A note on proofs of falsehood. Jan Krajicek (1987) Archiv für mathematische Logik und Grundlagenforschung
A sentence that is difficult to interpret Vítězslav Švejdar (1981) Commentationes Mathematicae Universitatis Carolinae
An interpretation of S 2 1 in Σ 1 b -NIA. Ferreira, Gilda, Oitavem, Isabel (2006) Portugaliae Mathematica. Nova Série