Completely regular growth solutions to linear differential equations with exponential polynomials coefficients

Consider the linear differential equation $$ f^{(n)}+A_{n-1}f^{(n-1)}+\cdots+A_{0}f=0 $$ where the coefficients $A_j,j=0,\ldots,n-1,$ are exponential polynomials. It is known that every solution is entire. This paper will show that all transcendental solutions of finite growth order are of completely regular growth. This problem was raised in Heittokangas et al.[8, p.33], which involves an extensive question about Gol'dberg-Ostrovski\v{i}'s Problem [5, p.300]. Moreover, we define functions in a generalized class concluding exponential polynomial functions, which are also of completely regular growth.