F. Arzarello, M. Bussi, and O. Robutti. Proceedings of the 28th International Conference of the International Group for the Psychology of Mathematics Education, 4, page 89-96. (2004)
K. Kahn, E. Sendova, A. Sacristán, and R. Noss. Proceedings of the 7th International Conference on Technology and Mathematics Teaching, Bristol, UK, (2005)