diff --git a/spaces-chow.tex b/spaces-chow.tex index 2c9fb9497..d1f964bc5 100644 --- a/spaces-chow.tex +++ b/spaces-chow.tex @@ -4363,6 +4363,179 @@ \section{Projective space bundle formula} +\section{The Chern classes of a vector bundle} +\label{section-chern-classes-vector-bundles} + +\noindent +This section is the analogue of Chow Homology, Sections +\ref{chow-section-chern-classes-vector-bundles} and +\ref{chow-section-intersecting-chern-classes}. +However, contrary to what is done there, we directly +define the chern classes of a vector bundle as bivariant classes. +This saves a considerable amount of work. + +\begin{lemma} +\label{lemma-segre-classes} +In Situation \ref{situation-setup} let $X/B$ be good. +Let $\mathcal{E}$ be a finite locally free sheaf of rank $r$ on $X$. +Let $(\pi : P \to X, \mathcal{O}_P(1))$ be the projective space +bundle associated to $\mathcal{E}$. For every +morphism $X' \to X$ of good algebraic spaces over $B$ +there are unique maps +$$ +c_i(\mathcal{E}) \cap - : A_k(X') \longrightarrow A_{k - i}(X'),\quad +i = 0, \ldots, r +$$ +such that for $\alpha \in A_k(X')$ we have +$c_0(\mathcal{E}) \cap \alpha = \alpha$ and +$$ +\sum\nolimits_{i = 0, \ldots, r} +(-1)^i c_1(\mathcal{O}_{P'}(1))^i \cap +(\pi')^*\left(c_{r - i}(\mathcal{E}) \cap \alpha\right) = 0 +$$ +where $\pi' : P' \to X'$ is the base change of $\pi$. +Moreover, these maps define a bivariant class +$c_i(\mathcal{E})$ of degree $i$ on $X$. +\end{lemma} + +\begin{proof} +Uniqueness and existence of the maps $c_i(\mathcal{E}) \cap -$ +follows immediately from Lemma \ref{lemma-chow-ring-projective-bundle} +and the given description of $c_0(\mathcal{E})$. For every $i \in \mathbf{Z}$ +the rule which to every morphism $X' \to X$ of good algebraic spaces +over $B$ assigns the map +$$ +t_i(\mathcal{E}) \cap - : +A_k(X') \longrightarrow A_{k - i}(X'),\quad +\alpha \longmapsto +\pi'_*(c_1(\mathcal{O}_{P'}(1))^{r - 1 + i} \cap (\pi')^*\alpha) +$$ +is a bivariant class\footnote{Up to signs these are +the Segre classes of $\mathcal{E}$.} by Lemmas \ref{lemma-cap-c1-bivariant}, +\ref{lemma-flat-pullback-bivariant}, and +\ref{lemma-push-proper-bivariant}. +By Lemma \ref{lemma-cap-projective-bundle} we have +$t_i(\mathcal{E}) = 0$ for $i < 0$ and $t_0(\mathcal{E}) = 1$. +Applying pushforward to the equation in the statement of the lemma +we find from Lemma \ref{lemma-cap-projective-bundle} that +$$ +(-1)^r t_1(\mathcal{E}) + (-1)^{r - 1}c_1(\mathcal{E}) = 0 +$$ +In particular we find that $c_1(\mathcal{E})$ is a bivariant class. +If we multiply the equation in the statement of the lemma by +$c_1(\mathcal{O}_{P'}(1))$ and push the result forward to $X'$ +we find +$$ +(-1)^r t_2(\mathcal{E}) + +(-1)^{r - 1} t_1(\mathcal{E}) \cap c_1(\mathcal{E}) + +(-1)^{r - 2} c_2(\mathcal{E}) = 0 +$$ +As before we conclude that $c_2(\mathcal{E})$ is a bivariant class. +And so on. +\end{proof} + +\begin{definition} +\label{definition-chern-classes} +In Situation \ref{situation-setup} let $X/B$ be