Two-parametric systems of three-dimensional spaces in a unimodular six-dimensional affine space Josef Srovnal (1986) Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica
Tzitzeica-Bäcklund theorems. Teleman, Kostake, Teleman, Ana-Maria (2005) Balkan Journal of Geometry and its Applications (BJGA)