Let $\left(W,S\right)$ be a non-affine, irreducible Coxeter system and assume that $W$ contains a copy of $\mathbb{Z}\oplus\mathbb{Z}$ (this is equivalent to $W$ being not word hyperbolic). Does this imply that $W$ contains a copy of $\mathbb{Z}\oplus\mathbb{F}\_{2}$ as well? Here $\mathbb{F}\_{2}$ denotes the free group in two generators. If this is not true: Is it at least true for $S$ large enough?



Copies of $\mathbb{Z}\oplus \mathbb{F}_2$ in non-affine, irreducible Coxeter groups
Consider the free product $W = \tilde A\_2 \* A\_1 \* A\_1 \* \ldots \* A\_1$. This Coxeter group is not affine, and it has a copy of $\mathbb Z^2$ within the $\tilde A\_2$ part, so it satisfies your condition.


Now assume $W$ contains a copy of $\mathbb F\_2 \oplus \mathbb Z$, such that $u,v$ generate $\mathbb F\_2$ and $w$ generates $\mathbb Z$. This means that $A := \langle u,w\rangle$ and $B := \langle v,w\rangle$ are two disjoint copies of $\mathbb Z^2$ whose intersection is $\langle w\rangle = \mathbb Z$.


Now I claim (proof in comments) that all copies of $\mathbb Z^2$ in $W$ are finite-index subgroups of some conjugate of the $\tilde A\_2$ part. So if $A$ and $B$ lie within the same conjugate, they intersect in another finite-index subgroup of that conjugate (which is $\mathbb Z^2$ again), but if $A$ and $B$ lie within different conjugates, they have trivial intersection. In neither case the intersection is $\mathbb Z$.


This is a contradiction to $A \cap B = \mathbb Z$, hence our assumption is false and $W$ does not contain a copy of $\mathbb F\_2 \oplus \mathbb Z$.


