R. Goldstein, I. Kalas, R. Noss, und D. Pratt. Cognitive Technology, Volume 2117 von Lecture Notes in Computer Science, Seite 267-281. Springer, (2001)
R. Goldstein, und D. Pratt. Proceedings of the 25th Conference of the International Group for the Psychology of Mathematics Education (PME25), 3, Seite 49--56. PO Box 9432, 3506 GK Utrecht, The Netherlands, Freudenthal Institute, Faculty of Mathematics and Computer Science, Uthrect University, (Juli 2001)