@article{10_2307_121117,
     author = {Luis A. Caffarelli and Lavi Karp and Henrik Shahgholian},
     title = {Regularity of a free boundary with application to the {Pompeiu} problem},
     journal = {Annals of mathematics},
     year = {2000},
     volume = {151},
     number = {1},
     doi = {10.2307/121117},
     zbl = {0960.35112},
     url = {http://geodesic.mathdoc.fr/articles/10.2307/121117/}
}