D. Carraher, A. Schliemann, and B. Brizuela. Proceedings of the XXV Conference of the International Group for the Psychology of Mathematics Education, (2001)
F. Arzarello. the 24th Conference of the International Group for the Psychology of Mathematics Education (PME), 1, page 23--38. Hiroshima, Japan, (2000)
L. Radford. Proceedings of the 29th Conference of the International Group for the Psychology of Mathematics Education, 1, page 143-145. Melbourne, Australia, University of Melbourne, (2005)