On Growth Functions of Coxeter Groups

Abstract

Let (W, S) be a Coxeter system of rank n, and let $p_{(W, S)}(t)$ be its growth function. It is known that $p_{(W, S)}(q^{-1}) \lt \infty$ holds for all $n \leq q \in \mathbb{N}$. In this paper, we will show that this still holds for $q = n-1$, if (W, S) is 2-spherical. Moreover, we will prove that $p_{(W, S)}(q^{-1}) = \infty$ holds for $q = n-2$, if the Coxeter diagram of (W, S) is the complete graph. These two results provide a complete characterization of the finiteness of the growth function in the case of 2-spherical Coxeter systems with a complete Coxeter diagram.

1. Introduction

One of the most central results in the theory of lattices is Margulis’ Normal Subgroup Theorem for irreducible lattices in connected semi-simple Lie groups of real rank $\geq 2$ with a finite centre and no non-trivial compact factor [16]. Among all the recent generalizations, let us mention that Bader and Shalom proved a version of the Normal Subgroup Theorem for irreducible cocompact lattices in a product of two locally compact, non-discrete, compactly generated groups [3]. Based on earlier results in [18], Caprace and Rémy applied the Normal Subgroup Theorem to show simplicity for Kac–Moody groups over finite fields of irreducible, non-spherical and non-affine types that are twin building lattices (cf. [10, Theorems 18, 19, 20]). Moreover, it can be used to prove virtual simplicity of certain twin tree lattices with non-trivial commutation relations (cf. [11]).

In [17] and [12], Rémy, and independently Carbone and Garland, proved that certain groups acting on (twin) buildings are lattices. To be more precise: Let (W, S) be a Coxeter system with $\vert S \vert \lt \infty$, and let $\Phi := \Phi(W, S)$ be its associated set of roots (viewed as half-spaces). Let $\mathcal{D} = (G, (U_{\alpha})_{\alpha \in \Phi})$ be an RGD system of type (W, S), i.e. a group G together with a family $(U_{\alpha})_{\alpha \in \Phi}$ of subgroups (which we call root groups) indexed by the set of roots Φ satisfying some combinatorial axioms (for the precise definition, we refer to [1, Ch. 7,8]). Then, there exists a twin building $\Delta = (\Delta_+, \Delta_-, \delta_*)$ such that G acts on Δ. It turns out that under some conditions, $G^{\dagger} := \langle U_{\alpha} \mid \alpha \in \Phi \rangle \leq \operatorname{Aut}(\Delta_+) \times \operatorname{Aut}(\Delta_-)$ and $U_+ := \langle U_{\alpha} \mid \alpha \in \Phi_+ \rangle \leq \operatorname{Aut}(\Delta_-)$ are lattices (cf. [17], [12]) – and in this case $G^{\dagger}$ is an example of a twin building lattice. Sufficient conditions are that every root group is finite, W is infinite and for $q_{\min} := \min\{\vert U_{\alpha} \vert \mid \alpha \in \Phi \}$, one has $p_{(W, S)}\left( \frac{1}{q_{\min}} \right) \lt \infty$, where $p_{(W, S)}(t)$ denotes the growth function of (W, S). Some authors call $p_{(W, S)}(t)$ the (spherical) growth series (cf. [13, Chapter 17] or [14, Chapter VI]) or the Poincaré series of (W, S) (cf. [6, Chapter 7.1]). It is clear that $p_{(W, S)}\left( \frac{1}{q_{\min}} \right) \lt \infty$ holds if $\vert S \vert \leq q_{\min}$. It is particularly unsatisfying that the criterion $\vert S \vert \leq q_{\min}$ does not apply to Coxeter systems of rank $n \geq 3$ and $q_{\min} = 2$. However, there are examples of Coxeter systems (W, S) of rank $n \geq 3$ with $p_{(W, S)}\left( \frac{1}{2} \right) \lt \infty$. Note that the growth function $p_{(W, S)}(t)$ applied to q ⁻¹ with $q\in \mathbb{N}$ and $q\geq 2$ is finite for spherical and affine Coxeter systems (cf. [7, Ch. VI, Exercises § 4, 10]).

Suppose (W, S) is of type $(4, 4, 4)$, that is, $\vert S \vert = 3$ and the order of st in W equals 4 for all $s\neq t\in S$. In [5] we constructed uncountably many new examples of RGD systems of type $(4, 4, 4)$ in which every root group has cardinality 2. As the criterion $\vert S \vert \leq q_{\min}$ does not apply to such RGD systems, we first asked the question whether $p_{(W, S)}\left( \frac{1}{2} \right) \lt \infty$ holds. It turns out that this is indeed the case (cf. Theorem A).

Main results

Let (W, S) be a Coxeter system and denote by m_st the order of st in W. The Coxeter system is called 2-spherical if $m_{st} \lt \infty$ for all $s\neq t \in S$. The rank of (W, S) is given by the cardinality of S. Throughout this paper we assume that all Coxeter systems under consideration are of finite rank. We prove the following (cf. Theorem 5.3):

Theorem A. Let (W, S) be a 2-spherical Coxeter system of rank n. Then $p_{(W, S)}\left( \frac{1}{n-1} \right) \lt \infty$.

Remark 1. After completion of this project, I was informed by Corentin Bodart that a more general version of Theorem A can be deduced from [2, Theorem 1] and we refer to Remark 3 at the end of the introduction for more details. Our methods of the proof are very different, and most of the results proved in the present paper are also used to prove Theorem C below. Our proofs are Coxeter group theoretic, while the proofs in [2] are for non-elementary word hyperbolic groups.

In view of the examples constructed in [5], Theorem A produces many new examples of lattices in (locally compact) automorphism groups of buildings and in a product of two automorphism groups of buildings. Combining Theorem A with [17, Théorème 1], we obtain that almost all RGD systems of 2-spherical type and rank 3 are twin building lattices:

Corollary B. Let (W, S) be a Coxeter system, and let $\mathcal{D} = (G, (U_{\alpha})_{\alpha \in \Phi})$ be an RGD system of type (W, S). Assume that the following are satisfied:

• • (W, S) is 2-spherical of rank 3 and W is infinite.

• • $G = \langle U_{\alpha} \mid \alpha \in \Phi \rangle$, $\vert Z(G) \vert \lt \infty$ and $\vert U_{\alpha} \vert \lt \infty$ for all $\alpha \in \Phi$.

Then, $\mathcal{D}$ is a twin building lattice.

