[1] Frege G., 1982, "Uber Sinn und Bedeutung", Zeitschrift für Philosophie und Philosophische Kritik, 22-50

[2] Arcavi, 1995, "Teaching and Learning Algebra: Past, Present and Future", The Journal of Mathematical Behavior", 14, 145-162

[3] Linchevski L., 1995, Algebra with numbers and arithmetic with letters: a definition of pre-algebra", The Journal of Mathematical Behavior, 14, 113-120.

[4] Linchevski L. and Sfard A., 1991, Rules without reasons as processes without objects. The case of equations and inequalities. Proc. PME XV, Assisi, 1991, 2, 1-36.