On the hyper order of solutions of a class of higher order linear differential equations

In this paper, we investigate the order and the hyper order of entire solutions of the higher order linear differential equation f +Ak−1 (z) e k−1f +...+A1 (z) e 1f ′ +A0 (z) e 0f = 0 (k ≥ 2) , where Pj (z) (j = 0, ..., k − 1) are nonconstant polynomials such that degPj = n (j = 0, ..., k − 1) and Aj (z) ( ≡ 0) (j = 0, ..., k − 1) are entire functions with ρ (Aj) < n (j = 0, ..., k − 1). Under some conditions, we prove that every solution f (z) ≡ 0 of the above equation is of infinite order and ρ2 (f) = n.