Corollary B. (Kac–Moody version)

Let (W, S) be a 2-spherical Coxeter system of rank 3 such that W is infinite, and let G be the Kac–Moody group (in the sense of [21]) of type (W, S). Then $\mathbf{G}(\mathbb{F}_q)$ is a twin building lattice, where $\mathbb{F}_q$ denotes the finite field with q elements.

Now the question is whether the finiteness still holds for some $q \lt n-1$. It turns out that in the class of Coxeter systems with $m_{st} \geq 3$ for all $s\neq t\in S$ this will not happen (cf. Theorem 5.5):

Theorem C. Let (W, S) be a Coxeter system of rank $n\geq 3$ such that $m_{st} \geq 3$ for all $s\neq t\in S$. Then, $p_{(W, S)}\left( \frac{1}{n-2} \right) = \infty$.

Suppose that the Coxeter diagram is 2-spherical, but the Coxeter diagram is not the complete graph. If the number of non-edges in the Coxeter diagram compared to the number of edges is large, then it is still possible that $p_{(W, S)} \left( \frac{1}{n-2} \right) \lt \infty$ holds (cf. [19]). We also remark that Theorem C can be used to exclude certain subdiagrams for twin building lattices, as parabolic subgroups of twin building lattices are again twin building lattices:

Corollary D. Let (W, S) be a Coxeter system, let $\mathcal{D}$ be an RGD system of type (W, S) with finite root groups and let $q_{\min} := \min\{\vert U_{\alpha} \vert \mid \alpha \in \Phi \}$. If $\mathcal{D}$ is a twin building lattice, then there does not exist a subdiagram of (W, S) with at least $q_{\min} +2$ vertices, whose underlying Coxeter diagram is the complete graph.

Several remarks of our main results are in order.

Remark 2. The proofs of Theorem A and Theorem C make essential use of a result of Terragni [20, Theorem A]. We recall this result in Subsection 5.1.

Remark 3. A more general version of Theorems A can be deduced from [2, Theorem 1]: Let (W, S) and $(W', S')$ be two Coxeter systems of rank $n \geq 3$. Suppose that $(W', S')$ is of universal type, i.e. $m_{st} = \infty$ for all $s\neq t\in S$. Note that W ^ʹ is word-hyperbolic (cf. [13, Corollary 12.6.3]) and non-elementary (in the sense of [2]; cf. [13, Theorem 8.6.1, 8.7.3]). Suppose that (W, S) is not of universal type. This means that $m_{st} \lt \infty$ for some $s\neq t \in S$. Let $\pi: W' \to W$ be a canonical homomorphism which induces a bijection between S ^ʹ and S. Then $N := \ker(\pi)$ is a normal subgroup which is infinite. We now use the notation from [2]. One can show $\lambda(W', S') = n-1$, and by [2, Theorem 1] we have $\lambda(W, S) \lt \lambda(W', S') = n-1$. We deduce from [14, Chapter VI.C, Observation 50] that $p_{(W, S)}\left( \frac{1}{n-1} \right) \lt \infty$. This implies that we can replace in Theorem A 2-spherical by non-universal.

Overview

In $\S$ 2, we fix notation and recall some basic results. In $\S$ 2.2, we define two subsets C_i and D_i of the Coxeter group W, which play a central role in this paper. In $\S$ 3, we recall the definition of reflection and combinatorial triangles and prove some results about them. In $\S$ 4, we establish some (in-)equalities concerning the cardinalities $\vert C_i \vert$ and $\vert D_i \vert$. In $\S$ 5, we recall a result due to Terragni and prove our main results.

2. Preliminaries

This section is devoted to fixing notation. In $\S$ 2.1 which is based on [20], we recall the notion of growth functions in finitely generated groups. In $\S$ 2.2 and $\S$ 2.3, we recall some basic definitions about Coxeter systems. Moreover, we introduce two sets C_i and D_i which play a central role in this paper. In $\S$ 2.4, we recall some basic results about roots and walls in Coxeter systems. $\S$ 2.2, $\S$ 2.3 and $\S$ 2.4 are based on [1, $\S$ 5].

2.1. Growth of finitely generated groups

Let G be a finitely generated group, and let $X = X^{-1} \subseteq G \backslash \{1\}$ be a finite, symmetric set of generators. The length of $g\in G$ with respect to X is the minimal n such that $g = x_1 \cdots x_n$ with $x_i \in X$; the length function will be denoted by $\ell_{(G, X)}: G \to \mathbb{N}$. For $n\in \mathbb{N}$, the sphere in $\mathrm{Cay}(G, X)$ centred around 1_G with radius n will be denoted by

\begin{equation*} C_n^{(G, X)} := \left\{g\in G \mid \ell_{(G, X)}(g) = n \right\}. \end{equation*}

The cardinalities are defined as $c_n^{(G, X)} := \vert C_n^{(G, X)} \vert$. The growth function of (G, X) is given by

\begin{equation*} p_{(G, X)}(t) := \sum_{n\geq 0} c_n^{(G, X)} t^n \in \mathbb{Z}[[t]]. \end{equation*}

2.2. Coxeter systems

Let W be a group, and let $S \subseteq W$ be a generating set of elements of order 2. For $s, t \in S$, we denote the order of st in W by m_st. Then, the pair (W, S) is called Coxeter system if the group W admits the presentation

\begin{equation*} W \cong \langle S \mid (st)^{m_{st}} = 1 \rangle, \end{equation*}

where there is one relation for each pair $s, t$ (possibly s = t) with $m_{st} \lt \infty$. Let (W, S) be a Coxeter system, and let $\ell := \ell_{(W, S)}$ be the corresponding length function. The Coxeter diagram corresponding to (W, S) is the labelled graph $(S, E(S))$, where $E(S) = \{\{s, t \} \mid m_{st} \gt 2 \}$ and where each edge $\{s,t\}$ is labelled by m_st for all $s, t \in S$. The rank of the Coxeter system is the cardinality of the set S. Recall from the introduction that in this paper all Coxeter systems under consideration are assumed to be of finite rank.

It is well-known that for each $J \subseteq S$, the pair $(\langle J \rangle, J)$ is a Coxeter system (cf. [7, Ch. IV, §1 Theorem 2]). A subset $J \subseteq S$ is called spherical if $\langle J \rangle$ is finite. The Coxeter system is called 2-spherical if $\langle J \rangle$ is finite for all $J \subseteq S$ containing at most 2 elements (i.e. $m_{st} \lt \infty$ for all $s, t \in S$). Given a spherical subset J of S, there exists a unique element of maximal length in $\langle J \rangle$, which we denote by r_J (cf. [1, Corollary 2.19]).

