The Stacks project

Lemma 90.10.1. Let $A \to B$ be a ring map with $p = 0$ in $A$. Let $P_\bullet $ be the standard resolution of $B$ over $A$. The map $P_\bullet \to P_\bullet $ induced by the diagram

\[ \xymatrix{ B \ar[r]_{F_ B} & B \\ A \ar[u] \ar[r]^{F_ A} & A \ar[u] } \]

discussed in Section 90.6 is homotopic to the Frobenius endomorphism $P_\bullet \to P_\bullet $ given by Frobenius on each $P_ n$.

Proof. Let $\mathcal{A}$ be the category of $\mathbf{F}_ p$-algebra maps $A \to B$. Let $\mathcal{S}$ be the category of pairs $(A, E)$ where $A$ is an $\mathbf{F}_ p$-algebra and $E$ is a set. Consider the adjoint functors

\[ V : \mathcal{A} \to \mathcal{S}, \quad (A \to B) \mapsto (A, B) \]

and

\[ U : \mathcal{S} \to \mathcal{A}, \quad (A, E) \mapsto (A \to A[E]) \]

Let $X$ be the simplicial object in in the category of functors from $\mathcal{A}$ to $\mathcal{A}$ constructed in Simplicial, Section 14.34. It is clear that $P_\bullet = X(A \to B)$ because if we fix $A$ then.

Set $Y = U \circ V$. Recall that $X$ is constructed from $Y$ and certain maps and has terms $X_ n = Y \circ \ldots \circ Y$ with $n + 1$ terms; the construction is given in Simplicial, Example 14.33.1 and please see proof of Simplicial, Lemma 14.34.2 for details.

Let $f : \text{id}_\mathcal {A} \to \text{id}_\mathcal {A}$ be the Frobenius endomorphism of the identity functor. In other words, we set $f_{A \to B} = (F_ A, F_ B) : (A \to B) \to (A \to B)$. Then our two maps on $X(A \to B)$ are given by the natural transformations $f \star 1_ X$ and $1_ X \star f$. Details omitted. Thus we conclude by Simplicial, Lemma 14.33.6. $\square$


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