3.
Arzarello, F., Micheletti, C., Olivero, F., & Robutti, O. (1998a). A model for analysing the transition to formal proofs in geometry. In A. Olivier & K. Newstead (Eds.), Proceedings of the Twentieth-second Annual Conference of the International Group for the Psychology of Mathematics Education (Vol. 2, pp. 24–31). Stellenbosch, South Africa.