[proofplan] We prove existence and uniqueness of $W(R)$ separately. Uniqueness follows immediately from the [Lifting Homomorphisms Between Strict $p$-Rings](/theorems/???) theorem: any two strict $p$-rings with residue ring $R$ are canonically isomorphic via the unique lift of the identity $R \to R$. For existence, we construct $W(R)$ by starting with a free perfect $\mathbb{F}_p$-algebra surjecting onto $R$, lifting to a strict $p$-ring via the $p$-adic completion of a polynomial ring over $\mathbb{Z}$, and then quotienting out the "Teichmüller ideal" to impose the correct residue ring. The functoriality bijection on $\operatorname{Hom}$-sets is a direct consequence of the lifting theorem. [/proofplan]

[step:Prove uniqueness: any two strict $p$-rings with residue ring $R$ are canonically isomorphic] Let $A$ and $C$ be strict $p$-rings with $A/pA \cong R \cong C/pC$. Fix an isomorphism $\bar{\varphi}: A/pA \xrightarrow{\sim} R$ and $\bar{\psi}: C/pC \xrightarrow{\sim} R$. Then $\bar{\psi}^{-1} \circ \bar{\varphi}: A/pA \to C/pC$ is a ring isomorphism. By the [Lifting Homomorphisms Between Strict $p$-Rings](/theorems/???), this lifts to a unique ring homomorphism $\Phi: A \to C$ with $\Phi \equiv \bar{\psi}^{-1} \circ \bar{\varphi} \pmod{p}$. Similarly, $\bar{\varphi}^{-1} \circ \bar{\psi}: C/pC \to A/pA$ lifts to a unique ring homomorphism $\Psi: C \to A$. The composition $\Psi \circ \Phi: A \to A$ lifts $(\bar{\varphi}^{-1} \circ \bar{\psi}) \circ (\bar{\psi}^{-1} \circ \bar{\varphi}) = \operatorname{id}_{A/pA}$. By the uniqueness clause of the lifting theorem, $\Psi \circ \Phi = \operatorname{id}_A$ (since $\operatorname{id}_A$ also lifts $\operatorname{id}_{A/pA}$). By the symmetric argument, $\Phi \circ \Psi = \operatorname{id}_C$. So $\Phi$ is an isomorphism. [/step]

[step:Construct a strict $p$-ring surjecting onto $R$ in residue]We build $W(R)$ in two stages: first construct a "free" strict $p$-ring $A$ with a surjection $A/pA \twoheadrightarrow R$, then quotient $A$ to make the surjection an isomorphism. **Stage 1: A free perfect $\mathbb{F}_p$-algebra.** For each $r \in R$, introduce an indeterminate $x_r$, and for each $n \geq 0$, introduce an indeterminate $x_r^{p^{-n}}$ (a formal symbol for the $p^{-n}$-th root). Define \begin{align*} P = \mathbb{F}_p[x_r^{p^{-n}} : r \in R,\, n \geq 0] \Big/ \bigl((\,x_r^{p^{-(n+1)}}\,)^p - x_r^{p^{-n}} : r \in R,\, n \geq 0\bigr). \end{align*} This is a perfect ring of characteristic $p$: every element has a $p$-th root. The map $P \to R$ sending $x_r^{p^{-n}} \mapsto r^{p^{-n}}$ (which exists uniquely since $R$ is perfect) is a surjective ring homomorphism. **Stage 2: A strict $p$-ring over $P$.** Let $\tilde{A} = \mathbb{Z}[x_r^{p^{-n}} : r \in R,\, n \geq 0] / ((\,x_r^{p^{-(n+1)}}\,)^p - x_r^{p^{-n}})$ and define $A = \varprojlim \tilde{A}/p^m \tilde{A}$, the $p$-adic completion of $\tilde{A}$. Then $A$ is $p$-adically complete by construction. Since $\tilde{A}$ is $p$-torsion free (it embeds into $\tilde{A}[1/p]$, a $\mathbb{Q}$-algebra), and the $p$-adic completion of a $p$-torsion free ring is $p$-torsion free, $A$ is $p$-torsion free. Finally, $A/pA \cong \tilde{A}/p\tilde{A} \cong P$, which is perfect. So $A$ is a strict $p$-ring with $A/pA \cong P$.[/step]

[guided]Why do we need the free perfect algebra $P$ as an intermediate step? Because we need a strict $p$-ring whose residue ring surjects onto $R$, and the simplest way to build one is to start with a polynomial ring (which is easy to lift from characteristic $p$ to characteristic $0$) and make it perfect by adjoining all $p$-power roots. The $p$-adic completion $A$ of $\tilde{A}$ is the characteristic-$0$ lift: $A/pA \cong P$ recovers the characteristic-$p$ ring, while $A$ itself has characteristic $0$ (it contains $\mathbb{Z}$) and is complete. The three defining properties of a strict $p$-ring -- $p$-torsion free, $p$-adically complete, perfect residue ring -- are all satisfied by construction.[/guided]

