Integral manifolds of the reduced system in the problem of inertial motion of a rigid body about a fixed point

Mekh. Tverd. Tela (Rus. J. Mechanics of Rigid Body) No. 8, 1976, pp. 18--23 The reduced system in the problem of the inertial motion of a rigid body with a fixed point (the Euler case) is equivalent, by the Maupertuis principle, to some geodesic flow on the 2-sphere. We describe the phase topology of this case including the types of the bifurcations of the integral tori. We establish the topology of the singular integral surfaces. Using the geometrical interpretation of $SO(3)$ as a 3-ball with opposite points of the boundary sphere identified we show how the singular integral surfaces and the families of 2-tori are settled in an iso-energy level.Using the contemporary language, one can say that this is the first description of the bifurcation of the type $C_2$ ($2T^2 \to 2T^2$).