On the formula for the PI-exponent of Lie algebras

We prove that one of the conditions in M.V. Zaicev’s formula for the PI-exponent and in its natural generalization for the Hopf PI-exponent, can be weakened. Using the modification of the formula, we prove that if a finite dimensional semisimple Lie algebra acts by derivations on a finite dimensional Lie algebra over a field of characteristic $0$, then the differential PI-exponent coincides with the ordinary one. Analogously, the exponent of polynomial $G$-identities of a finite dimensional Lie algebra with a rational action of a connected reductive affine algebraic group $G$ by automorphisms, coincides with the ordinary PI-exponent. In addition, we provide a simple formula for the Hopf PI-exponent and prove the existence of the Hopf PI-exponent itself for $H$-module Lie algebras whose solvable radical is nilpotent, assuming only the $H$-invariance of the radical, i.e. under weaker assumptions on the $H$-action, than in the general case. As a consequence, we show that the analog of Amitsur’s conjecture holds for $G$-codimensions of all finite dimensional Lie $G$-algebras whose solvable radical is nilpotent, for an arbitrary group $G$.