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Feb 3, 2013 at 21:54 comment added Will Sawin "that quotient = $\hat{\mathbb Z}$.
Feb 3, 2013 at 21:53 comment added Will Sawin The ramification subgroups of the Galois group. The map $Gal(\mathbb Q_p) = Gal(\mathbb Q)$ is defined up to conjugacy. There is an extension $1 \to I\to Gal(\mathbb Q_p) \to \hat{\mathbb Z} \to 1$ for a subgroup $I$ called the inertia or ramification group. The Frobenius element is the generator of that quotient. On any representation on which $I$ acts trivially, there is a well-defined conjugacy class of Frobenius, with a well-defined characteristic polynomial.
Feb 3, 2013 at 21:22 comment added Nicole The second part of your answer makes perfect sense to me. Can you explain a little more (or point me to a place that explains) the first part of your answer? Ramification of what over what?
Feb 3, 2013 at 20:55 history answered Will Sawin CC BY-SA 3.0