## Abstract

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 Γ_{loc}^{1, 1} (the analogue of the well-known C_{loc}^{1, 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) C^{1, 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 language | English (US) |
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Pages (from-to) | 485-516 |

Number of pages | 32 |

Journal | Advances in Mathematics |

Volume | 211 |

Issue number | 2 |

DOIs | |

State | Published - Jun 1 2007 |

Externally published | Yes |

## Keywords

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

## ASJC Scopus subject areas

- General Mathematics

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