Flexible cross-polytopes in the Euclidean 4-space.
Infinite number of spheres through four tangents. (Unendlich viele Kugeln durch vier Tangenten.)
Napoleon's theorem and generalizations through linear maps.
Ivory's theorem in the Minkowski plane.
Ivory's theorem in hyperbolic spaces.
Almost-orthonormal vector systems.
Generating solids by sweeping polyhedra.
The development of the oloid.
Über zweiparametrige ebene Bewegungsvorgänge.
Quasielliptische Schraubschiebflächen.
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