On a conjecture of Wan about limiting Newton polygons

We show that for a monic polynomial f(x)f(x) over a number field K   containing a global permutation polynomial of degree >1 as its composition factor, the Newton Polygon of fmodp does not converge for pp passing through all finite places of K. In the rational number field case, our result is the “only if” part of a conjecture of Wan about limiting Newton polygons.