My work spans mathematics, computer science and the didactics of both. I studied in Münster and at ETH Zürich and held professorships at TU Berlin, PH Schwäbisch Gmünd, PH Karlsruhe, MLU Halle-Wittenberg and the University of Potsdam. Since 1 April 2026 I have been at Humboldt-Universität zu Berlin.
In research and teaching, my group and I focus on building sustainable basic mental models (“Grundvorstellungen”) and on connecting virtual and physical spaces for action. It helps that we do not only evaluate digital and hands-on teaching materials but also create them ourselves, bringing new, creative ideas to the mathematics classroom.
I am Professor of Mathematics and Mathematics Education at Humboldt-Universität zu Berlin. There I do research and teach on the teaching and learning of mathematics, from primary school to university.
Together with Jürgen Richter-Gebert (TU Munich) I have been developing the interactive geometry software Cinderella since 1996. Besides non-Euclidean geometries, it also supports physics simulations. The open-source project CindyJS brings Cinderella and its built-in language CindyScript to HTML5/JavaScript. Some exhibits of the mathematics exhibition Imaginary are based on Cinderella.
The German Centre for Mathematics Teacher Education (DZLM) is a joint project initiated by the Deutsche Telekom Stiftung. It develops, offers and researches professional development courses for teachers all over Germany, with a particular focus on programmes for teacher educators and for teachers teaching mathematics out of field. Since 2021, the DZLM has been part of the Leibniz Institute for Science and Mathematics Education (IPN) as its department of subject-specific knowledge transfer.
I am an active member of the Gesellschaft für Didaktik der Mathematik (GDM) and was a member of its advisory board from 2011 to 2020. For many years I chaired the GDM working group on mathematics education and computer science (AKMUI). I am currently responsible for the reference wiki Madipedia. In 2017, my group in Potsdam organised the annual GDM conference.
The interactive place value chart (“Stellenwerttafel”) is an app for iOS devices and is available on the App Store. In the project Digitales Lernen Grundschule (digital learning in primary school) we used this and other apps to develop concepts for coherent, digitally supported teaching. The project pages also offer OER materials (in German) for teachers on using the app.
Besides software, physical materials matter to me. That is why I designed number walls (“Zahlenmauern”) and bracket mountains (“Klammergebirge”) made of building blocks, which offer many different learning opportunities. The number walls are available directly from the manufacturer Hubelino; bracket mountains are only available as remaining stock. On Vimeo you can find recorded teacher training sessions (in German) on both topics.