For $i \in \mathbb{N}$, we define

• • $C_i := C_i^{(W, S)} = \{w\in W \mid \ell(w) = i \}$ and $c_i := \vert C_i \vert = c_i^{(W, S)}$;

• • $D_i := \{w\in C_i \mid \exists! s\in S: \ell(ws) \lt \ell(w) \}$ and $d_i := \vert D_i \vert$.

The set C_i consists of all elements $w\in W$ of length i. The set D_i consists of all elements $w\in W$ of length i whose right descent set contains a single element of S.

2.3. The chamber system $\Sigma(W, S)$

Let (W, S) be a Coxeter system. Defining $w \sim_s w'$ if and only if $w^{-1}w' \in \langle s \rangle$, we obtain a chamber system (for the definition of a chamber system, see [1, Definition 5.21]) with chamber set W and equivalence relations $\sim_s$ for $s\in S$, which we denote by $\Sigma(W, S)$. We call two chambers $w, w'$ s-adjacent if $w \sim_s w'$ and adjacent if they are s-adjacent for some $s\in S$. A gallery of length n from w ₀ to w_n is a sequence $(w_0, \ldots, w_n)$ of chambers, where w_i and $w_{i+1}$ are adjacent for each $0 \leq i \lt n$. A gallery $(w_0, \ldots, w_n)$ is called minimal if there exists no gallery from w ₀ to w_n of length k < n, and we denote the length of a minimal gallery from w ₀ to w_n by $\ell(w_0, w_n)$. For $J \subseteq S$, we define the J-residue of a chamber $c\in W$ to be the set $R_J(c) := c \langle J \rangle$. A residue R is a J-residue for some $J \subseteq S$; we call J the type of R, and the cardinality of J is called the rank of R. A residue is called spherical if its type is a spherical subset of S. Let R be a spherical J-residue. Two chambers $x, y \in R$ are called opposite in R if $x^{-1} y = r_J$. Two residues $P, Q \subseteq R$ are called opposite in R if for each $p\in P$ there exists $q\in Q$ such that $p, q$ are opposite in R. A panel is a residue of rank 1. It is a fact that for every chamber $x\in W$ and every residue R, there exists a unique chamber $z\in R$ such that $\ell(x, y) = \ell(x, z) + \ell(z, y)$ holds for each chamber $y\in R$. The chamber z is called the projection or the gate of x onto R and is denoted by $z = \operatorname{proj}_R x$.

A subset $\Sigma \subseteq W$ is called convex if for any two chambers $c, d \in \Sigma$ and any minimal gallery $(c_0 = c, \ldots, c_k = d)$, we have $c_i \in \Sigma$ for all $0 \leq i \leq k$. Note that residues are convex by [1, Example 5.44(b)].

For two residues R and T, we define $\operatorname{proj}_T R := \{\operatorname{proj}_T r \mid r\in R \}$. By [1, Lemma 5.36(2)], $\operatorname{proj}_T R$ is a residue contained in T. The residues R and T are called parallel if $\operatorname{proj}_T R = T$ and $\operatorname{proj}_R T = R$.

2.4. Roots and walls

Let (W, S) be a Coxeter system. A reflection is an element of W that is conjugate to an element of S. For $s\in S$ we let $\alpha_s := \{w\in W \mid \ell(sw) \gt \ell(w) \}$ be the simple root corresponding to s. A root is a subset $\alpha \subseteq W$ such that $\alpha = v\alpha_s$ for some $v\in W$ and $s\in S$. We denote the set of all roots by $\Phi := \Phi(W, S)$. The set $\Phi_+ := \{\alpha \in \Phi \mid 1_W \in \alpha \}$ is the set of all positive roots, and $\Phi_- := \{\alpha \in \Phi \mid 1_W \notin \alpha \}$ is the set of all negative roots. For each root $\alpha \in \Phi$, we denote the opposite root by $-\alpha$ and we denote the unique reflection which interchanges these two roots by r_α. For $\alpha \in \Phi$, we denote by $\partial \alpha$ (respectively, $\partial^2 \alpha$) the set of all panels (respectively, spherical residues of rank 2) stabilized by r_α. Furthermore, we define $\mathcal{C}(\partial \alpha) := \bigcup_{P \in \partial \alpha} P$ and $\mathcal{C}(\partial^2 \alpha) := \bigcup_{R \in \partial^2 \alpha} R$.

The set $\partial \alpha$ is called the wall associated with α. Let $G = (c_0, \ldots, c_k)$ be a gallery with $c_{i-1} \neq c_i$ for each $1 \leq i \leq k$. We say that G crosses the wall $\partial \alpha$ if there exists $1 \leq i \leq k$ such that $\{c_{i-1}, c_i \} \in \partial \alpha$. It is a basic fact that a minimal gallery crosses a wall at most once (cf. [1, Lemma 3.69]). Moreover, a gallery which crosses each wall at most once is already minimal.

A pair $\{\alpha, \beta \} \subseteq \Phi$ of roots is called prenilpotent, if $\alpha \cap \beta \neq \emptyset \neq (-\alpha) \cap (-\beta)$. For a prenilpotent pair $\{\alpha, \beta \}$ of roots, we will write $\left[\alpha,\beta\right]:=\{\gamma\in\Phi\mid\alpha\cap\beta\subseteq\gamma\;\text{and}\ (-\alpha)\cap(-\beta)\subseteq(-\gamma)\}$ and $(\alpha, \beta) := \left[ \alpha, \beta \right] \backslash \{\alpha, \beta \}$. We note that roots are convex (cf. [1, Lemma 3.44]).

Let $(c_0, \ldots, c_k)$ and $(d_0 = c_0, \ldots, d_k = c_k)$ be two minimal galleries from c ₀ to c_k, and let $\alpha \in \Phi$. Then, $\partial \alpha$ is crossed by the minimal gallery $(c_0, \ldots, c_k)$ if and only if it is crossed by the minimal gallery $(d_0, \ldots, d_k)$.

Lemma 2.1. Let R be a spherical residue of $\Sigma(W, S)$ of rank 2, and let $\alpha \in \Phi$. Then, exactly one of the following holds:

1. (a) $R \subseteq \alpha$;

2. (b) $R \subseteq (-\alpha)$;

3. (c) $R \in \partial^2 \alpha$.

