Necessary and sufficient conditions for existence of minimizers for vector problems in the Calculus of Variations

We report our work about the main necessary and sufficient conditions for weak lower semi-continuity of integral functionals in vector Calculus of Variations.

In particular we provide tools to investigate rank-one convexity of functions defined on $2\times 2$-matrices. Furthermore, we explore some consequences and examples.

We also explore the quasiconvexity condition in the case where the integrand of an integral functional is a fourth-degree homogeneous polynomial.