Conservative Extension in Relevant Arithmetic. - Robert K. Meyer; Igor Urbas (Math. Log. Q., 1986)

Paraconsistency and the ℂ-Systems of da Costa. - Igor Urbas (Notre Dame J. Formal Log., 1989)

On subsystems of the system J1 of Arruda and Da Costa. - Igor Urbas (Math. Log. Q., 1990)

Dual-Intuitionistic Logic. - Igor Urbas (Notre Dame J. Formal Log., 1996)