The strong soundness theorem for real closed fields and Hilbert's Nullstellensatz in second order arithmetic. - Nobuyuki Sakamoto; Kazuyuki Tanaka (Arch. Math. Log., 2004)

Uniform versions of some axioms of second order arithmetic. - Nobuyuki Sakamoto; Takeshi Yamazaki (Math. Log. Q., 2004)

The Jordan curve theorem and the Schönflies theorem in weak second-order arithmetic. - Nobuyuki Sakamoto; Keita Yokoyama (Arch. Math. Log., 2007)