@conference {19807,
	title = {Conceptualization of function as covariation through the use of learning trajectories},
	booktitle = {Proceedings of the Fifth ERME Topic Conference (ETC 5) on Mathematics Education in the Digital Age (MEDA)},
	year = {2018},
	pages = {139-146},
	publisher = {University of Copenhagen},
	organization = {University of Copenhagen},
	address = {Copenhagen, Denmark},
	author = {G-I Kafetzopoulos and G Psycharis},
	editor = {H-G. Weigand and A. Clark-Wilson and A. Donevska-Todorova and E. Faggiano and N Gr{\o}nb{\ae}k and J Trgalova}
}
