diff --git a/2021_Algebra_I.tex b/2021_Algebra_I.tex index 79bb718..b475dc4 100644 --- a/2021_Algebra_I.tex +++ b/2021_Algebra_I.tex @@ -30,14 +30,12 @@ Fields which are not assumed to be algebraically closed have been renamed (usually to $\mathfrak{l}$). \pagebreak - \section{Finiteness conditions} \input{inputs/finiteness_conditions} \section{The Nullstellensatz and the Zariski topology} \input{inputs/nullstellensatz_and_zariski_topology} - % Lecture 11 \section{Projective spaces} \input{inputs/projective_spaces} @@ -46,4 +44,9 @@ Fields which are not assumed to be algebraically closed have been renamed (usual \section{Varieties} \input{inputs/varieties} +\iffalse +\section{Übersicht} +\input{inputs/uebersicht} +\fi + \end{document} diff --git a/inputs/uebersicht.tex b/inputs/uebersicht.tex new file mode 100644 index 0000000..fa705ea --- /dev/null +++ b/inputs/uebersicht.tex @@ -0,0 +1,125 @@ +% TODO REMARK ABOUT ZORNS LEMMA (LECTURE 1) + +% TODO REMARK ABOUT FIN PRESENTED MODULES (LECTURE 2) + + +% TODO: LECTURE 9 LEMMA + + + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +% ÜBERSICHT % +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + + +% List of forms of HNS + + +\begin{itemize} + \item[HNS2 $\implies$ HNS1b] Let $I \subseteq \mathfrak{l}[X_1,\ldots,X_n]$. $I \subseteq \mathfrak{m}$ maximal. $R / \mathfrak{m}$ is isomorphic to a field extension of $\mathfrak{l}$. Finite by HNS2. + \item[NNT $\implies$ HNS2] Apply NNT to $L / K$ $\leadsto$ alg. independent $a_i$ such that $L$ is finite over the image of $K[X_1,\ldots,X_n] \xrightarrow{\ev_a} L$ ($\ev_a : P \to P(a_1,\ldots,a_n)$). + $\ev_a(K[X_1,\ldots,X_n])$ is a field by fact about integrality and fields \ref{fintaf}. Hence $n = 0$ and $L / K$ is finite. + \item[UNCHNS2] $K$ uncountable, $L / K$ fin. type. Then $\dim_K L$ is countable. Suppose $l \in L$ is not integral. Then $K(l) \cong K(T)$ and $\dim_K L \ge \dim_K K(T) \ge \aleph_1$. + Thus $L / K$ algebraic $\implies$ integral $\implies$ finite. + \item[HNS3] ($V(I) \subseteq V(f) \iff f \in \sqrt{I} $). Suppose $V(I) \subseteq V(f)$. $R' \coloneqq \mathfrak{k}[X_1,\ldots,X_n, T], J \subseteq R'$ the ideal generated by $I$ and $g(X_1,\ldots,X_n,T) \coloneqq 1 - Tf(X_1,\ldots,X_n)$. +\end{itemize} + + +% Proofs +Def of integrality (<=>) + + +Fact about integrality and field: + % TODO + + +Technical lemma for Noether normalization: For $S \subseteq \N^n$ finite, there exists $k \in \N^n$ such that $k_1 = 1$ and $s_1 \neq s_2 \in S \implies \langle k, s_1 \rangle \neq \langle k, s_2 \rangle$: +For $s_1 \neq s_2$, % TODO + +Noether normalization: +$a_i \in A$ minimal such that $A$ is integral over the subalgebra genereted by the $a_i$. +Suppose $\exists P \in K[X_1,\ldots,X_n] \setminus \{ 0\} ~ P(a_1,\ldots,a_n) = 0$. $P = \sum_{\alpha \in \N^n} p_\alpha X^\alpha, S \coloneqq \{ \alpha \in \N^n | p_\alpha \neq 0\}$. +Choose $k$ as in the lemma. +$b_i \coloneqq a_{i+1} - a_1^{k_{i+1}}, 1 \le i \trdeg(\mathfrak{k}(\fq) / \mathfrak{l})$: +\Wlog $\fp = \{0\}$ and $A$ a domain ($A' \coloneqq A / \fp$). +For $\fq \in \MaxSpec A$, $\mathfrak{k}(\fq) = A / \fq$ finite type, hence finite (HNS) $\implies \trdeg(\mathfrak{k}(\fq) / \mathfrak{l}) = 0$. +$\trdeg(Q(A) / \mathfrak{l}) = 0 \implies A$ integral over $\mathfrak{k}$ $\implies$ $A$ a field $\implies \fp = \fq \lightning$. + +If $\fq \not\in \MaxSpec A$, let $a_1,\ldots,a_n \in A$ alg. independent such that the $\overline{a_i}$ are a transcendence base for $\mathfrak{k}(\fq) / \mathfrak{k}$ +Let $R$ be the ring generated by $\mathfrak{l}$ and the $a_i$. Localize with respect to $S \coloneqq R \setminus \{0\}$. +%TODO +% TODO: LERNEN + + +% Dim k^n +$\dim(\mathfrak{k}^n)$ +$ \ge n$ build chian +$\le n$ a first result in dim T ($\fp \subsetneq \fq \implies \trdeg(\mathfrak{k}(\fq) / \mathfrak{l}) < \trdeg(\mathfrak{k}(\fp) / \mathfrak{l})$. Thus $\codim(X,Y) \le \trdeg(\mathfrak{K}(Y) / \mathfrak{l}) - \trdeg(\mathfrak{K}(X) / \mathfrak{l})$. + +TODO +% List of proofs of HNS + + +% Going up + + +% TODO proof of dim Y = trdeg(K(Y) / k) +$\dim Y \ge \trdeg(\mathfrak{k}(Y) / \mathfrak{k})$: Noether normalization. Subalgebra $\cong \mathfrak{k}[X_1,\ldots,X_d]$. Lift chain of prime ideals using going up. + +% TODO prime avoidance + + +Action of $\Aut(L/K)$ on prime ideals of a normal ring extension. $A$ normal domain, $L / Q(A)$ normal field extension, $B$ int closure of $A$ in $L$, $\fp \in \Spec A$. +Then $\Aut(L / K)$ transitively acts on $\{\fq \in \Spec B | \fq \cap A = \fp\}$ : + +\begin{itemize} + \item $\fq, \fr \in \Spec B$ lying over $\fp$. + \item only need to show $\fq \subseteq \sigma(\fr)$ for some $\sigma \in G$ (Krull going-up, no inclusions) + \item Suppose not. Then $x \in \fq \setminus \bigcup_{\sigma \in G} \sigma(\fr)$ (prime aviodance) + \item $y = \prod_{\sigma \in G} \sigma(x) \in \fq \setminus \fr$ ($\fr$ prime ideal) + \item $\exists k \in \N$ s.t. $y^k \in K$ ($y \in L^G$) + \item $y^k \in K \cap B = A $ ($A$ normal). Thus $y^k \in (A \cap \fq) \setminus (A \cap \fr) = \fp \setminus \fp$. + \item $L / K$ infinite: Apply Zorn to pairs $(M, \sigma)$ where $K \subseteq M \subseteq L$ and $\sigma \in \Aut(M /K)$ s.t. $\sigma(\fr \cap M) = \fq \cap M$. +\end{itemize} + + +Going down Krull %TODO + +The ht p and trdeg +================== +% TODO % TODO % TODO % + +% Definitions +Zariski-Topology, Spec, $\mathfrak{k}^n$ +Residue field $\mathfrak{k}(\fp) \coloneqq Q(A / \fp), \mathfrak{K}(V(\fp)) \coloneqq \mathfrak{k}(\fp)$. TODO? +% Counterexamples + no going-up +% list of definitions of codim, dim, trdeg, ht +Original (Noether normalization) +Artin-Tate +Uncountable fields +\begin{landscape} +\section{Übersicht} +{\rowcolors{2}{gray!10}{white} + \begin{longtable}{lll} + \end{longtable} +} + +\end{landscape} +