Invariante Gruppenpartitionen und Doppelräume.
Inzidenzgruppen der Ordnung 3.
Kategorizitätsbeweise in der Geometrie
Kleinsche Geometrie und Theorie der geometrischen Objekte
Normalteiler und Faktorstrukturen geschlitzter Inzidenzgruppen.
On a Generalization of the Gallai-Sylvester Theorem.
On free constructions.
On S. Gołąb's characterization of the anharmonic ratio
Orientierung in angeordneten affinen Räumen
Ovals and non-ovoidal Laguerre planes.
Pentagramma mirificum and elliptic functions (Napier, Gauss, Poncelet, Jacobi, ...)
We give an exposition of unpublished fragments of Gauss where he discovered (using a work of Jacobi) a remarkable connection between Napier pentagons on the sphere and Poncelet pentagons on the plane. As a corollary we find a parametrization in elliptic functions of the classical dilogarithm five-term relation.
Planes with analogues to Euclidean angular bisectors.
Relativistische Mechanik mit klassicher Geometrie und Kinematik.
Remark on "Planes with analogues to Eucledian angular bisectors".
Remarks on projectivities.
Replaceable nets and improper collineations
Special automorphisms of nets with Baer subnets
Strongly n-Uniform and Level n-Hjelmslev Planes.
The envelope of the Wallace-Simpson lines of a triangle. A simple proof of the Steiner problem on the deltoid.
A simple proof is presented of a famous, and difficult, theorem by Jakob Steiner. By means of a straightforward transformation of the triangle, the proof of the theorem is reduced to the case of the equilateral triangle. Several relations of the Steiner deltoid with the Feuerbach circle and the Morley triangle appear then as obvious.
The number of triangles in a tringulation of a set of points in the plane.