[1] Katsumi Sasaki, "A construction of an exact model for S4", Technical Report of the Nanzan Academic Society Information Sciences and Engineering, NANZAN-TR-2009-06, Seto, Japan: Nanzan University, pp. 1-9

[2] Katsumi Sasaki, "Constructions of normal forms in modal logic K4", Technical Report of the Nanzan Academic Society Information Sciences and Engineering, NANZAN-TR-2010-01, Seto, Japan: Nanzan University, pp. 1-14

[3] Katsumi Sasaki, "A study of formulas in modal logic S4", Technical Report of the Nanzan Academic Society Information Sciences and Engineering, NANZAN-TR-2010-02, Seto, Japan: Nanzan University, pp. 1-12 [corrections]

[4] Katsumi Sasaki, "Exact models in modal logic K4", Technical Report of the Nanzan Academic Society Information Sciences and Engineering, NANZAN-TR-2010-03, Seto, Japan: Nanzan University, pp. 1-16 [corrections]

[5] Katsumi Sasaki, "Formulas in modal logic S4", The Review of Symbolic Logic, The Review of Symbolic Logic, 3, 2010, pp. 600--627. [corrections March 2011][corrections Aug 2011]