Recall that abelian is contained in nilpotent is contained in solvable.
There is a unique 3 dimensional connected simply connected abelian Lie group $ \mathbb{R}^3 $.
And there is a unique connected simply connected nonabelian nilpotent 3 dimensional Lie group, the Heisenberg group
$$
Nil=\{ \begin{pmatrix} 1 & a & b \\ 0 & 1 & c \\ 0 & 0 & 1 \end{pmatrix} : a,b,c \in \mathbb{R} \}
$$
However there are lots of (simply connected, connected, 3 dimensional) solvable non nilpotent groups.
The two solvable (non nilpotent) groups I can think of are, first,
$$
{SE}_2= \{
\begin{bmatrix}
a & b & x \\
-b & a & y \\
0 & 0 & 1
\end{bmatrix} : a^2+b^2=1 \}
$$
which is the connected component of the identity in the Euclidean group $ E_2= \mathbb{R}^2 \rtimes O_2 $. And, second, the isometry group of the Minkowski plane $ E_{1,1}= \mathbb{R}^2 \rtimes O_{1,1} $
$$
E_{1,1}= \{
\begin{bmatrix}
a & b & x \\
-b & a & y \\
0 & 0 & 1
\end{bmatrix} : a^2-b^2=1 \}
$$
which is isomorphic to
$$
\{
\begin{bmatrix}
a & 0 & x \\
0 & b & y \\
0 & 0 & 1
\end{bmatrix} : ab=1 \}
$$
For solvable non nilpotent Lie groups the Bianchi classification https://en.wikipedia.org/wiki/Bianchi_classification lists 6 distinct types $ 3,4,5,6,6_0,7_0 $ as well as an infinite family of of distinct groups, type $ 7 $.
Which of these groups $ 3,4,5,6,6_0,7_0,7 $ admit cocompact discrete subgroups? A group admitting a cocompact discrete subgroup must be unimodular (see for example https://arxiv.org/pdf/0903.2926.pdf) and of these solvable groups only $ 6_0 $ (corresponding to $ E_{1,1} $) and $ 7_0 $ (corresponding to $ E_2 $) are unimodular and thus possibly contain cocompact discrete subgroups.
For both unimodular groups $ E_2 $ and $ E_{1,1} $ do there exist cocompact discrete subgroups?
If not why not? If so what is an example of a cocompact discrete subgroup?
And which of these groups is the Sol geometry for 3 manifolds based on?
Best Answer
The group $6_0$ is the isometry group of solv geometry, which is the geometry of $\mathbb R^3$ equipped with the Riemannian metric $$e^{2t} dx^2 + e^{-2t} dy^2 + dt^2 $$ The lattices in $6_0$ correspond to those compact Riemannian 3-manifolds whose universal cover is isometric to solv geometry. These 3-manifolds are the mapping tori of Anosov homeomorphisms of the torus $T^2$, described as follows. Start with a matrix $A = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in SL_2(\mathbb Z)$ with $\left|\text{trace}(A)\right| = \left|a+d\right| > 2$, and so $A$ has an independent pair of eigenvectors with eigenvalues $\lambda, \frac{1}{\lambda}$ with $\left|\lambda\right| > 1$. The matrix $A$ preserves the lattice $\mathbb Z \oplus \mathbb Z$ and so its action on $\mathbb R^2$ descends to an Anosov diffeomorphism $f : T^2 \to T^2$. Using the mapping torus construction one obtains the desired 3-manifold $M = T^2 \times [0,1] / (x,1) \sim (f(x),0)$. The fact that the eigenvalues of $M$ form a multiplicative inverse pair allows you to diagonalize $A$ to the form $\begin{pmatrix} \lambda & 0 \\ 0 & 1/\lambda\end{pmatrix}$ and using that to construct a deck transformation invariant solv-geometry metric on the universal cover of $M$.
The group $7_0$ is actually a subgroup of the full isometry group of Euclidean geometry $E^3$. The lattices in that full isometry group are the famous 3-dimensional crystallographic groups, also known as the space groups, and their number is known (look up that link to see). When you add the constraint of being a torsion free subgroup of the Lie subgroup $7_0$ (torsion free is needed for the quotient of $\mathbb R^3$ to be a 3-manifold), then the number of possibilities is cut wayyyy down. These 3-manifolds can in fact be described almost exactly as above, i.e. as certain mapping toruses of $T^2$ diffeomorphisms, with the difference that one requires $\left|\text{trace}(A)\right| = \left|a+d\right| < 1$. The 3-manifolds that you obtain are exactly the mapping toruses of (orientation preserving) finite order diffeomorphisms $f : T^2 \to T^2$. Up to isometry I count 5 of these: the identity (whose mapping torus is $T^3$); an order 2 rotation with 4 fixed points, and an order 4 rotation with 2 fixed points and 2 points of order 2 (picture rotations of a square with opposite sides glued); an order 3 rotation with 3 fixed points, and an order 6 rotation with 1 fixed point, 2 points of order 2, and 3 points of order 3 (picture rotations of a regular hexagon with opposite sides glued).