Proof. It is clear that the three cases are exclusive. Suppose that $R \not\subseteq \alpha$ and $R \not\subseteq (-\alpha)$. Then, there exist $c \in R \cap (-\alpha)$ and $d \in R \cap \alpha$. Let $(c_0 = c, \ldots, c_k = d)$ be a minimal gallery. As residues are convex, we have $c_i \in R$ for each $0 \leq i \leq k$. As $c\in (-\alpha), d\in \alpha$, there exists $1 \leq i \leq k$, with $c_{i-1} \in (-\alpha), c_i \in \alpha$. In particular, $\{c_{i-1}, c_i \} \in \partial \alpha$ and hence $R \in \partial^2 \alpha$.

Lemma 2.2. Let $R, T$ be two spherical residues of $\Sigma(W, S)$. Then, the following are equivalent:

1. (i) $R, T$ are parallel;

2. (ii) a reflection of $\Sigma(W, S)$ stabilizes R if and only if it stabilizes T;

3. (iii) there exist two sequences $R_0 = R, \ldots, R_n = T$ and $T_1, \ldots, T_n$ of residues of spherical type such that for each $1 \leq i \leq n$, the rank of T_i is equal to $1+\mathrm{rank}(R)$, the residues $R_{i-1}, R_i$ are contained and opposite in T_i and moreover, we have $\operatorname{proj}_{T_i} R = R_{i-1}$ and $\operatorname{proj}_{T_i} T = R_i$.

Proof. This is [9, Proposition 2.7].

Lemma 2.3. Let $\alpha \in \Phi$ be a root, and let $x, y \in \alpha \cap \mathcal{C}(\partial \alpha)$. Then, there exists a minimal gallery $(c_0 = x, \ldots, c_k = y)$ such that $c_i \in \mathcal{C}(\partial^2 \alpha)$ for each $0 \leq i \leq k$. Moreover, for each $1 \leq i \leq k$, there exists $L_i \in \partial^2 \alpha$ with $\{c_{i-1}, c_i \} \subseteq L_i$.

Proof. This is a consequence of [8, Lemma 2.3] and its proof.

Lemma 2.4. Let $\alpha, \beta \in \Phi, \alpha \neq \pm \beta$ be two roots, and let $R, T \in \partial^2 \alpha \cap \partial^2 \beta$.

1. (a) The residues R and T are parallel.

2. (b) If $\vert \langle J \rangle \vert = \infty$ holds for all $J \subseteq S$ containing three elements, then R = T.

Proof. As $R, T \in \partial^2 \alpha \cap \partial^2 \beta$, there exist panels $P_1, Q_1 \in \partial \alpha$ and $P_2, Q_2 \in \partial \beta$ such that $P_1, P_2 \subseteq R$ and $Q_1, Q_2 \subseteq T$ (as in the proof of Lemma 2.1). By Lemma 2.2, the panels $P_i, Q_i$ are parallel for both $i \in \{1, 2\}$. Now [15, Lemma 17] yields that $P_i, \operatorname{proj}_T P_i$ are parallel, and hence $\operatorname{proj}_T P_1 \in \partial \alpha, \operatorname{proj}_T P_2 \in \partial \beta$ by Lemma 2.2. As $\alpha \neq \pm \beta$, we deduce $\operatorname{proj}_T P_1 \neq \operatorname{proj}_T P_2$, and hence $\operatorname{proj}_T R$ contains the two different panels $\operatorname{proj}_T P_1$ and $\operatorname{proj}_T P_2$. In particular, $\operatorname{proj}_T R$ is not a panel. Since $\operatorname{proj}_T R$ is a residue contained in T, we deduce $\operatorname{proj}_T R = T$. Using similar arguments, we found that $\operatorname{proj}_R T = R$ and $R, T$ are parallel. This proves (a). Moreover, Lemma 2.2 yields R = T, as there are no spherical residues of rank 3 by assumption. This finishes the proof.

3. Reflection and combinatorial triangles in $\Sigma(W, S)$

Reflection triangles and combinatorial triangles were introduced in [8]. A reflection triangle is a set $\mathcal{R}$ of three reflections such that the order of tt ^ʹ is finite for all $t, t' \in \mathcal{R}$, and $\bigcap_{t\in \mathcal{R}} \partial^2 \beta_t = \emptyset$, where β_t is one of the two roots associated with the reflection t. Note that $\partial^2 \beta_t = \partial^2 (-\beta_t)$. A set of three roots $\mathcal{T}$ is called a combinatorial triangle (or simply triangle) if the following holds:

1. (CT1) The set $\{r_{\alpha} \mid \alpha \in \mathcal{T} \}$ is a reflection triangle.

2. (CT2) For each $\alpha \in \mathcal{T}$, there exists $\sigma \in \partial^2 \beta \cap \partial^2 \gamma$ such that $\sigma \subseteq \alpha$, where $\{\beta, \gamma \} = \mathcal{T} \backslash \{\alpha \}$.

Lemma 3.1. Suppose that (W, S) is 2-spherical and the Coxeter diagram is the complete graph. If $\mathcal{T}$ is a triangle, then $(-\alpha, \beta) = \emptyset$ holds for all $\alpha \neq \beta \in \mathcal{T}$.

Proof. This is [4, Proposition 2.3].

Proposition 3.2. Assume that (W, S) is 2-spherical and the Coxeter diagram is the complete graph. Let R ≠ T be two residues of rank 2 such that $P := R \cap T$ is a panel. If $\ell(1_W, \operatorname{proj}_R 1_W) \lt \ell(1_W, \operatorname{proj}_T 1_W)$, then $\operatorname{proj}_T 1_W = \operatorname{proj}_P 1_W$.

Proof. We let $\alpha \in \Phi_+$ be the root with $P \in \partial \alpha$. Let $(c_0 = 1_W, \ldots, c_{k'} = \operatorname{proj}_P c_0)$ be a minimal gallery with $c_k = \operatorname{proj}_R c_0$ for some $0 \leq k \leq k'$ and $c_k, \ldots, c_{k'} \in R$.

