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Published online by Cambridge University Press: 20 November 2018
G. Hessenberg (2) showed that euclidean plane geometry can be realized on the surface of the sphere without assuming the parallel axiom. This geometry will be called the pseudo-euclidean geometry due to G. Hessenberg. In the present paper we give a slightly different treatment, which is perhaps simpler than that of Hessenberg and which has a greater transparency.