diff --git a/2021_Algebra_I.tex b/2021_Algebra_I.tex index 092461a..a2fb884 100644 --- a/2021_Algebra_I.tex +++ b/2021_Algebra_I.tex @@ -2465,7 +2465,7 @@ The following is somewhat harder than in the affine case: \item The category of sets. \item The category of groups. \item The category of rings. - \item If $R$ is a ring, the category of $R$-modules and the category $\foralllg_R$ of $R$-algebras + \item If $R$ is a ring, the category of $R$-modules and the category $\Alg_R$ of $R$-algebras \item The category of topological spaces \item The category $\Var_\mathfrak{k}$ of varieties over $\mathfrak{k}$ (see \ref{defvariety}) \item If $\mathcal{A}$ is a category, then the \vocab{opposite category} or \vocab{dual category} is defined by $\Ob(\mathcal{A}\op) = \Ob(\mathcal{A})$ and $\Hom_{\mathcal{A}\op}(X,Y) = \Hom_\mathcal{A}(Y,X)$. @@ -2482,7 +2482,7 @@ The following is somewhat harder than in the affine case: \item The category of abelian groups is a full subcategory of the category of groups. It can be identified with the category of $\Z$-modules. \item The category of finitely generated $R$-modules as a full subcategory of the category of $R$-modules. - \item The category of $R$-algebras of finite type as a full subcategory of $\foralllg_R$. + \item The category of $R$-algebras of finite type as a full subcategory of $\Alg_R$. \item The category of affine varieties over $\mathfrak{k}$ as a full subcategory of the category of varieties over $\mathfrak{k}$. \end{itemize} \end{example} @@ -2563,17 +2563,17 @@ The following is somewhat harder than in the affine case: \begin{itemize} \item Let $X,Y$ be varieties over $\mathfrak{k}$. Then the map \begin{align} - \phi: \Hom_{\Var_\mathfrak{k}}(X,Y) &\longrightarrow \Hom_{\foralllg_\mathfrak{k}}(\mathcal{O}_Y(Y), \mathcal{O}_X(X)) \\ + \phi: \Hom_{\Var_\mathfrak{k}}(X,Y) &\longrightarrow \Hom_{\Alg_\mathfrak{k}}(\mathcal{O}_Y(Y), \mathcal{O}_X(X)) \\ (X \xrightarrow{f} Y) &\longmapsto (\mathcal{O}_Y(Y) \xrightarrow{f\st} \mathcal{O}_X(X)) \end{align} is injective when $Y$ is quasi-affine and bijective when $Y$ is affine. \item The contravariant functor \begin{align} - F: \Var_\mathfrak{k} &\longrightarrow \foralllg_\mathfrak{k} \\ + F: \Var_\mathfrak{k} &\longrightarrow \Alg_\mathfrak{k} \\ X &\longmapsto \mathcal{O}_X(X)\\ (X\xrightarrow{f} Y) &\longmapsto (\mathcal{O}_X(X) \xrightarrow{f\st} \mathcal{O}_Y(Y)) \end{align} - restricts to an equivalence of categories between the category of affine varieties over $\mathfrak{k}$ and the full subcategory $\mathcal{A}$ of $\foralllg_\mathfrak{k}$, + restricts to an equivalence of categories between the category of affine varieties over $\mathfrak{k}$ and the full subcategory $\mathcal{A}$ of $\Alg_\mathfrak{k}$, having the $\mathfrak{k}$-algebras $A$ of finite type with $\nil A = \{0\} $ as objects. \end{itemize} \end{proposition} @@ -2630,7 +2630,7 @@ The following is somewhat harder than in the affine case: \end{proof} \begin{remark} - Note that giving a contravariant functor $\cC \to \cD$ is equivalent to giving a functor $\cC \to \cD\op$. We have thus shown that the category of affine varieties is equivalent to $\mathcal{A}\op$, where $\mathcal{A} \subsetneq \foralllg_\mathfrak{k}$ is the full subcategory of $\mathfrak{k}$-algebras $A$ of finite type with $\nil(A) = \{0\}$. + Note that giving a contravariant functor $\cC \to \cD$ is equivalent to giving a functor $\cC \to \cD\op$. We have thus shown that the category of affine varieties is equivalent to $\mathcal{A}\op$, where $\mathcal{A} \subsetneq \Alg_\mathfrak{k}$ is the full subcategory of $\mathfrak{k}$-algebras $A$ of finite type with $\nil(A) = \{0\}$. \end{remark} \subsubsection{Affine open subsets are a topology base}