We assume that $\operatorname{proj}_T c_0 \neq \operatorname{proj}_P c_0$ holds. Then, we have $k' \gt \ell(1_W, \operatorname{proj}_T 1_W) \gt \ell(1_W, \operatorname{proj}_R 1_W) = k$. Let $(d_0 = 1_W, \ldots, d_{m'} = \operatorname{proj}_P d_0)$ be a minimal gallery with $d_m = \operatorname{proj}_T c_0$ for some $0 \leq m \leq m'$ and $d_m, \ldots, d_{m'} \in T$. We let $\beta \in \Phi_+$ be the root with $\{d_m, d_{m+1} \} \in \partial \beta$, and we let $\gamma \in \Phi_+$ be the root with $\{c_k, c_{k+1} \} \in \partial \gamma$. We will show that $\{\alpha, -\beta, -\gamma \}$ is a triangle. Thus, we first show that $\{r_{\alpha}, r_{\beta}, r_{\gamma} \}$ is a reflection triangle. We have $T \in \partial^2 \alpha \cap \partial^2 \beta$, and, as a minimal gallery crosses a wall at most once, we deduce $\alpha \neq \beta$. Note that the wall $\partial \beta$ is crossed by the minimal gallery $(c_0, \ldots, c_{k'})$. Since $\partial^2 \alpha \ni R \neq T \in \partial^2 \alpha \cap \partial^2 \beta$ and $\alpha \neq \pm \beta$, Lemma 2.4(b) implies $R \notin \partial^2 \beta$, and hence $\partial \beta$ is crossed by $(c_0, \ldots, c_k)$. As $k \lt k'$, we have $\operatorname{proj}_R 1_W \neq \operatorname{proj}_P 1_W$ and hence $\alpha \neq \gamma$. As $\alpha, \gamma \in \Phi_+$, we have $\alpha \neq \pm \gamma$.

Assume that $o(r_{\beta} r_{\gamma}) = \infty$. We deduce $\beta \subseteq \gamma$. But $\partial\gamma$ has to be crossed by the gallery $(d_0, \ldots, d_{m'})$. Since $\partial^2 \alpha \ni T \neq R \in \partial^2 \alpha \cap \partial^2 \gamma$ and $\alpha \neq \pm \gamma$, we have $T \notin \partial \gamma^2$ by Lemma 2.4(b) as before. This implies that $(d_0, \ldots, d_m)$ crosses the wall $\partial \beta$ and hence $\gamma \subseteq \beta$. This yields a contradiction, and we have $o(r_{\beta} r_{\gamma}) \lt \infty$.

As $R \in \partial^2 \alpha \cap \partial^2 \gamma$, Lemma 2.4(b) implies $\partial^2 \alpha \cap \partial^2 \gamma = \{R \}$. As $R \notin \partial^2 \beta$, we deduce $\partial^2 \alpha \cap \partial^2 \beta \cap \partial^2 \gamma = \emptyset$, and hence $\{r_{\alpha}, r_{\beta}, r_{\gamma} \}$ is a reflection triangle.

Now we have to verify (CT2). As $\partial^2 \gamma \not\ni T \in \partial^2 \alpha \cap \partial^2 \beta$ and $P \subseteq T \cap (-\gamma)$, we have $T \subseteq (-\gamma)$ by Lemma 2.1. As $\partial^2 \beta \not\ni R \in \partial^2 \alpha \cap \partial^2 \gamma$ and $P \subseteq R \cap (-\beta)$, we have $R \subseteq (-\beta)$. Let $1 \leq i \leq k$ be such that $\{c_{i-1}, c_i \} \in \partial \beta$. Note that $\{d_m, d_{m+1} \} \in \partial \beta, d_{m+1} \in (-\beta) \cap T \subseteq (-\gamma)$ and $c_i \in (-\beta) \cap \gamma$. By Lemma 2.3 there exists a minimal gallery $(e_0 = d_{m+1}, \ldots, e_z = c_i)$ such that $e_j \in \mathcal{C}(\partial^2 \beta)$. As $d_{m+1} \in (-\gamma)$ and $c_i \in \gamma$, there exists $1 \leq p \leq z$ such that $e_{p-1} \in (-\gamma)$ and $e_p \in \gamma$. Again, by Lemma 2.3, there exists $L \in \partial^2 \beta$ such that $\{e_{p-1}, e_p \} \subseteq L$, and hence $L \in \partial^2 \beta \cap \partial^2 \gamma$. As roots are convex and $e_0 = d_{m+1}, e_z = c_i \in \alpha$, we have $e_p \in L \cap \alpha$. As $\{r_{\alpha}, r_{\beta}, r_{\gamma} \}$ is a reflection triangle (and hence $L \notin \partial^2 \alpha$), we obtain $L \subseteq \alpha$ by Lemma 2.1. This implies that $\{\alpha, -\beta, -\gamma \}$ is a triangle, and hence $(\alpha, \gamma) = \emptyset$ holds by Lemma 3.1. In particular, $k+1 = k'$ and $\ell(1_W, \operatorname{proj}_R 1_W) = \ell(1_W, \operatorname{proj}_P 1_W) -1 \geq \ell(1_W, \operatorname{proj}_T 1_W)$. This is a contradiction to the assumption, and we conclude $\operatorname{proj}_T 1_W = \operatorname{proj}_P 1_W$.

Corollary 3.3. Assume that (W, S) is 2-spherical and that the underlying Coxeter diagram is the complete graph. Suppose $w\in W$ and $s\neq t \in S$ with $\ell(ws) = \ell(w) +1 = \ell(wt)$ and suppose $w' \in \langle s, t \rangle$ with $\ell(w') \geq 2$. Then we have $\ell(ww'r) = \ell(w) + \ell(w') +1$ for each $r\in S \backslash \{s, t \}$.

Proof. Suppose $r\in S \backslash \{s, t\}$, and assume that $\ell(ww'r) = \ell(ww') -1$ holds for some $w' \in \langle s, t \rangle$ with $\ell(w') \geq 2$. Suppose $w'$ starts with s, i.e. $w' = sw^{\prime\prime}$ for some $w^{\prime\prime} \in \langle s, t \rangle$ with $\ell(w^{\prime\prime}) = \ell(w') -1$. As $\ell(ww'r) = \ell(ww') -1$, one easily sees that $\ell(wstr) = \ell(wst) -1$ and $\ell(wsr) = \ell(ws) -1$ hold, too. We define $R := R_{\{r, t\}}(ws), T := R_{\{s, t\}}(w)$ and $P := R \cap T = \mathcal{P}_t(ws)$. Clearly, $\operatorname{proj}_T 1_W \neq \operatorname{proj}_P 1_W$. As $m_{rt} \geq 3$, we deduce $\ell(1_W, \operatorname{proj}_R 1_W) \lt \ell(1_W, \operatorname{proj}_T 1_W)$ and Proposition 3.2 yields a contradiction.

Lemma 3.4. Assume that (W, S) is 2-spherical and that $m_{st} \geq 4$ holds for all $s\neq t \in S$. Suppose $w \in W$ and $s\neq t \in S$ with $\ell(ws) = \ell(w) +1 = \ell(wt)$. Then we have $\ell(w) +2 \in \{\ell(wsr), \ell(wtr) \}$ for all $r\in S \backslash \{s, t\}$.

