64.5 Why derived categories?
With this definition of the trace, let us now discuss another issue with the formula as stated. Let C be a smooth projective curve over k. Then there is a correspondence between finite locally constant sheaves \mathcal{F} on C_{\acute{e}tale} whose stalks are isomorphic to {(\mathbf{Z}/\ell ^ n\mathbf{Z})}^{\oplus m} on the one hand, and continuous representations \rho : \pi _1 (C, \bar c) \to \text{GL}_ m(\mathbf{Z}/\ell ^ n\mathbf{Z})) (for some fixed choice of \bar c) on the other hand. We denote \mathcal{F}_\rho the sheaf corresponding to \rho . Then H^2 (C_{\bar k}, \mathcal{F}_\rho ) is the group of coinvariants for the action of \rho (\pi _1 (C, \bar c)) on {(\mathbf{Z}/\ell ^ n\mathbf{Z})}^{\oplus m}, and there is a short exact sequence
For instance, let \mathbf{Z} = \mathbf{Z} \sigma act on \mathbf{Z}/\ell ^2\mathbf{Z} via \sigma (x) = (1+\ell ) x. The coinvariants are (\mathbf{Z}/\ell ^2\mathbf{Z})_{\sigma } = \mathbf{Z}/\ell \mathbf{Z}, which is not a flat \mathbf{Z}/\ell ^2\mathbf{Z}-module. Hence we cannot take the trace of some action on H^2(C_{\bar k}, \mathcal{F}_\rho ), at least not in the sense of the previous section.
In fact, our goal is to consider a trace formula for \ell -adic coefficients. But \mathbf{Q}_\ell = \mathbf{Z}_\ell [1/\ell ] and \mathbf{Z}_\ell = \mathop{\mathrm{lim}}\nolimits \mathbf{Z}/\ell ^ n\mathbf{Z}, and even for a flat \mathbf{Z}/\ell ^ n\mathbf{Z} sheaf, the individual cohomology groups may not be flat, so we cannot compute traces. One possible remedy is consider the total derived complex R\Gamma (C_{\bar k}, \mathcal{F}_\rho ) in the derived category D(\mathbf{Z}/\ell ^ n\mathbf{Z}) and show that it is a perfect object, which means that it is quasi-isomorphic to a finite complex of finite free module. For such complexes, we can define the trace, but this will require an account of derived categories.
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