The sub-elliptic obstacle problem: C1, α regularity of the free boundary in Carnot groups of step two

Donatella Danielli, Nicola Garofalo, Arshak Petrosyan

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


The sub-elliptic obstacle problem arises in various branches of the applied sciences, e.g., in mechanical engineering and robotics, mathematical finance, image reconstruction and neurophysiology. In the recent paper [Donatella Danielli, Nicola Garofalo, Sandro Salsa, Variational inequalities with lack of ellipticity. I. Optimal interior regularity and non-degeneracy of the free boundary, Indiana Univ. Math. J. 52 (2) (2003) 361-398; MR1976081 (2004c:35424)] it was proved that weak solutions to the sub-elliptic obstacle problem in a Carnot group belong to the Folland-Stein (optimal) Lipschitz class Γloc1, 1 (the analogue of the well-known Cloc1, 1 interior local regularity for the classical obstacle problem). However, the regularity of the free boundary remained a challenging open problem. In this paper we prove that, in Carnot groups of step r = 2, the free boundary is (Euclidean) C1, α near points satisfying a certain thickness condition. This constitutes the sub-elliptic counterpart of a celebrated result due to Caffarelli [Luis A. Caffarelli, The regularity of free boundaries in higher dimensions, Acta Math. 139 (3-4) (1977) 155-184; MR0454350 (56 #12601)].

Original languageEnglish (US)
Pages (from-to)485-516
Number of pages32
JournalAdvances in Mathematics
Issue number2
StatePublished - Jun 1 2007
Externally publishedYes


  • Carnot groups
  • NTA domains
  • Regularity of free boundary
  • Sub-elliptic obstacle problem

ASJC Scopus subject areas

  • Mathematics(all)


Dive into the research topics of 'The sub-elliptic obstacle problem: C<sup>1, α</sup> regularity of the free boundary in Carnot groups of step two'. Together they form a unique fingerprint.

Cite this