1. Aldenderfer, M. S., & Blashfield, R. K. (1984). Cluster analysis. Beverly Hills: Sage Publications.
2. Altiparmak, K., & Özdoğan, E. (2010) A study on the teaching of the concept of negative numbers. International Journal of Mathematical Education in Science and Technology, 41(1), 31-47.
3. Artut, P. D., & Tarım, K. (2006). Elementary students’ comprehension of place value concept. Journal of Theory and Practice in Education, 2(1), 26-36.
4. Asworth, P., & Lucas, U. (1998). What is ‘world’ of phenomenography? Scandinavian Journal of Educational Research, 42(4), 415-431.
5. Barnard, A. D., & Tall, D. O. (1997). Cognitive units, connections, and mathematical proof. In E. Pehkonen, (Ed.), Proceedings of the 21’ Annual Conference for the Psychology of Mathematics Education, Vol. 2 (pp. 41-48). Lahti, Finland.