My concern arises about the existence of unramified primes in infinite extensions of $\mathbb Q$, particularly in $\overline{\mathbb Q}$.
Now, in general there exist be a definition of ramification for the ring $\mathbb E$ the all algebraic integers in $\overline{\mathbb Q}$?
In this MO post is defined the descomposition group of primes in infinite extensions.
Now, suppose that $p$ is a prime number and $\mathfrak p$ is a prime ideal of $\mathbb E$ such that $\mathfrak p\cap \mathbb Z=(p)$ (exist infinitely prime ideals with this feature), and define $k(\mathfrak p)=\mathbb E/\mathfrak p$, which is a algebraic clousure of $\mathbb F_p$.
Let $G(\mathfrak p)$ and $I(\mathfrak p)$ the descomposition group and the inercia group respectively of $\mathfrak p$, both respect to the absolute Galois group of $\mathbb Q$.
This gives the following sequence
$$0\rightarrow I(\mathfrak p) \stackrel{ }{\rightarrow} G(\mathfrak p) \stackrel{}{\rightarrow} Gal(k(\mathfrak p)/\mathbb F_p) \rightarrow 0$$
- My first question is about the definining a unramified prime number.
A prime number $p$ is unramified in $\mathbb E$ if the inercia group $I(\mathfrak p)$ is trivial
Now we have to see that the definition is not empty.
- My second question is how to prove that the definition is not empty? i.e there exist prime numbers $p$ such that $I(\mathfrak p)$ is trivial.
Thanks you all.