diff --git a/ws2022/rav/lecture/rav.pdf b/ws2022/rav/lecture/rav.pdf index 1c286af..139345e 100644 Binary files a/ws2022/rav/lecture/rav.pdf and b/ws2022/rav/lecture/rav.pdf differ diff --git a/ws2022/rav/lecture/rav11.pdf b/ws2022/rav/lecture/rav11.pdf index 25b2a96..b259c89 100644 Binary files a/ws2022/rav/lecture/rav11.pdf and b/ws2022/rav/lecture/rav11.pdf differ diff --git a/ws2022/rav/lecture/rav11.tex b/ws2022/rav/lecture/rav11.tex index 0f2a0af..65e8292 100644 --- a/ws2022/rav/lecture/rav11.tex +++ b/ws2022/rav/lecture/rav11.tex @@ -40,23 +40,21 @@ at $x$: For all $h \in k^{n}$: \end{bem} \begin{definition} - We set - \[ - \mathcal{I}(X)_x^{*} \coloneqq \{ P_x^{*} \colon P \in \mathcal{I}(X) \} - .\] + We set $\mathcal{I}(X)_x^{*}$ to be the ideal generated + by $P_x^{*}$ for all $P \in \mathcal{I}(X)$. \end{definition} -\begin{satz} - The set $\mathcal{I}(X)_x^{*}$ is an ideal of $k[T_1, \ldots, T_n]$. -\end{satz} - -\begin{proof} - By definition, $0 \in \mathcal{I}(X)_x^{*}$. Let $P_x^{*}, Q_x^{*}$ be elements - of $\mathcal{I}(X)_x^{*}$ coming from $P, Q \in \mathcal{I}(X)$. Then - $P_x^{*} - Q_x^{*}$ is of the form $R_x^{*}$ for some $R \in \mathcal{I}(X)$, where - $R = 0$, $R = P$, $R = Q$ or $R = P-Q$. Moreover, for $Q \in k[T_1, \ldots, T_n]$, - we have $P_x^{*} Q = (PQ)_x^{*} \in \mathcal{I}(X)_x^{*}$. -\end{proof} +%\begin{satz} +% The set $\mathcal{I}(X)_x^{*}$ is an ideal of $k[T_1, \ldots, T_n]$. +%\end{satz} +% +%\begin{proof} +% By definition, $0 \in \mathcal{I}(X)_x^{*}$. Let $P_x^{*}, Q_x^{*}$ be elements +% of $\mathcal{I}(X)_x^{*}$ coming from $P, Q \in \mathcal{I}(X)$. Then +% $P_x^{*} - Q_x^{*}$ is of the form $R_x^{*}$ for some $R \in \mathcal{I}(X)$, where +% $R = 0$, $R = P$, $R = Q$ or $R = P-Q$. Moreover, for $Q \in k[T_1, \ldots, T_n]$, +% we have $P_x^{*} Q = (PQ)_x^{*} \in \mathcal{I}(X)_x^{*}$. +%\end{proof} \begin{bem}[] The ideal $\mathcal{I}(X)^{*}$ is finitely generated. However, @@ -324,29 +322,29 @@ consider the Zariski tangent space to $X$ at a point $x \in X$. may vary with $x$. \end{bem} -\begin{satz}[a Jacobian criterion] - If $(P_1, \ldots, P_m)$ are polynomials such that - $\mathcal{I}(X) = (P_1, \ldots, P_m)$ and $\operatorname{rk } P'(x) = m$, where - $P = (P_1, \ldots, P_m)$, then $x$ is a non-singular point of $X$. -\end{satz} - -\begin{proof} - By \ref{kor:cone-in-tangent-space} and \ref{kor:tangent-kernel-jacobian} it suffices to show that - \[ - \mathcal{C}_x(X) \supseteq x + \bigcap_{i=1} ^{m} \text{ker } P_i'(x) - .\] By definition - \[ - \mathcal{C}_x(X) = x + \mathcal{V}_{k^{n}}(\mathcal{I}(X)_x^{*}) - \] and $\mathcal{I}(X)_x^{*} = \{ Q_x^{*} : Q \in \mathcal{I}(X)\} $. If $Q \in \mathcal{I}(X)$, - there exist polynomials $Q_1, \ldots, Q_m$ such that - $Q = \sum_{i=1}^{m} Q_i P_i$, so $Q_x^{*}$ is a linear combination of the $(P_i)_x^{*}$. - Since $\text{rk }(P_1'(x), \ldots, P_m'(x)) = m$, we have - $P_i'(x) \neq 0$ for all $i$. So $(P_i)_x^{*} = P_i'(x)$ in the Taylor expansion - of $P_i$ at $x$. So $Q_x^{*}$ is a linear combination - of $(P_1'(x), \ldots, P_m'(x))$, - which proves that if $h \in \bigcap_{i=1}^{m} \text{ker } P_i'(x)$, then - $Q_x^{*}(h) = 0$ for all $Q \in \mathcal{I}(X)$, hence - $x + h \in \mathcal{C}_x(X)$. -\end{proof} +%\begin{satz}[a Jacobian criterion] +% If $(P_1, \ldots, P_m)$ are polynomials such that +% $\mathcal{I}(X) = (P_1, \ldots, P_m)$ and $\operatorname{rk } P'(x) = m$, where +% $P = (P_1, \ldots, P_m)$, then $x$ is a non-singular point of $X$. +%\end{satz} +% +%\begin{proof} +% By \ref{kor:cone-in-tangent-space} and \ref{kor:tangent-kernel-jacobian} it suffices to show that +% \[ +% \mathcal{C}_x(X) \supseteq x + \bigcap_{i=1} ^{m} \text{ker } P_i'(x) +% .\] By definition +% \[ +% \mathcal{C}_x(X) = x + \mathcal{V}_{k^{n}}(\mathcal{I}(X)_x^{*}) +% \] and $\mathcal{I}(X)_x^{*} = \{ Q_x^{*} : Q \in \mathcal{I}(X)\} $. If $Q \in \mathcal{I}(X)$, +% there exist polynomials $Q_1, \ldots, Q_m$ such that +% $Q = \sum_{i=1}^{m} Q_i P_i$, so $Q_x^{*}$ is a linear combination of the $(P_i)_x^{*}$. +% Since $\text{rk }(P_1'(x), \ldots, P_m'(x)) = m$, we have +% $P_i'(x) \neq 0$ for all $i$. So $(P_i)_x^{*} = P_i'(x)$ in the Taylor expansion +% of $P_i$ at $x$. So $Q_x^{*}$ is a linear combination +% of $(P_1'(x), \ldots, P_m'(x))$, +% which proves that if $h \in \bigcap_{i=1}^{m} \text{ker } P_i'(x)$, then +% $Q_x^{*}(h) = 0$ for all $Q \in \mathcal{I}(X)$, hence +% $x + h \in \mathcal{C}_x(X)$. +%\end{proof} \end{document}