Y. Hirata, K. Imai
A Necessary Condition for existence of Lie Symmetries in Quasihomogeneous Systems of Ordinary Differential Equations
(37K, AMS-TeX)

ABSTRACT.  Lie symmetries for ordinary differential equations are studied. 
In systems of ordinary differential equations, there do not always exist non-trivial Lie symmetries around equilibrium points. 
We present a necessary condition for existence of Lie symmetries analytic in the neighbourhood of an equilibrium point. 
In addition, this result can be applied to a necessary condition for existence of a Lie symmetry in quasihomogeneous systems of ordinary differential equations. 
With the help of our main theorem, it is proved that several systems do not possess any analytic Lie symmetries.