Two Classical Theorems for Dynamics of Rigid Bodies Gu DeganIn this paper, we discuss the two theorems on. the necessary condition of integrability of Euler-Poisson's equations in the dynamics of a rigid body.Simple examples are introduced to show that the multivalued functions may be related with bifurcations in motion. Based on the absence of multivalued complex logarithmic function in the solutions the method used by Liapunov has no meanings in mechanics. Therefore, the study of necessary condition for integrability is unfinished yet.