good. +Let $\mathcal{E}$ be a finite locally free sheaf of rank $r$ on $X$. +For $i = 0, \ldots, r$ the {\it $i$th chern class of $\mathcal{E}$} +is the bivariant class $c_i(\mathcal{E}) \in A^i(X)$ of degree $i$ +constructed in Lemma \ref{lemma-segre-classes}. +\end{definition} + +\noindent +For convenience we often set $c_i(\mathcal{E}) = 0$ +for $i > r$ and $i < 0$. By definition +we have $c_0(\mathcal{E}) = 1 \in A^0(X)$. +Here is a sanity check. + +\begin{lemma} +\label{lemma-first-chern-class} +In Situation \ref{situation-setup} let $X/B$ be good. +Let $\mathcal{L}$ be an invertible $\mathcal{O}_X$-module. +The first chern class of $\mathcal{L}$ on $X$ of +Definition \ref{definition-chern-classes} +is equal to the bivariant class of Lemma \ref{lemma-cap-c1-bivariant}. +\end{lemma} + +\begin{proof} +Namely, in this case $P = \mathbf{P}(\mathcal{L}) = X$ with +$\mathcal{O}_P(1) = \mathcal{L}$ by our normalization of the +projective bundle, see Section \ref{section-projective-space-bundle-formula}. +Hence the equation in Lemma \ref{lemma-segre-classes} +reads +$$ +(-1)^0 c_1(\mathcal{L})^0 \cap c^{new}_1(\mathcal{L}) \cap \alpha + +(-1)^1 c_1(\mathcal{L})^1 \cap c^{new}_0(\mathcal{L}) \cap \alpha = 0 +$$ +where $c_i^{new}(\mathcal{L})$ is as in +Definition \ref{definition-chern-classes}. +Since $c_0^{new}(\mathcal{L}) = 1$ and $c_1(\mathcal{L})^0 = 1$ +we conclude. +\end{proof} + +\noindent +Next we see that chern classes are in the center of the bivariant +Chow cohomology ring $A^*(X)$. + +\begin{lemma} +\label{lemma-cap-commutative-chern} +In Situation \ref{situation-setup} let $X/B$ be good. +Let $\mathcal{E}$ be a locally free $\mathcal{O}_X$-module of rank $r$. +Then $c_j(\mathcal{L}) \in A^j(X)$ commutes with every +element $c \in A^p(X)$. In particular, if $\mathcal{F}$ is a +second locally free $\mathcal{O}_X$-module on $X$ of rank $s$, then +$$ +c_i(\mathcal{E}) \cap c_j(\mathcal{F}) \cap \alpha += +c_j(\mathcal{F}) \cap c_i(\mathcal{E}) \cap \alpha +$$ +as elements of $A_{k - i - j}(X)$ for all $\alpha \in A_k(X)$. +\end{lemma} + +\begin{proof} +Let $X' \to X$ be a morphism of good algebraic spaces over $B$. +Let $\alpha \in A_k(X')$. Write $\alpha_j = c_j(\mathcal{E}) \cap \alpha$, so +$\alpha_0 = \alpha$. By Lemma \ref{lemma-segre-classes} we have +$$ +\sum\nolimits_{i = 0}^r +(-1)^i c_1(\mathcal{O}_{P'}(1))^i \cap +(\pi')^*(\alpha_{r - i}) = 0 +$$ +in the chow group of the projective bundle +$(\pi' : P' \to X', \mathcal{O}_{P'}(1))$ +associated to $(X' \to X)^*\mathcal{E}$. +Applying $c \cap -$ and using Lemma \ref{lemma-c1-center} +and the properties of bivariant classes we obtain +$$ +\sum\nolimits_{i = 0}^r +(-1)^i c_1(\mathcal{O}_{P'}(1))^i \cap +\pi^*(c \cap \alpha_{r - i}) = 0 +$$ +in the Chow group of $P'$. Hence we see that $c \cap \alpha_j$ is +equal to $c_j(\mathcal{E}) \cap (c \cap \alpha)$ by the uniqueness in +Lemma \ref{lemma-segre-classes}. This proves the lemma. +\end{proof} + + + + + + + + + + + + + + + + + + + + \input{chapters}