Reidemeister studied both with Landau and Hecke, as a younger mathematician, his focus was on algebraic number theory and then later became interested in differential geometry. With this interest in geometry, Reidemeister wrote a sort of Principia Mathematica for knot theory, called Knoten und gruppen in 1926. The branch of geometry and topology that he created, knot theory, is based on group theory without the concept of a limit. His most signifigant contributions to knot theory are the Reidemeister moves, which are way of manipulating knots without changing their isotopic class.