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Fix statement Jacobi-Zariski sequence
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aisejohan committed Apr 11, 2024
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Expand Up @@ -34941,7 +34941,8 @@ \section{The naive cotangent complex}
$$
of $C$-modules extending the sequence of
Lemma \ref{lemma-exact-sequence-differentials}.
If $\text{Tor}_1^B(\Omega_{B/A}, C) = 0$, then
If $\text{Tor}_1^B(\Omega_{B/A}, C) = 0$ and
$\text{Tor}_2^B(\Omega_{B/A}, C) = 0$, then
$H_1(\NL_{B/A} \otimes_B C) = H_1(L_{B/A}) \otimes_B C$.
\end{lemma}

Expand All @@ -34961,8 +34962,27 @@ \section{The naive cotangent complex}
Lemma \ref{lemma-NL-homotopy}
to identify the terms as homology groups of the naive
cotangent complexes.
The final assertion follows as the degree $0$ term of the complex
$\NL_{B/A}$ is a free $B$-module.

\medskip\noindent
The final assertion is a statement in homological algebra.
Recall that $\NL_{B/A} = (N^{-1} \to N^0)$ is a two term
complex of $B$-modules with $N^0$ free and cohomology modules
$H^0 = \Omega_{B/A}$ and $H^{-1} = H_1(L_{B/A})$.
Write $M \subset N^0$ for the image of the differential.
If $\text{Tor}_1^B(H^0, C) = 0$, then we have an exact sequence
$$
0 \to M \otimes_B C \to N^0 \otimes_B C \to N^0 \otimes_B C \to 0
$$
Since $N^0$ is free, we also see that $\text{Tor}_2^B(H^0, C) =
\text{Tor}_1^B(M, C)$. Hence if $\text{Tor}_2^B(H^0, C) = 0$
then we also have an exact sequence
$$
0 \to H^{-1} \otimes_B C \to N^{-1} \otimes_B C \to M \otimes_B C \to 0
$$
Putting everything together we see that if
$\text{Tor}_1^B(H^0, C) = 0$ and $\text{Tor}_2^B(H^0, C) = 0$,
then $H^{-1} \otimes_B C$ is the kernel of
$N^{-1} \otimes_B C \to N^0 \otimes_B C$ as desired.
\end{proof}

\begin{remark}
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