Sharp estimates of the curvature of some free boundaries in two dimensions. Gustafsson, Björn; Sakai, Makoto — 2003 Annales Academiae Scientiarum Fennicae. Mathematica
Existence and geometric properties of solutions of a free boundary problem in potential theory. Björn Gustafsson; Henrik Shahgholian — 1995 Journal für die reine und angewandte Mathematik
An exponential transform and regularity of free boundaries in two dimensions Björn Gustafsson; Mihai Putinar — 1998 Annali della Scuola Normale Superiore di Pisa - Classe di Scienze