A couple of months ago I was in Oberwolfach, where Tomer Schlank told me about the following conjecture:
Conjecture. (Boston-Markin) Let G be a finite group. Then there exists a Galois extension of Q with Galois group G ramified over exactly d(G) primes, where d(G) is the minimal number of generators of Gab. (Here we violate convention and say that d({1})=1.)
It's not hard to see that d(G) is a lower bound for the minimal number of primes over which a G-extension ramifies, but I don't see much evidence for the conjecture (except for some results in the case of e.g. solvable groups). Here is a topological analogue (translating the conjecture through the usual primes-vs.-knots dictionary).
Question. Let G be a finite group and M a 3-manifold. Does there exist a link L in M with d(G) components, and a G-cover of S3 branched only over L?
The case most analogous to the original conjecture is M=S3 (or in general, the case where M is simply connected), where one can see that d(G) is a lower bound for the number of components of a link with the desired properties. We weren't able to make much progress with S3, but Tomer, Brian Lawrence, and I were able to show that the answer is yes if G is a finite simple group and M is S2×S1, or indeed any surface bundle over a circle! (Of course such manifolds have infinite fundamental group, so the problem is strictly easier in this case.) If I have time, I'll sketch the argument in another post. Please let me know if you have any thoughts about this question!