Shaffer, D. W. (1995). Exploring trigonometry with the Geometer’s Sketchpad. Berkeley: Key Curriculum Press.
Shaffer, D. W. (1995). Symmetric Intuitions: Dynamic Geometry/Dynamic Art. Symmetry: Culture and Science, 6(3), 476-479. http://epistemicgames.org/cv/papers/SymmetricIntuitions95.pdf