@Article{ 
  SorTio21,
  author = { Sorella, Massimo and Tione, Riccardo },
  title = { The four-state problem and convex integration for linear differential  operators },
  journal = { Journal of Functional Analysis },
  year = { 2021 },
  doi = { https://doi.org/10.1016/j.jfa.2022.109785 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/5204/ },
  note = { cvgmt preprint}
}