This was work which he had undertaken with his student and friend Werner Fenchel, the author of the articles and, and was based on a course of lectures Nielsen gave on the theory of discontinuous groups of isometries of the hyperbolic plane in 1938-39.
Ito ay trabaho kung saan siya ay nagtangka sa kanyang estudyante at kaibigan Werner Fenchel, ang mga may-akda ng mga artikulo at, at noon ay batay sa isang kurso ng lektura Nielsen ibinigay sa teorya ng mga patlang na grupo ng isometries ng hayperboliko eroplano sa 1938-39.