Proof. Assume that $\ell(wsr) = \ell(w) = \ell(wtr)$. Then, $\ell(wr) = \ell(w)-1$ and $\ell(wrs) = \ell(w) -2 = \ell(wrt)$. Let R be the $\{r, s \}$-residue containing w. As $m_{rs} \geq 4$, we deduce $\ell(wrsr) = \ell(wrs) -1$. Let $w' \in \langle s, t \rangle$ be such that $wr = (\operatorname{proj}_R 1_W) w'$. Then, $\ell(w') \geq 2$ and the previous corollary implies $\ell(wrt) = \ell(wr) +1$, which is a contradiction. This finishes the proof.

Remark 3.5. Note that Lemma 3.4 is false without the assumption $m_{st} \geq 4$. To see this, one can consider the Coxeter system of type $\tilde{A}_2$.

4. Some (in-)equalities

To show the two main results (Theorem 5.3 and 5.5), we will apply the ratio test. In order to do so, we need a few inequalities, which we establish in this and the next section. We recall that for $i\in \mathbb{N}$ we have

• • $C_i := \{w\in W \mid \ell(w) = i \}$ and $c_i := \vert C_i \vert$;

• • $D_i := \{w\in C_i \mid \exists! s\in S: \ell(ws) \lt \ell(w) \}$ and $d_i := \vert D_i \vert$;

Lemma 4.1. Suppose that the Coxeter diagram of (W, S) is the complete graph. Then, for each $w\in W \backslash \{1_W\}$, there is either a unique element $s_w \in S$ with $\ell(w s_w) = \ell(w) -1$, or else there are exactly two elements $s_w \neq t_w \in S$ with $\ell(w s_w) = \ell(w) -1 = \ell(w t_w)$.

Proof. Let $J \subseteq S$ with $\ell(wj) \lt \ell(w)$ for each $j \in J$. Then [1, Corollary 2.18] implies that J is spherical. As the underlying Coxeter diagram is the complete graph, it follows that each subset of S containing at least three elements is non-spherical. This finishes the proof.

Convention 4.2.

In this section, we assume that (W, S) is of rank $n \geq 3$ and that there exists $m\geq 3$ such that $m_{st} = m$ holds for all $s\neq t \in S$. Moreover, we let i > m.

Lemma 4.3. $c_i - d_i = \begin{pmatrix} n-2 \\ 2 \end{pmatrix} c_{i-m} + (n-2) d_{i-m}$.

Proof. Let $v \in C_i \backslash D_i$ be an element. By Lemma 4.1, there exist exactly two elements $s\neq t \in S$ with $\ell(vs) = \ell(v) -1 = \ell(vt)$. We define $R_v := R_{\{s, t\}}(v)$. Then, we consider the mapping

\begin{equation*} f: C_i \backslash D_i \to C_{i-m}, v \mapsto \operatorname{proj}_{R_v} 1_W \end{equation*}

Note that $C_{i-m} = D_{i-m} \cup C_{i-m} \backslash D_{i-m}$. If $w\in C_{i-m} \backslash D_{i-m}$, Lemma 4.1 implies that there are exactly two elements in S, say $s_w \neq t_w \in S$, which decreases the length of w (as i > m). Any other element $r\in S \backslash \{s_w, t_w \}$ increases the length of w. For n > 3 and $r_1 \neq r_2 \in S \backslash \{s_w, t_w\}$, we have $f(wr_{\{r_1, r_2\}}) = w$. For n = 3, we have $f^{-1}(w) = \emptyset$. In both cases, w has $\begin{pmatrix} n-2 \\ 2 \end{pmatrix}$ many preimages. If $w \in D_{i-m}$ is, there exists a unique $s_w \in S$ which decreases the length of w and (similarly as before) w has $\begin{pmatrix} n-1 \\ 2 \end{pmatrix}$ many preimages. Note that $\begin{pmatrix} n-1 \\ 2 \end{pmatrix} - \begin{pmatrix} n-2 \\ 2 \end{pmatrix} = n-2$. We conclude

\begin{align*} c_i - d_i = \vert C_i \backslash D_i \vert &= \sum_{w\in C_{i-m}} \vert f^{-1}(w) \vert \\ &= \sum_{w\in C_{i-m} \backslash D_{i-m}} \vert f^{-1}(w) \vert + \sum_{w\in D_{i-m}} \vert f^{-1}(w) \vert \\ &= \begin{pmatrix} n-2 \\ 2 \end{pmatrix} \left( c_{i-m} - d_{i-m} \right) + \begin{pmatrix} n-1 \\ 2 \end{pmatrix} d_{i-m} \\ &= \begin{pmatrix} n-2 \\ 2 \end{pmatrix} c_{i-m} + (n-2) d_{i-m}. \end{align*}

Lemma 4.4. $2c_{i+1} - d_{i+1} = (n-2) c_i + d_i$.

Proof. We put $M_i := \{(w, s) \in C_i \times S \mid ws \in C_{i+1} \}$. We prove the claim by showing that both sides of the equation are equal to $\vert M_i \vert$.

1. (a) $2c_{i+1} - d_{i+1} = \vert M_i \vert$: We consider the mapping

   \begin{equation*} \pi: M_i \to C_{i+1}, (w, s) \mapsto ws. \end{equation*}

   Clearly, π is surjective. We define

   \begin{align*} &C_{i+1}^1 := \{w\in C_{i+1} \mid \vert \pi^{-1}(w) \vert = 1 \} &&\text{and} &&C_{i+1}^{ \gt 1} := \{w\in C_{i+1} \mid \vert \pi^{-1}(w) \vert \gt 1 \}. \end{align*}

