The Stacks project

63.33 Precise form of Chebotarev

As a first application let us prove a precise form of Chebotarev for a finite ├ętale Galois covering of curves. Let $\varphi : Y \to X$ be a finite ├ętale Galois covering with group $G$. This corresponds to a homomorphism

\[ \pi _1(X) \longrightarrow G = \text{Aut}(Y/X) \]

Assume $Y_{\overline{k}} = $ irreducible. If $C\subset G$ is a conjugacy class then for all $n > 0$, we have

\[ | \# \{ x \in X(k_ n) \mid F_ x \in C\} - \frac{\# C}{\# G} \cdot \# X(k_ n) | \leq (\# C)(2g - 2) \sqrt{q^ n} \]

(Warning: Please check the coefficient $\# C$ on the right hand side carefuly before using.)

Sketch. Write

\[ \varphi _*(\overline{\mathbf{Q}_ l}) = \oplus _{\pi \in \widehat{G}} \mathcal{F}_{\pi } \]

where $\widehat{G}$ is the set of isomorphism classes of irred representations of $G$ over $\overline{\mathbf{Q}}_ l$. For $\pi \in \widehat{G}$ let $\chi _{\pi }: G \to \overline{\mathbf{Q}}_ l$ be the character of $\pi $. Then

\[ H^*(Y_{\overline{k}}, \overline{\mathbf{Q}}_ l) = \oplus _{\pi \in \widehat{G}} H^*(Y_{\overline{k}}, \overline{\mathbf{Q}}_ l)_\pi =_{(\varphi \text{ finite })} \oplus _{\pi \in \widehat{G}} H^*(X_{\overline{k}}, \mathcal{F}_\pi ) \]

If $\pi \neq 1$ then we have

\[ H^0(X_{\overline{k}}, \mathcal{F}_\pi ) = H^2(X_{\overline{k}}, \mathcal{F}_\pi ) = 0,\quad \dim H^1(X_{\overline{k}}, \mathcal{F}_\pi ) = (2g_ X - 2)d_\pi ^2 \]

(can get this from trace formula for acting on ...) and we see that

\[ |\sum _{x \in X(k_ n)} \chi _\pi (\mathcal{F}_ x)| \leq (2g_ X - 2) d_\pi ^2\sqrt{q^ n} \]

Write $1_ C = \sum _\pi a_\pi \chi _\pi $, then $a_\pi = \langle 1_ C, \chi _\pi \rangle $, and $a_1 = \langle 1_ C, \chi _1\rangle = \frac{\# C}{\# G}$ where

\[ \langle f, h\rangle = \frac{1}{\# G}\sum _{g \in G} f(g)\overline{h(g)} \]

Thus we have the relation

\[ \frac{\# C}{\# G} = ||1_ C||^2 = \sum |a_\pi |^2 \]

Final step:

\begin{align*} \# \left\{ x \in X(k_ n) \mid F_ x \in C\right\} & = \sum _{x \in X(k_ n)} 1_ C(x) \\ & = \sum _{x \in X(k_ n)} \sum _\pi a_\pi \chi _\pi (F_ x) \\ & = \underbrace{\frac{\# C}{\# G} \# X(k_ n)}_{ \text{term for }\pi = 1} + \underbrace{\sum _{\pi \neq 1}a_\pi \sum _{x\in X(k_ n)}\chi _\pi (F_ x)}_{ \text{ error term (to be bounded by }E)} \end{align*}

We can bound the error term by

\begin{align*} |E| & \leq \sum _{\pi \in \widehat{G}, \atop \pi \neq 1} |a_\pi | (2g - 2) d_\pi ^2 \sqrt{q^ n} \\ & \leq \sum _{\pi \neq 1} \frac{\# C}{\# G} (2g_ X - 2) d_\pi ^3 \sqrt{q^ n} \end{align*}

By Weil's conjecture, $\# X(k_ n)\sim q^ n$. $\square$


Comments (3)

Comment #867 by Emmanuel Kowalski on

There is a TeXing issue here, at east in the online version.

Comment #868 by on

Indeed, thanks!

It's probably due to the presence of greater-than and lesser-than symbols. On the other hand, the correct LaTeX in this case would be \langle and \rangle, so I hope making the LaTeX more correct also fixes this render bug. If not I'll look into the parsing code.

Comment #870 by on

OK, that seems to have fixed it (refresh the page if it doesn't for you). The change is here. Thanks!


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