@article{BaerHanke2022,
  author    = {B{\"a}r, Christian and Hanke, Bernhard},
  title     = {Local flexibility for open partial differential relations},
  series = {Communications on pure and applied mathematics / issued by the Courant Institute of Mathematical Sciences, New York Univ.},
  volume    = {75},
  journal   = {Communications on pure and applied mathematics / issued by the Courant Institute of Mathematical Sciences, New York Univ.},
  number    = {6},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {0010-3640},
  doi       = {10.1002/cpa.21982},
  pages     = {1377 -- 1415},
  year      = {2022},
  abstract  = {We show that local deformations, near closed subsets, of solutions to open partial differential relations can be extended to global deformations, provided all but the highest derivatives stay constant along the subset. The applicability of this general result is illustrated by a number of examples, dealing with convex embeddings of hypersurfaces, differential forms, and lapse functions in Lorentzian geometry. The main application is a general approximation result by sections that have very restrictive local properties on open dense subsets. This shows, for instance, that given any K is an element of Double-struck capital R every manifold of dimension at least 2 carries a complete C-1,C- 1-metric which, on a dense open subset, is smooth with constant sectional curvature K. Of course, this is impossible for C-2-metrics in general.},
  language  = {en}
}
@article{BaerBandara2022,
  author    = {B{\"a}r, Christian and Bandara, Lashi},
  title     = {Boundary value problems for general first-order elliptic differential operators},
  series = {Journal of functional analysis},
  volume    = {282},
  journal   = {Journal of functional analysis},
  number    = {12},
  publisher = {Elsevier},
  address   = {Amsterdam [u.a.]},
  issn      = {0022-1236},
  doi       = {10.1016/j.jfa.2022.109445},
  pages     = {69},
  year      = {2022},
  abstract  = {We study boundary value problems for first-order elliptic differential operators on manifolds with compact boundary. The adapted boundary operator need not be selfadjoint and the boundary condition need not be pseudo-local.We show the equivalence of various characterisations of elliptic boundary conditions and demonstrate how the boundary conditions traditionally considered in the literature fit in our framework. The regularity of the solutions up to the boundary is proven. We show that imposing elliptic boundary conditions yields a Fredholm operator if the manifold is compact. We provide examples which are conveniently treated by our methods.},
  language  = {en}
}