   We show that $C_{i+1}^{ \gt 1} = C_{i+1} \backslash D_{i+1}$. Let $\bar{w} \in C_{i+1}^{ \gt 1}$ be an element. Then, there exist $(w, s) \neq (w', s') \in \pi^{-1}(\bar{w})$. It follows that $s\neq s'$, and hence $\bar{w} \in C_{i+1} \backslash D_{i+1}$. Now, let $w\in C_{i+1} \backslash D_{i+1}$. By Lemma 4.1, there exist exactly two elements $s_w \neq t_w \in S$, which decreases the length of w. This implies $(ws_w, s_w) \neq (wt_w, t_w) \in \pi^{-1}(w)$. As $\vert \langle J \rangle \vert = \infty$ for all $J \subseteq S$ containing three elements, we deduce for every $1 \neq w\in W$ that

   \begin{equation*} \vert \pi^{-1}(w) \vert \in \{1, 2 \}. \end{equation*}

   We infer $C_{i+1}^1 = C_{i+1} \backslash C_{i+1}^{ \gt 1} = C_{i+1} \backslash \left( C_{i+1} \backslash D_{i+1} \right) = D_{i+1}$ and compute

   \begin{align*} \vert M_i \vert = \sum_{w\in C_{i+1}} \vert \pi^{-1}(w) \vert &= \sum_{w\in D_{i+1}} \vert \pi^{-1}(w) \vert + \sum_{w\in C_{i+1} \backslash D_{i+1}} \vert \pi^{-1}(w) \vert \\ &= d_{i+1} + 2(c_{i+1} - d_{i+1}) \\ &= 2c_{i+1} - d_{i+1}. \end{align*}

2. (b) $(n-2) c_i + d_i = \vert M_i \vert$: For a subset $T \subseteq C_i$, we define

   \begin{align*} M_{i, T} &:= \{(w, s) \in M_i \mid w \in T \}. \end{align*}

   For $w\in D_i$ there are exactly n − 1 elements which increase the length of w. Thus, we have $\vert M_{i, D_i} \vert = (n-1)d_i$. For $w\in C_i \backslash D_i$, there are exactly n − 2 elements in S, which increases the length of w (cf. Lemma 4.1). Thus, we have $\vert M_{i, C_i \backslash D_i} \vert = (n-2)(c_i - d_i)$. We conclude

   \begin{equation*} \vert M_i \vert = \vert M_{i, C_i \backslash D_i} \vert + \vert M_{i, D_i} \vert = (n-2) (c_i - d_i) + (n-1)d_i = (n-2) c_i + d_i. \end{equation*}

Lemma 4.5. $c_{i+1} \leq (n-1) c_i - (n-2)d_{i-m+1} \leq (n-1) c_i$.

Proof. The last inequality is obvious. Using Lemma 4.3 and 4.4, we deduce the following:

\begin{align*} c_{i+1} + (n-2) d_{i-m+1} &\leq 2c_{i+1} - d_{i+1} = (n-2) c_i + d_i \leq (n-1) c_i. \end{align*}

Lemma 4.6. Suppose m > 3. Then, the following holds:

1. (a) $(n-2) c_i \leq c_{i+1}$;

2. (b) $(n-2) d_i \leq d_{i+1}$;

Proof. We define $N_i := \{(w, s) \in C_i \times S \mid ws \in D_{i+1} \}$. Then, $N_i \to D_{i+1}, (w, s) \mapsto ws$ is a bijection, and hence $\vert N_i \vert = d_{i+1}$. As in the proof of Lemma 4.4, we define for a subset $T \subseteq C_i$:

\begin{equation*} N_{i, T} := \{(w, s) \in N_i \mid w\in T \}. \end{equation*}

We see that $c_{i+1} \geq d_{i+1} = \vert N_i \vert = \vert N_{i, D_i} \vert + \vert N_{i, C_i \backslash D_i} \vert$. Let $w\in C_i$. We now count pairs $(w, s) \in N_i$. We distinguish the following two cases:

1. (i) $w\in D_i$: Let $s_w \in S$ be the unique element with $\ell(ws_w) \lt \ell(w)$. Let $t\in S \backslash \{s_w \}$. Then, $wt \in C_{i+1}$. Suppose $wt \notin D_{i+1}$. Then, there exists $t\neq r \in S$ with $\ell(wtr) \lt \ell(wt)$. This implies $\ell(wr) \lt \ell(w)$, and the uniqueness of s_w yields $r = s_w$. Now, let $r \in S \backslash \{s_w, t\}$. Then $wr \in C_{i+1}$. Again, if $wr \notin D_{i+1}$, then s_w would decrease the length of wr. But this is a contradiction to Lemma 3.4. This implies $(w, r) \in N_{i, D_i}$ for all $r\in S \backslash \{s_w, t\}$. This shows (b).

2. (ii) $w\in C_i \backslash D_i$: Let $s_w \neq t_w \in S$ be the two elements with $\ell(w s_w) = \ell(w t_w) \lt \ell(w)$. Now let $r\in S \backslash \{s_w, t_w \}$. Then, $wr \in C_{i+1}$. We assume by contradiction that $wr \notin D_{i+1}$. Then, there would exist $u \in S \backslash \{r \}$ with $\ell(wr u) = \ell(w)$, and hence $\ell(wu) \lt \ell(w)$. As s_w and t_w are the only two elements in S with the property that they decrease the length of w, we obtain $u \in \{s_w, t_w \}$. But then, we obtain a contradiction to Corollary 3.3. We conclude $(w, r) \in N_{i, C_i \backslash D_i}$.

We infer $c_{i+1} \geq \vert N_{i, D_i} \vert + \vert N_{i, C_i \backslash D_i} \vert \geq (n-2) d_i + (n-2) (c_i - d_i) = (n-2) c_i$.

5. Main results

In this section, we prove our main results. In $\S$ 5.1, we recall a reduction result due to Terragni. In $\S$ 5.2, we use the reduction result to prove convergence of $p_{(W, S)}\left(\frac{1}{n-1}\right)$, where (W, S) is 2-spherical of rank $n \geq 3$. In $\S$ 5.3, we use the reduction result to prove divergence of $p_{(W, S)}\left( \frac{1}{n-2} \right)$, where (W, S) is of rank $n \geq 4$ and the underlying Coxeter diagram is the complete graph.

5.1. Reduction step

Let (W, S) and $(W', S')$ be two Coxeter systems. Following [20], we define $(W, S) \preceq (W', S')$ if there exists an injective map $\varphi: S \to S'$ satisfying $m_{st} \leq m'_{\varphi(s)\varphi(t)}$ for all $s, t\in S$.

Theorem 5.1. Let (W, S) and $(W', S')$ be two Coxeter systems, and let $c_n := c_n^{(W, S)}$ and $c_n' := c_n^{(W', S')}$. If $(W, S) \preceq (W', S')$, then $c_n \leq c_n'$.

Proof. This is [20, Theorem A].

5.2. Convergence

Lemma 5.2. Let (W, S) be of rank $n \geq 3$, and assume that there exists $m \geq 4$ such that $m_{st} = m$ holds for all $s\neq t \in S$. Then, there exists $k \in \mathbb{R}$ such that $\frac{d_i}{c_i} \geq k \gt 0$ holds for all i > m.

