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Jan 17 at 11:00 answer added Carl-Fredrik Nyberg Brodda timeline score: 7
Jan 17 at 10:48 comment added Carl-Fredrik Nyberg Brodda Note: Behr & Mennicke's paper actually gives $n=2$ and all prime $k$, not just $k=2$.
Jun 20, 2018 at 7:10 vote accept ahulpke
Jun 19, 2018 at 15:24 answer added Matthias Wendt timeline score: 7
Jun 18, 2018 at 20:38 comment added Luc Guyot Maybe "settles" is wrong. Serre recovered the presentations of Behr and Mennick for $n = 2$ and $k = 2, 3$ in "Cohomologie des groupes discrets", 1969. Those presentations seem to be the only explicit presentations at that time. (L&S mention that Proposition 13.1 can be used to compute presentations, but it becomes soon "unwieldy", even in the simplest cases).
Jun 18, 2018 at 20:07 comment added Luc Guyot Proposition 13.3 of Lyndon & Schupp's "Combinatorial Group Theory" settles the case $n = 2$ and $k$ is any prime number. The result is attributed to Yasutaka Ihara, "On discrete subgroups of the two by two projective linear group over $\mathfrak{p}$ -adic fields", 1966. (I presume that it can be inferred from Remark 3 and §5 where the structure of $PGL(2, \mathbb{Q}_p))$ as an amalgam is given.
Jun 18, 2018 at 14:16 history edited ahulpke
Added general group theory tag
Jun 17, 2018 at 19:51 history edited ahulpke CC BY-SA 4.0
Corrected localization as pointed out in comment.
Jun 17, 2018 at 19:49 comment added ahulpke @JulianRosen Yes, will correct.
Jun 17, 2018 at 19:46 comment added Julian Rosen The set $\frac{1}{k}\mathbb{Z}$ is not a ring. Do you want $\mathbb{Z}[1/k]$
Jun 17, 2018 at 19:19 history asked ahulpke CC BY-SA 4.0