  Let p be a prime and let F_pbar be the algebraic closure of the finite field
of p elements. Let f(x) be any one variable rational function over F_pbar with
n poles of orders d_1, ...,d_n. Suppose p is coprime to d_i for every i. We
prove that there exists a Hodge polygon, depending only on d_i's, which is a
lower bound to the Newton polygon of L functions of exponential sums of f(x).
Moreover, we show that these two polygons coincide if p=1 mod d_i for every
i=1,...,n. As a corollary, we obtain a tight lower bound of Newton polygon of
Artin-Schreier curve.
