@inbook {805,
	title = {Artifact-Centric Activity Theory - A Framework for the Analysis of the Design and Use of Virtual Manipulatives},
	booktitle = {International Perspectives on Teaching and Learning Mathematics with Virtual Manipulatives},
	year = {2016},
	pages = {25-40},
	publisher = {Springer Verlag},
	organization = {Springer Verlag},
	address = {Heidelberg},
	author = {Ladel, Silke and Kortenkamp, Ulrich},
	editor = {Moyer-Packenham, Patricia}
}
@article {731,
	title = {An activity-approach to multi-touch tools in early maths learning},
	journal = {The International Journal for Technology in Mathematics Education},
	volume = {20},
	year = {2013},
	pages = {3-8},
	author = {Kortenkamp, Ulrich and Ladel, Silke}
}
@article {757,
	title = {An activity-theoretic approach to multi-touch tools in early maths learning},
	journal = {The International Journal for Technoloy in Mathematics Education},
	volume = {20},
	year = {2013},
	pages = {3-8},
	author = {Ladel, Silke and Kortenkamp, Ulrich}
}
