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Optimal proof of a lemma
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aisejohan committed Oct 23, 2018
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Expand Up @@ -23245,33 +23245,15 @@ \section{Flatness criteria over Artinian rings}

\begin{proof}
The implication (2) $\Rightarrow$ (1) is immediate.
We will prove the other implication by using induction on $n$ to show that
$\{x_\alpha\}_{\alpha \in A}$ forms a basis for
$M/\mathfrak m^nM$ over $R/\mathfrak m^n$. The case $n = 1$ holds by
assumption (1). Assume the statement holds for some $n \geq 1$. By
Nakayama's Lemma \ref{lemma-NAK}
the elements $x_\alpha$ generate $M$, in particular $M/\mathfrak m^{n + 1}M$.
The exact sequence
$0 \to \mathfrak m^n/\mathfrak m^{n + 1} \to R/\mathfrak m^{n + 1} \to
R/\mathfrak m^n \to 0$
gives on tensoring with $M$ the exact sequence
$$
0 \to \mathfrak m^nM/\mathfrak m^{n + 1}M \to
M/\mathfrak m^{n + 1}M \to
M/\mathfrak m^nM \to 0
$$
Here we are using that $M$ is flat.
Moreover, we have $\mathfrak m^nM/\mathfrak m^{n + 1}M =
M/\mathfrak mM \otimes_{R/\mathfrak m} \mathfrak m^n/\mathfrak m^{n + 1}$
by flatness of $M$ again.
Now suppose that $\sum f_\alpha x_\alpha = 0$ in $M/\mathfrak m^{n + 1}M$.
Then by induction hypothesis $f_\alpha \in \mathfrak m^n$ for each $\alpha$.
By the short exact sequence above we then conclude that
$\sum \overline{f}_\alpha \otimes \overline{x}_\alpha$ is zero in
$\mathfrak m^n/\mathfrak m^{n + 1} \otimes_{R/\mathfrak m} M/\mathfrak mM$.
Since $\overline{x}_\alpha$ forms a basis we conclude that each of the
congruence classes $\overline{f}_\alpha \in \mathfrak m^n/\mathfrak m^{n + 1}$
is zero and we win.
Assume (1). By Nakayama's Lemma \ref{lemma-NAK}
the elements $x_\alpha$ generate $M$. Then one gets a short exact
sequence
$$
0 \to K \to \bigoplus\nolimits_{\alpha \in A} R \to M \to 0
$$
Tensoring with $R/\mathfrak m$ and using Lemma \ref{lemma-flat-tor-zero}
we obtain $K/\mathfrak mK = 0$. By Nakayama's Lemma \ref{lemma-NAK}
we conclude $K = 0$.
\end{proof}

\begin{lemma}
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