[1] P. Asquith, A. C. Stephens, E. J. Knuth and M. W. Alibali, Middle school mathematics teachers� knowledge of students� understanding of core algebraic concepts: equal sign and variable, Mathematical Thinking and Learning 9 (2007), 249-272.

[2] I. Attorps, Mathematics teachers� conceptions about equations, University of Helsinki, Faculty of Behavioural Sciences, Research Report 266, Helsinki, 2006.

[3] I. Attorps and T. Tossavainen, Is there equality in equation?, D. Pitta-Pantazi and G. Philippou, eds., The Proceedings of CERME 2007 Fifth Congress of the European Society for Research in Mathematics Education, 22-26 February 2007 in Larnaca, Cyprus, pp. 2250-2259, ERME, 2008.

[4] I. Attorps and T. Tossavainen, Is there always truth in equation?, C. Winsl�w, ed., Nordic Research in Mathematics Education, Proceedings from NORMA08 in Copenhagen, April 21 - April 25, 2008, pp. 143-150, Rotterdam, Sense Publishers, 2009.

[5] E. J. Borowski and J. M. Borwein, Dictionary of Mathematics, Unwin Hyman � Bookmart, Leicester, 1999.

[6] E. Filloy and T. Rojano, Solving equations: the transition from arithmetic to algebra, For the Learning of Mathematics 9 (1989), 19-25.

[7] E. J. Knuth, A. C. Stephens, N. M. McNeil and M. W. Alibali, Does understanding the equal sign matter? Evidence from solving equations, Journal for Research in Mathematics Education 37 (2006), 297-312.

[8] X. Li, Cognitive analysis of students� errors and misconceptions in variables, equations, and functions, Texas A&M University, 2006. Available at http://hdl.handle.net/1969.1/ETD-TAMU-1098.

[9] F. Marton, Phenomenography: Exploring different conceptions of reality, D. Fetterman, ed., Qualitative Approaches to Evaluation in Education � The Silent Scientific Revolution, pp. 176-205, Praeger, New York, 1989.

[10] F. Marton and S. Booth, Learning and Awareness, Lawrence Erlbaum, Mahwah, NJ, USA, 1997.

[11] N. M. McNeil, L. Grandau, E. J. Knuth, M. W. Alibali, A. C. Stephens, S. Hattikudur and D. E. Krill, Middle-school students� understanding of the equal sign: The books they read can�t help, Cognition and Instruction 24 (2006), 367-385.

[12] R. Nogueira de Lima and D. Tall, Procedural embodiment and magic in linear equations, Educational Studies in Mathematics 67 (2008), 3-18.

[13] S. Pirie and L. Martin, The equation, the whole equation and nothing but the equation! One approach to the teaching of linear equations, Educational Studies in Mathematics 34 (1997), 159-181.

[14] A. S�enz-Ludlow and C. Walgamuth, Third graders� interpretations of equality and the equal symbol, Educational Studies in Mathematics 35 (1998), 153-187.

[15] A. Sfard, On the dual nature of mathematical conceptions: Reflections on processes and objects as different sides of the same coin, Educational Studies in Mathematics 22 (1991), 1-36.

[16] R. S�lj�, Learning in educational settings: Methods of inquiry, P. Ramsden, ed., Improving Learning. New perspectives, pp. 32-48, Kogan Page, London, 1988.

[17] D. Tall and S. Vinner, Concept image and concept definition with particular reference to limits and continuity, Educational Studies in Mathematics 12 (1981), 151-169.

[18] Wolfram MathWorld. http://mathworld.wolfram.com/Equation.html. (Retrieved in 2011-03-25).