P. Thompson, and L. Saldanha. Research companion to the Principles and Standards for School Mathematics, National Council of Teachers of Mathematics, Reston, VA, (2003)
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)
L. Saldanha, and P. Thompson. Proceedings of the Annual Meeting of the Psychology of Mathematics Education - North America, Raleigh, NC, North Carolina State University, (1998)