Johannes Hjelmslev – mathematician and mathematics educator

Johannes Hjelmslev (1873 - 1950) was a renowned Danish mathematician and one of the most remarkable mathematicians of his time. A special feature is that he was active in two fields, mathematics (the foundations of geometry) and the didactics of mathematics (the didactics of geometry). He not only contributed original and fruitful ideas to both areas, but also had a significant impact on the concept of mathematics and school mathematics until the present day. While historical appraisals have already been published on individual topics of his research work - for example on his so-called geometry of reality ( Lützen, 2021 ) - a summarizing presentation and appraisal of his work in both fields in a historical context is still lacking. This is the aim of this article, which, incidentally, is a response on the less appreciative assessment of Hjelmslev's achievements and his geometry of reality by Lützen (2021) .