[step:Quotient $A$ by the Teichmüller ideal to obtain $W(R)$] Let $I = \ker(P \to R)$ be the kernel of the surjection $A/pA \cong P \twoheadrightarrow R$. Define \begin{align*} J = \Bigl\{ \sum_{n=0}^\infty [a_n]_A p^n \in A : a_n \in I \text{ for all } n \geq 0 \Bigr\}. \end{align*} [claim:$J$ is an ideal of $A$] For closure under addition: let $\alpha = \sum [a_n] p^n$ and $\beta = \sum [b_n] p^n$ with all $a_n, b_n \in I$. The Witt coefficient of $\alpha + \beta$ at position $n$ is $S_n(a_0, \ldots, a_n, b_0, \ldots, b_n)$, a universal polynomial over $\mathbb{Z}$ evaluated in $P$. Since $I$ is an ideal of $P$ and $S_n(0, \ldots, 0, 0, \ldots, 0) = 0$, each $S_n$ vanishes when all inputs lie in $I$ (because $S_n$ has no constant term as a polynomial in the $a_i, b_i$ -- this follows from $S_n(a, 0, \ldots, b, 0, \ldots) = a + b$ for $n = 0$ and the recursive structure). More precisely, since $I$ is an ideal and the $S_n$ are polynomial expressions in the $a_i, b_i$ with each monomial containing at least one of these variables, each $S_n(a_0, \ldots, a_n, b_0, \ldots, b_n)$ lies in $I$. For closure under multiplication by $A$: let $\alpha = \sum [a_n] p^n \in J$ and $\gamma = \sum [c_n] p^n \in A$. The Witt coefficient of $\alpha \gamma$ at position $n$ is $P_n(a_0, \ldots, a_n, c_0, \ldots, c_n)$. Since $a_i \in I$ and $I$ is an ideal, each monomial in $P_n$ that involves at least one $a_i$ lands in $I$. Since every monomial in $P_n$ involves at least one $a_i$ (because $P_n(0, \ldots, 0, c_0, \ldots, c_n) = 0$), we get $P_n(\ldots) \in I$. [/claim] [proof] Proved inline above. [/proof] Set $W(R) = A/J$. We verify the three properties of a strict $p$-ring. **Residue ring.** The composition $A \to A/J = W(R) \to W(R)/pW(R)$ has kernel $J + pA$. Under the identification $A/pA \cong P$, the image of $J$ in $P$ is $I$ (since $\sum [a_n] p^n \equiv [a_0] \equiv a_0 \pmod{p}$, and $a_0 \in I$). So $W(R)/pW(R) \cong A/(J + pA) \cong P/I \cong R$. **$p$-torsion free.** Suppose $px \in J$ for some $x \in A$. Write $x = \sum [x_n] p^n$. Then $px = \sum [x_n] p^{n+1} = p[x_0] + \sum_{n \geq 1} [x_{n-1}] p^n$ (reindexing: the Witt coefficient of $px$ at position $0$ is $0$ and at position $n$ is $x_{n-1}$). Since $px \in J$, the zeroth Witt coefficient of $px$ lies in $I$, giving $0 \in I$ (which is automatic), and for $n \geq 1$ the coefficient $x_{n-1} \in I$. So $x_n \in I$ for all $n \geq 0$, meaning $x \in J$. Therefore $W(R)$ is $p$-torsion free. **$p$-adically complete.** For each $m \geq 1$, the [Witt Expansion](/theorems/???) gives $A = \sum_{n=0}^{m-1} [P]_A p^n + p^m A$, where $[P]_A = \{[a]_A : a \in P\}$. Projecting to $W(R) = A/J$: $W(R) = \sum_{n=0}^{m-1} \overline{[R]} \, p^n + p^m W(R)$, where $\overline{[R]}$ denotes the image of the Teichmüller lifts of $R$ in $W(R)$. This shows $W(R)/p^m W(R) \cong A/(J + p^m A)$ is a quotient of a finite product, and the natural map $W(R) \to \varprojlim W(R)/p^m W(R)$ is an isomorphism (injectivity: $\bigcap_m (J + p^m A) = J$ by completeness of $A$; surjectivity: by the density condition above). [/step]

[step:Establish the bijection on $\operatorname{Hom}$-sets] For perfect rings $R$ and $R'$, we show \begin{align*} \operatorname{Hom}_{\mathrm{Ring}}(W(R), W(R')) \xrightarrow{\;\sim\;} \operatorname{Hom}_{\mathrm{Ring}}(R, R') \end{align*} via the reduction-mod-$p$ map $F \mapsto \bar{F}$, where $\bar{F}: R \cong W(R)/pW(R) \to W(R')/pW(R') \cong R'$ is the induced map on residue rings. **Surjectivity.** Given $f: R \to R'$, the [Lifting Homomorphisms](/theorems/???) theorem produces a unique ring homomorphism $F: W(R) \to W(R')$ with $\bar{F} = f$ (since $W(R)$ and $W(R')$ are strict $p$-rings). **Injectivity.** If $F, G: W(R) \to W(R')$ satisfy $\bar{F} = \bar{G} = f$, then uniqueness in the lifting theorem gives $F = G$. [/step]