Thomsen's theorem [SEP] named after
Thomsen's theorem
Thomsen's theorem, named after Gerhard Thomsen, is a theorem in elementary geometry. It shows that a certain path constructed by line segments being parallel to the edges of a triangle always ends up at its starting point. 
Consider an arbitrary triangle "ABC" with a point "P" on its edge "BC". A sequence of points and parallel lines is constructed as follows. The parallel line to "AC" through "P" intersects "AB" in "P" and the
physics as well. The latter were mostly written in Italian rather than in German. Thomsen also wrote a book on the foundations of elementary geometry.
In elementary geometry Thomsen's theorem is named after him.