@inbook {LadKor-EML-2014, title = {Number concepts {\textendash}- processes of internalization and externalization by the use of multi-touch technology Silke Ladel and Ulrich Kortenkamp}, booktitle = {Early Mathematics Learning}, year = {Submitted}, chapter = {3-3}, keywords = {ACAT}, author = {Silke Ladel and Ulrich Kortenkamp} }