Ihara’s theorem
Let be a discrete, torsion-free subgroup of (where is the field of -adic numbers (http://planetmath.org/PAdicIntegers)). Then is free.
Proof, or a sketch thereof.
There exists a regular tree on which acts, with stabilizer
(here, denotes the ring of -adic integers (http://planetmath.org/PAdicIntegers)). Since is compact in its profinite topology, so is . Thus, must be compact, discrete and torsion-free. Since compact and discrete implies finite, the only such group is trivial. Thus, acts freely on . Since groups acting freely on trees are free, is free.∎