The Classical and the omega-Complete Arithmetic. - Andrzej Grzegorczyk; Andrzej Mostowski; Czeslaw Ryll-Nardzewski (J. Symb. Log., 1958)

Decidability without mathematics. - Andrzej Grzegorczyk (Ann. Pure Appl. Log., 2004)

Undecidability without Arithmetization. - Andrzej Grzegorczyk (Stud Logica, 2005)