@Article{ 
  condelmue,
  author = { Conti, Sergio and De Lellis, Camillo and Mueller, Stefan and Romeo, Mario },
  title = { Polyconvexity equals rank-one convexity for connected isotropic sets in $M^{2\times 2}$ },
  journal = { C. R. Math. Acad. Sci. Paris },
  year = { 2003 },
  volume = { 337 },
  number = { 4 },
  pages = { 233-238 },
  URL = { http://cvgmt.sns.it/paper/1511/ },
  note = { cvgmt preprint}
}