Skip to main content
added 813 characters in body
Source Link
Todd Trimble
  • 53.3k
  • 6
  • 205
  • 322

If "rational rank 1" simply means that the underlying group is isomorphic to $\mathbb{Q}$, then every such lattice-ordered group structure $(\mathbb{Q}, \preceq)$ is totally ordered, in fact isomorphic to $\mathbb{Q}$ with its usual order $\leq$.

By a classical result (see the first sentence here), any abelian lattice-ordered group ($l$-group) $G$ can be embedded (by an $l$-group homomorphism) into a direct product of totally ordered non-trivial abelian groups. By projecting to any one of the factors, there exists a totally ordered $l$-group $G'$ and a non-trivial $l$-group homomorphism $f: G \to G'$. By an epi-mono factorization, we may assume $f$ is surjective.

In the case $G = \mathbb{Q}$, any group quotient $G'$ is divisible, and it is well-known that an $l$-group $G'$ is torsionfree (see e.g. here, corollary 1.2.6 page 16). But divisible torsionfree abelian groups are rational vector spaces, so in this situation the quotient $G'$ is isomorphic to $\mathbb{Q}$ and $f: G \to G'$ is a group isomorphism. But $f$ is also a lattice homomorphism. Since bijective lattice homomorphisms are lattice isomorphisms, $f: G \to G'$ is a lattice isomorphism, and hence $G$ is totally ordered, as desired.

Finally, a totally ordered $l$-group structure $(\mathbb{Q}, \preceq)$ is given by two submonoids $P, N = -P$ whose intersection is $\{0\}$ and whose union is $\mathbb{Q}$. To show $(\mathbb{Q}, \preceq)$ is isomorphic to $(\mathbb{Q}, \leq)$, we need only show that $P$ contains every multiple $\frac{m}{n}p$ where $p \in P$ is any chosen non-zero element and $0 < m, n$. But clearly the submonoid generated by $\frac{m}{n}p$ contains $0 \neq mp \in P$. Thus $\frac{m}{n}p \in N$ is impossible. This completes the proof.

Edit: Following on Andreas's first interpretation of "rational rank 1" (subgroup $B \subseteq \mathbb{Q}$), the same results hold. For again, we have a surjective $l$-group homomorphism $f: B \to B'$ to a totally ordered $l$-group $B'$, where $B'$ is nontrivial and torsionfree. If $A = \ker(f)$, we have $\mathrm{rank}(B) = \mathrm{rank}(A) + \mathrm{rank}(B')$, where $\mathrm{rank}(B') = 1$ since $B'$ is torsionfree. Thus $\mathrm{rank}(A) = 0$, meaning $A$ is a torsion subgroup of a torsionfree group, meaning $A = 0$. Thus $f: B \to B'$ is a group isomorphism, and we deduce as before it is a lattice isomorphism, so $B$ is totally ordered. We deduce that $B$ is isomorphic to $B$ with its standard lattice structure, by essentially the same argument as in the last paragraph before this edit.

If "rational rank 1" simply means that the underlying group is isomorphic to $\mathbb{Q}$, then every such lattice-ordered group structure $(\mathbb{Q}, \preceq)$ is totally ordered, in fact isomorphic to $\mathbb{Q}$ with its usual order $\leq$.

By a classical result (see the first sentence here), any abelian lattice-ordered group ($l$-group) $G$ can be embedded (by an $l$-group homomorphism) into a direct product of totally ordered non-trivial abelian groups. By projecting to any one of the factors, there exists a totally ordered $l$-group $G'$ and a non-trivial $l$-group homomorphism $f: G \to G'$. By an epi-mono factorization, we may assume $f$ is surjective.

In the case $G = \mathbb{Q}$, any group quotient $G'$ is divisible, and it is well-known that an $l$-group $G'$ is torsionfree (see e.g. here, corollary 1.2.6 page 16). But divisible torsionfree abelian groups are rational vector spaces, so in this situation the quotient $G'$ is isomorphic to $\mathbb{Q}$ and $f: G \to G'$ is a group isomorphism. But $f$ is also a lattice homomorphism. Since bijective lattice homomorphisms are lattice isomorphisms, $f: G \to G'$ is a lattice isomorphism, and hence $G$ is totally ordered, as desired.

Finally, a totally ordered $l$-group structure $(\mathbb{Q}, \preceq)$ is given by two submonoids $P, N = -P$ whose intersection is $\{0\}$ and whose union is $\mathbb{Q}$. To show $(\mathbb{Q}, \preceq)$ is isomorphic to $(\mathbb{Q}, \leq)$, we need only show that $P$ contains every multiple $\frac{m}{n}p$ where $p \in P$ is any chosen non-zero element and $0 < m, n$. But clearly the submonoid generated by $\frac{m}{n}p$ contains $0 \neq mp \in P$. Thus $\frac{m}{n}p \in N$ is impossible. This completes the proof.

