diff --git a/commutative-algebra-notes.pdf b/commutative-algebra-notes.pdf index a0c973b..46139b6 100644 Binary files a/commutative-algebra-notes.pdf and b/commutative-algebra-notes.pdf differ diff --git a/commutative-algebra-notes.tex b/commutative-algebra-notes.tex index 67b8d83..73efa22 100644 --- a/commutative-algebra-notes.tex +++ b/commutative-algebra-notes.tex @@ -1347,6 +1347,31 @@ Recall: a $K$-algebra $A$ is fingen over $K$ if $A=K[y_1, \ldots, y_n]$ for some thus there exists inverse in $A$, so $A$ is a field too. \end{proof} +\begin{thm}{4.10.prev}[Zariski's lemma] \label{zariski} + $k$ a field, $L$ fingen $k$-algebra and a field. Then $L$ is a finite algebraic extension of $k$. + + If $k$ is algebraically closed, then $L=k$. +\end{thm} + +\vspace{0.4cm} +From Zariski's lemma we can see that:\\ +let maximal ideal $m \subset k[X_1, \ldots, X_n]$, then $k[X_1, \ldots, X_n]/m$ is a (quotient) field. + +It's a fingen $k$-algebra, so by Zariski's lemma, $k[X_1, \ldots, X_n]/m$ is algebraic over $k$. + +Since $k[X_1, \ldots, X_n]/m$ is algebraically closed, it must equal $k$; ie. +$$\frac{k[X_1, \ldots, X_n]}{m}=k$$ +Thus $m=(X_1 - a_1, \ldots, X_n - a_n)$ (shown at \ref{5.2}). + +\vspace{0.4cm} +Then: +For $k$ algebraically closed, every maximal ideal of $k[X_1, \ldots, X_n]$ has the form +$(X_1 - a_1, \ldots, X_n - a_n)$ for some $a_i \in k$. + +Equivalently, $V(I) = \emptyset ~~\Longleftrightarrow 1 \in I$. + +\vspace{0.4cm} + \begin{thm}{R.4.10}[Weak Nullstellensatz - Zariski's lemma] \label{zariski} let $k$ a field, $K$ a $k$-algebra which \begin{enumerate} @@ -1545,18 +1570,21 @@ Note: for $k$ a field, $k[X_1, \ldots, X_n]$, $m$ maximal ideal; the residue fie \item if $J \subsetneq k[X_1, \ldots, X_n]$ then $V(J) \neq \emptyset$:\\ Let $m \subset k[X_1, \ldots, X_n]$ be a maximal ideal.\\ + Now, \begin{itemize} - \item Since $m$ maximal, $L= k[X_1, \ldots, X_n]/m$ is a field. - \item Since $k[X_1, \ldots, X_n]$ is a fingen $k$-algebra, $L$ is a fingen $k$-algebra. + \item[-] since $m$ maximal, $L= k[X_1, \ldots, X_n]/m$ is a field. + \item[-] since $k[X_1, \ldots, X_n]$ is a fingen $k$-algebra, $L$ is a fingen $k$-algebra. \end{itemize} $\Longrightarrow~$ thus $L$ is a finite field extension of $k$. - (Recall: if $k$ algebraically closed and $L$ a fingen field extension of $k$, then $L=k$.) + (Recall: if $k$ algebraically closed and $L$ a fingen field extension of $k$, + then $L \cong k$.) Therefore, $$L=\frac{k[X_1, \ldots, X_n]}{m} \cong k$$ - Since $k[X_1, \ldots, X_n]$ is a $k$-algebra, $\exists$ a surjective homomorphism $\psi: k[X_1, \ldots, X_n] \longrightarrow k$. + Since $k[X_1, \ldots, X_n]$ is a $k$-algebra, $\exists$ a surjective homomorphism + $$\psi: k[X_1, \ldots, X_n] \longrightarrow k$$ Let $a_i = \psi(x_i)$. Then $x_i - a_i \in ker(\psi) = m ~\forall~ i \in [n]$.