Sharp estimates of the curvature of some free boundaries in two dimensions. Gustafsson, Björn; Sakai, Makoto — 2003 Annales Academiae Scientiarum Fennicae. Mathematica
Regularity of free boundaries in two dimensions Makoto Sakai — 1993 Annali della Scuola Normale Superiore di Pisa - Classe di Scienze
An Approximation Theorem for Integrable Harmonic Vector Fields. B. Gustafsson; Makoto Sakai — 1992 Mathematica Scandinavica