Factorization of skew polynomials over k((u))

Let $k$ be a perfect field of characteristic $p>0$, and let $K=k((u))$ be the field of Laurent series over $K$. We study the skew polynomial ring $K[T,\mathrm{\Phi }]$, where $\mathrm{\Phi }$ is an endomorphism of $K$ that extends a Frobenius endomorphism of $k$. We give a description of the irreducible skew polynomials, develop an analogue of the theory of the Newton polygon in this context, and classify the similarity classes of irreducible elements.