Proof. Using Lemma 4.3 and 4.6, we compute

\begin{align*} 1 = \frac{c_i -d_i +d_i}{c_i} &= \frac{1}{c_i}\left( \begin{pmatrix} n-2 \\ 2 \end{pmatrix}c_{i-m} + (n-2)d_{i-m} + d_i \right) \\ &\leq \frac{1}{c_i}\left( \begin{pmatrix} n-2 \\ 2 \end{pmatrix} \frac{1}{(n-2)^m} c_i + \left( \frac{1}{(n-2)^{m-1}} +1 \right) d_i \right) \\ &= \frac{1}{c_i}\left( \frac{(n-3)}{2(n-2)^{m-1}} c_i + \left( \frac{1}{(n-2)^{m-1}} +1 \right) d_i \right) \\ &\leq \frac{1}{2 (n-2)^{m-2}} + \left( \frac{1}{(n-2)^{m-1}} +1 \right) \frac{d_i}{c_i}. \end{align*}

We put

\begin{equation*} k := \left( 1 - \frac{1}{2 (n-2)^{m-2}} \right) \cdot \left( \frac{1}{(n-2)^{m-1}} + 1 \right)^{-1}. \end{equation*}

As $n \geq 3$ and $m \geq 4$, we have k > 0. This proves the claim.

Theorem 5.3. Let (W, S) be 2-spherical and of rank $n \geq 3$. Then, $p_{(W, S)}\left(\frac{1}{n-1}\right) \lt \infty$.

Proof. Let $m := \max \{4, m_{st} \mid s, t \in S \}$, and let $(W', S')$ be the Coxeter system of rank n with $m_{st}' = m$ for all $s\neq t \in S'$. Using Theorem 5.1, it suffices to show that

\begin{equation*} p_{(W', S')}\left( \frac{1}{n-1} \right) \lt \infty. \end{equation*}

By Lemma 5.2, there exists $k\in \mathbb{R}$ such that $\frac{d_i}{c_i} \geq k \gt 0$ holds for all i > m. We apply the ratio test. We use Lemma 4.5 and compute for $i \gt 2m-1$ and $t = \frac{1}{n-1}$:

\begin{align*} \frac{c_{i+1}t^{i+1}}{c_i t^i} \leq \frac{(n-1) c_i - (n-2)d_{i-m+1}}{(n-1) c_i} \leq 1 - \frac{(n-2) d_{i-m+1}}{(n-1)^m c_{i-m+1}} \leq 1- \frac{n-2}{(n-1)^m} k \lt 1. \end{align*}

5.3. Divergence

In this subsection, we prove that the new lower bound $\frac{1}{n-1}$ for the finiteness of the growth function is optimal for the class of 2-spherical Coxeter systems with a complete Coxeter diagram.

Lemma 5.4. Let (W, S) be 2-spherical and of rank $n \geq 4$, and assume that the underlying Coxeter diagram is the complete graph. Then $(n-2) c_i \leq d_i + d_{i+1}$.

Proof. For i = 0, we have $c_0 = 1$, $d_0 = 0$ and $d_1 = n$, and the claim follows. Thus, we can assume i > 0. As in Lemma 4.6, we define $N_i := \{(w, s) \in C_i \times S \mid ws \in D_{i+1} \}$ as well as $N_{i, T} := \{(w, s) \in N_i \mid w\in T \}$ for $T \subseteq C_i$. We consider the mapping

\begin{equation*} \pi: N_i \to D_{i+1}, (w,s) \mapsto ws. \end{equation*}

As before, π is a bijection and we have $\vert N_i \vert = d_{i+1}$. Moreover, we have $N_i = N_{i, D_i} \cup N_{i, C_i \backslash D_i}$ and this union is disjoint. We now count pairs (w, s) in N_i.

We fix $w\in D_i$, and we let $s_w \in S$ be the unique element with $\ell(w s_w) = \ell(w) -1$. Assume that there are $r, s, t \in S \backslash \{s_w \}$ pairwise distinct with $wr, ws, wt \in C_{i+1} \backslash D_{i+1}$. Similarly, as in Lemma 4.6(b), we deduce $\ell(wz s_w) = \ell(w)$ for each $z\in \{r, s, t\}$. As $m_{pq} \geq 3$ holds for all $p \neq q \in S$, we infer $\ell(w s_w z) = \ell(w s_w) -1$. As $\{r, s, t \}$ is not spherical, this is a contradiction and we have for a fixed $w\in D_i$ at least n − 3 tuples (w, s) in N_i.

We fix $w\in C_i \backslash D_i$, and we let $s_w \neq t_w \in S$ be the two elements with $\ell(w s_w) = \ell(w)- 1 = \ell(w t_w)$. Assume that there is $s\in S \backslash \{s_w, t_w \}$ with $ws \in C_{i+1} \backslash D_{i+1}$. Then, $\ell(w) \in \{\ell(wss_w), \ell(wst_w) \}$. W.l.o.g. we assume $\ell(wss_w) = \ell(w)$. But then Corollary 3.3 implies $\ell(wt_w) = \ell(w) +1$, which is a contradiction. Thus, we have for a fixed $w\in C_i \backslash D_i$ exactly n − 2 tuples (w, s) in N_i (cf. Lemma 4.1). This implies that $(n-2) c_i - d_i = (n-3) d_i + (n-2) (c_i - d_i) \leq d_{i+1}$.

Theorem 5.5. Let (W, S) be of rank $n \geq 4$, and assume that the underlying Coxeter diagram is the complete graph. Then, $p_{(W, S)}\left( \frac{1}{n-2} \right) = \infty$.

Proof. Let $(W', S')$ be the Coxeter system of rank n with $m_{st}' = 3$ for all $s\neq t \in S'$. Using Theorem 5.1, it suffices to show that

\begin{equation*} p_{(W', S')} \left( \frac{1}{n-2} \right) = \infty. \end{equation*}

As before, we apply the ratio test. Using Lemmas 4.4 and 5.4, we deduce the following for $i \gt m = 3$ and $t = \frac{1}{n-2}$:

\begin{align*} \frac{c_{i+1} t^{i+1}}{c_i t^i} &= \frac{(n-2) c_i + d_i + d_{i+1}}{2 (n-2) c_i} = \frac{1}{2} + \frac{d_i + d_{i+1}}{2(n-2)c_i} \geq \frac{1}{2} + \frac{1}{2} = 1. \end{align*}