Euler's Three Biquadrate Problem.

1 Kasner, "Conformal Classification of Analytic Arcs or Elements, Poincare's Local Problem of Conformal Geometry," Trans. Am. Math. Soc., 16, 333-349 (1915). The theory of a pair of regular arcs, including the horn angle, is given in "Conformal Geometry," Proceedings Cambridge International Congress Mathematicians, 1912, and a paper appearing in Scripta Mathematica, 1945. 2 Kasner and De Cicco, "The General Invariant Theory of Irregular Analytic Arcs or Elements," Ibid., 51, 232-254 (1942). Also in Publications of the Illinois Institute of Technology (1943.) 3 See for regular curves Halhhen's dissertation 1878, and his collected works Vol. 2. Also Lane, "Projective Differential Geometry," Chicago Press, 1942.