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Let $k$ be a file of prime characteristic $p$. Let $W$ denote the Witt ring over Z?
The Witt ring over $k$ denoted by is defined by the coefficient extension $W_k:=W\otimes_\mathbb{Z} k$. and $W_{nk}:=W_n\otimes_\mathbb{Z}k$
The phantom-components? $W_k\to \alpha_k$ reduce now to $(a_i)_{0\le i\le n}\mapsto a_0^{p^n}$.
Since $W_k=W_{F_p}\otimes_{F_p}k$ we can identify $W_k^{(p)}$, see Definition Frobenius morphism, and $W_k$ and the Frobenius morphism becomes the endomorphism
This is a ring morphism since since $F$ commutes with products. Similar statements are true for $W_{nk}$ and the affine $k$-group $\Lambda_k$ defined in Artin-Hasse exponential series?.
Let $k$ be perfect. Then
$W(k)$ is a discrete valuation ring.
$W(k)$ is complete.
$W(k)/p W(k)=k$
(Witt) Let $k$ be perfect, let $A$ be compete, noetherian local ring with residue field $k$. Let $\pi:A\to k$ be the canonical projection. There exists a unique ring morphism
which is compatible with the projections $W(k)\to k$ and $\pi:A\to k$.
If moreover $A$ is a discrete valuation ring with $p\cdot 1_A\not 0$, then $A$ is a free finite $W(k)$-module of rank $[A/p : A]$.
In particular if $pA=A$, then $u$ is an isomorphism.