If "rational rank 1" simply means that the underlying group is isomorphic to $\mathbb{Q}$, then every such lattice-ordered group structure $(\mathbb{Q}, \preceq)$ is totally ordered, in fact isomorphic to $\mathbb{Q}$ with its usual order $\leq$.

By a classical result (see the first sentence here), any abelian lattice-ordered group ($l$-group) $G$ can be embedded (by an $l$-group homomorphism) into a direct product of totally ordered non-trivial abelian groups. By projecting to any one of the factors, there exists a totally ordered $l$-group $G'$ and a non-trivial $l$-group homomorphism $f: G \to G'$. By an epi-mono factorization, we may assume $f$ is surjective.

In the case $G = \mathbb{Q}$, any group quotient $G'$ is divisible, and it is well-known that an $l$-group $G'$ is torsionfree (see e.g. here, corollary 1.2.6 page 16). But divisible torsionfree abelian groups are rational vector spaces, so in this situation the quotient $G'$ is isomorphic to $\mathbb{Q}$ and $f: G \to G'$ is a group isomorphism. But $f$ is also a lattice homomorphism. Since bijective lattice homomorphisms are lattice isomorphisms, $f: G \to G'$ is a lattice isomorphism, and hence $G$ is totally ordered, as desired.

Finally, a totally ordered $l$-group structure $(\mathbb{Q}, \preceq)$ is given by two submonoids $P, N = -P$ whose intersection is $\{0\}$ and whose union is $\mathbb{Q}$. To show $(\mathbb{Q}, \preceq)$ is isomorphic to $(\mathbb{Q}, \leq)$, we need only show that $P$ contains every multiple $\frac{m}{n}p$ where $p \in P$ is any chosen non-zero element and $0 < m, n$. But clearly the submonoid generated by $\frac{m}{n}p$ contains $0 \neq mp \in P$. Thus $\frac{m}{n}p \in N$ is impossible. This completes the proof.

Edit: Following on Andreas's first interpretation of "rational rank 1" (subgroup $B \subseteq \mathbb{Q}$), the same results hold. For again, we have a surjective $l$-group homomorphism $f: B \to B'$ to a totally ordered $l$-group $B'$, where $B'$ is nontrivial and torsionfree. If $A = \ker(f)$, we have $\mathrm{rank}(B) = \mathrm{rank}(A) + \mathrm{rank}(B')$, where $\mathrm{rank}(B') = 1$ since $B'$ is torsionfree. Thus $\mathrm{rank}(A) = 0$, meaning $A$ is a torsion subgroup of a torsionfree group, meaning $A = 0$. Thus $f: B \to B'$ is a group isomorphism, and we deduce as before it is a lattice isomorphism, so $B$ is totally ordered. We deduce that $B$ is isomorphic to $B$ with its standard lattice structure, by essentially the same argument as in the last paragraph before this edit.

Source Link
Todd Trimble
  • 53.3k
  • 6
  • 205
  • 322

If "rational rank 1" simply means that the underlying group is isomorphic to $\mathbb{Q}$, then every such lattice-ordered group structure $(\mathbb{Q}, \preceq)$ is totally ordered, in fact isomorphic to $\mathbb{Q}$ with its usual order $\leq$.

By a classical result (see the first sentence here), any abelian lattice-ordered group ($l$-group) $G$ can be embedded (by an $l$-group homomorphism) into a direct product of totally ordered non-trivial abelian groups. By projecting to any one of the factors, there exists a totally ordered $l$-group $G'$ and a non-trivial $l$-group homomorphism $f: G \to G'$. By an epi-mono factorization, we may assume $f$ is surjective.

In the case $G = \mathbb{Q}$, any group quotient $G'$ is divisible, and it is well-known that an $l$-group $G'$ is torsionfree (see e.g. here, corollary 1.2.6 page 16). But divisible torsionfree abelian groups are rational vector spaces, so in this situation the quotient $G'$ is isomorphic to $\mathbb{Q}$ and $f: G \to G'$ is a group isomorphism. But $f$ is also a lattice homomorphism. Since bijective lattice homomorphisms are lattice isomorphisms, $f: G \to G'$ is a lattice isomorphism, and hence $G$ is totally ordered, as desired.

Finally, a totally ordered $l$-group structure $(\mathbb{Q}, \preceq)$ is given by two submonoids $P, N = -P$ whose intersection is $\{0\}$ and whose union is $\mathbb{Q}$. To show $(\mathbb{Q}, \preceq)$ is isomorphic to $(\mathbb{Q}, \leq)$, we need only show that $P$ contains every multiple $\frac{m}{n}p$ where $p \in P$ is any chosen non-zero element and $0 < m, n$. But clearly the submonoid generated by $\frac{m}{n}p$ contains $0 \neq mp \in P$. Thus $\frac{m}{n}p \in N$ is impossible. This completes the proof.