@inbook {LadKor-DTBLEPVUAAT-2013, title = {Designing a technology based learning environment for place value using artifact-centric activity theory}, booktitle = {Proceedings of the 37th conference of the International Group for the Psychology of Mathematics Education. Mathematics learning across the life span}, volume = {1}, year = {2013}, pages = {188-192}, keywords = {refereed}, author = {Silke Ladel and Ulrich Kortenkamp}, editor = {Anke M. Lindmeier and Aiso Heinze} }