This question is motivated by a computational issue. Suppose $R$ is a product of orders in numberfields such that there is no ring homomorphism $R \to \mathbb Z$, then can one write an algorithm that actually proves that there is no ring homomorphism $R \to \mathbb Z$ if one is only able to compute $R/pR$ for primes $p$. Sidenote: in this question one should think of $R$ as terribly complicated such that it is not possible to directly work with it in characteristic 0. For so far the motivation now to the mathematics :)


If for a prime $p$ there there is no map $R/p \to \mathbb F\_p$ then there is also no map $R \to \mathbb Z$. So restricting for simplicity to the case where $R$ is an order instead of a product of orders one then naturally arrives at the question in the title.



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> Does there exist an order $R\neq \mathbb Z$ that has a map to $\mathbb F\_p$ for all primes p.
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Now I already have done a bit of thinking about this question but have not been able to solve it yet. However using galois theory one can reduce it to a question of finite groups.


For simplicity let's assume $R=O\_K$ is a maximal order in a numberfield $K$ and also ignore the primes in $\mathbb Z$ above which ramification occurs. Now let $L$ be the galois closure of $K$, $G=Gal(L/\mathbb Q)$ and $H=Gal(L/K)$. Let $q \mid pO\_K$ be a prime, $q' \subset O\_L$ a prime lying above $q$ and let $D\_{q'} \subset G$ (this is step is where I ignore the ramified primes for simplicity) be the decomposition group of $q'$, then $$O\_K/q \cong \mathbb F\_p \Leftrightarrow f(q)=1 \Leftrightarrow D\_{q'} \subset H.$$ So one sees that there is a map $O\_K \to \mathbb F\_p$ if and only if $H \cap \text{Frob}\_p \neq \emptyset$ where $\text{Frob}\_p \subset G$ denotes the conjugacy class of frobenius.
By Chebotarevs density theorem all conjugacy classes will occur as $\text{Frob}\_p$ for a positive density of the primes. So this leads to the following group theoretic question:



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> Let $H \subset G$ be two finite groups with $H \cap C \neq \emptyset$ for all conjugacy classes $C$ of $G$ then does this imply that $G=H$?
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The above at least feels like the answer might be yes, because $H$ should be at least very big if it is to contain an element of every conjugacy class. Using a small computer search I proved that the answer is yes if $G \subset S(9)$. So an order as in my first question has to have at least rank 10 over $\mathbb Z$.



Does there exist an order in a number field of deg>1 with a map to F_p for all p?
Since the answer is already in the comments I'm just putting it here as community wiki.


As Noam Elkies noted:



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> Yes, a proper subgroup of a finite group G cannot intersect every conjugacy class. This has appeared here at least once before: [mathoverflow.net/questions/2697](https://mathoverflow.net/questions/26979)
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