that can be interpreted as a subset of the effective conductivities of a polycrystal made by a single, arbitrarily fixed basic crystal. Constructed in the companion paper [1], such a set constitutes a two-dimensional portion of the boundary of the pertinent upper G G G -closure because it corresponds to materials saturating a well-known polyconvex lower bound; we prove that laminations between any two points of such a set still belong to the set.