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Ian Agol
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The third example is a spine of the Gieseking manifold. The existence of a spine of this sort follows from a result of Iain Aitchison and the fact that the Gieseking is make of a single regular ideal tetrahedron. I think one could get this directly from the combinatorial description, but I also checked that it's a 1-relator group that fibers with fiber with the same monodromy as the Gieseking manifold.

The group has a presentation $\langle a, b | a^2b^2a^{-1}b^{-1} \rangle$. Letting $u=ab$, we can change to a presentation $ \langle a , u | a^2ua^{-1}ua^{-1} u^{-1} \rangle$. This has abelianization $\mathbb{Z}$ with $a\to 1, u\to 0$. Hence the kernel is generated by $u_i=a^i u a^{-i}$, and the relator shows that $u_0=a(u_1 u_0)a^{-1}, u_1=au_0a^{-1}$, hence this is a free-by-cyclic group with the same monodromy as the Gieseking.

The fourth complex has fundamental group a 1-relator group $\langle a, b| a^2b^2a^{-1}b\rangle$. One may see also that this is a rank-3-free-by-cyclic group. It also has an automorphism given by $(a,b) \mapsto (a, a^{-1}ba )$. Making the substitution $u=b^3a$, one gets a presentation $\langle b,u| bub^{-3}ub^2u^{-1}\rangle$. If $u_i=b^iub^{-i}$, then one has the relations $u_0=b^{-1}(u_0 u_{-2}^{-1})b, u_{-1}=b^{-1}u_0b, u_{-2}=b^{-1}u_{-1}b$, and hence this is the mapping torus of the free group automorphism $(u_0,u_{-1},u_{-2}) \mapsto (u_0u_{-2}^{-1},u_0,u_{-1})$.

Since the ranks of the kernels of the maps to $\mathbb{Z}$ are different, the groups are not isomorphic.

I'm guessing that you can find these examples in the literature on 1-relator groups, but I'm not sure if they have names in that context.

The third example is a spine of the Gieseking manifold. The existence of a spine of this sort follows from a result of Iain Aitchison and the fact that the Gieseking is make of a single regular ideal tetrahedron. I think one could get this directly from the combinatorial description, but I also checked that it's a 1-relator group that fibers with fiber with the same monodromy as the Gieseking manifold.

The group has a presentation $\langle a, b | a^2b^2a^{-1}b^{-1} \rangle$. Letting $u=ab$, we can change to a presentation $ \langle a , u | a^2ua^{-1}ua^{-1} u^{-1} \rangle$. This has abelianization $\mathbb{Z}$ with $a\to 1, u\to 0$. Hence the kernel is generated by $u_i=a^i u a^{-i}$, and the relator shows that $u_0=a(u_1 u_0)a^{-1}, u_1=au_0a^{-1}$, hence this is a free-by-cyclic group with the same monodromy as the Gieseking.

The fourth complex has fundamental group a 1-relator group $\langle a, b| a^2b^2a^{-1}b\rangle$. One may see also that this is a rank-3-free-by-cyclic group. It also has an automorphism given by $(a,b) \mapsto (a, a^{-1}ba )$. Making the substitution $u=b^3a$, one gets a presentation $\langle b,u| bub^{-3}ub^2u^{-1}\rangle$. If $u_i=b^iub^{-i}$, then one has the relations $u_0=b^{-1}(u_0 u_{-2}^{-1})b, u_{-1}=b^{-1}u_0b, u_{-2}=b^{-1}u_{-1}b$, and hence this is the mapping torus of the free group automorphism $(u_0,u_{-1},u_{-2}) \mapsto (u_0u_{-2}^{-1},u_0,u_{-1})$.

I'm guessing that you can find these examples in the literature on 1-relator groups, but I'm not sure if they have names in that context.

The third example is a spine of the Gieseking manifold. The existence of a spine of this sort follows from a result of Iain Aitchison and the fact that the Gieseking is make of a single regular ideal tetrahedron. I think one could get this directly from the combinatorial description, but I also checked that it's a 1-relator group that fibers with fiber with the same monodromy as the Gieseking manifold.

The group has a presentation $\langle a, b | a^2b^2a^{-1}b^{-1} \rangle$. Letting $u=ab$, we can change to a presentation $ \langle a , u | a^2ua^{-1}ua^{-1} u^{-1} \rangle$. This has abelianization $\mathbb{Z}$ with $a\to 1, u\to 0$. Hence the kernel is generated by $u_i=a^i u a^{-i}$, and the relator shows that $u_0=a(u_1 u_0)a^{-1}, u_1=au_0a^{-1}$, hence this is a free-by-cyclic group with the same monodromy as the Gieseking.

The fourth complex has fundamental group a 1-relator group $\langle a, b| a^2b^2a^{-1}b\rangle$. One may see also that this is a rank-3-free-by-cyclic group. It also has an automorphism given by $(a,b) \mapsto (a, a^{-1}ba )$. Making the substitution $u=b^3a$, one gets a presentation $\langle b,u| bub^{-3}ub^2u^{-1}\rangle$. If $u_i=b^iub^{-i}$, then one has the relations $u_0=b^{-1}(u_0 u_{-2}^{-1})b, u_{-1}=b^{-1}u_0b, u_{-2}=b^{-1}u_{-1}b$, and hence this is the mapping torus of the free group automorphism $(u_0,u_{-1},u_{-2}) \mapsto (u_0u_{-2}^{-1},u_0,u_{-1})$.

Since the ranks of the kernels of the maps to $\mathbb{Z}$ are different, the groups are not isomorphic.

I'm guessing that you can find these examples in the literature on 1-relator groups, but I'm not sure if they have names in that context.

Source Link
Ian Agol
  • 68.8k
  • 3
  • 194
  • 358

The third example is a spine of the Gieseking manifold. The existence of a spine of this sort follows from a result of Iain Aitchison and the fact that the Gieseking is make of a single regular ideal tetrahedron. I think one could get this directly from the combinatorial description, but I also checked that it's a 1-relator group that fibers with fiber with the same monodromy as the Gieseking manifold.

The group has a presentation $\langle a, b | a^2b^2a^{-1}b^{-1} \rangle$. Letting $u=ab$, we can change to a presentation $ \langle a , u | a^2ua^{-1}ua^{-1} u^{-1} \rangle$. This has abelianization $\mathbb{Z}$ with $a\to 1, u\to 0$. Hence the kernel is generated by $u_i=a^i u a^{-i}$, and the relator shows that $u_0=a(u_1 u_0)a^{-1}, u_1=au_0a^{-1}$, hence this is a free-by-cyclic group with the same monodromy as the Gieseking.

The fourth complex has fundamental group a 1-relator group $\langle a, b| a^2b^2a^{-1}b\rangle$. One may see also that this is a rank-3-free-by-cyclic group. It also has an automorphism given by $(a,b) \mapsto (a, a^{-1}ba )$. Making the substitution $u=b^3a$, one gets a presentation $\langle b,u| bub^{-3}ub^2u^{-1}\rangle$. If $u_i=b^iub^{-i}$, then one has the relations $u_0=b^{-1}(u_0 u_{-2}^{-1})b, u_{-1}=b^{-1}u_0b, u_{-2}=b^{-1}u_{-1}b$, and hence this is the mapping torus of the free group automorphism $(u_0,u_{-1},u_{-2}) \mapsto (u_0u_{-2}^{-1},u_0,u_{-1})$.

I'm guessing that you can find these examples in the literature on 1-relator groups, but I'm not sure if they have names in that context.