Abstract. Let G be the fundamental group of the complement of the torus knot of type (m, n). This has a presentation G = x, y | x m = y n . We find the geometric description of the character variety X(G) of characters of representations of G into SL(3, C), GL(3, C) and PGL(3, C).
IntroductionSince the foundational work of Culler and Shalen [1], the varieties of SL(2, C)-characters have been extensively studied. Given a manifold M , the variety of representations of π 1 (M ) into SL(2, C) and the variety of characters of such representations both contain information of the topology of M . This is specially interesting for 3-dimensional manifolds, where the fundamental group and the geometrical properties of the manifold are strongly related. This can be used to study knots K ⊂ S 3 , by analysing the SL(2, C)-character variety of the fundamental group of the knot complement S 3 − K (these are called knot groups).For a very different reason, the case of fundamental groups of surfaces has also been extensively analysed [2,5,9], in this situation focusing more on geometrical properties of the moduli space in itself (cf. non-abelian Hodge theory).However, much less is known of the character varieties for other groups, notably for SL(r, C) with r ≥ 3. The character varieties for SL(3, C) for free groups have been described in [7,8]. In the case of 3-manifolds, little has been done. In this paper, we study the case of the torus knots K m,n of any type (m, n), which are the first family of knots where the computations are rather feasible. The case of SL(2, C)-character varieties of torus knots was carried out in [11,12]. For SL(3, C), the torus knot K 2,3 has been done in [3].In the case of SL(2, C)-character varieties of torus knot groups, only one-dimensional irreducible components appear. However, when we move to SL(3, C), we see components of different dimensions. In the case of torus knots, we shall see components of dimension 4 and of dimension 2. Our main result is an explicit geometrical description of the SL(3, C)-character variety of torus knots. Theorem 1.1. Let m, n be coprime positive integers. By swapping them if necessary, assume that n is odd. The SL(3, C)-character variety X 3 of the torus knot K m,n ⊂ S 3 is stratified into the following components:• One component consisting of totally reducible representations, isomorphic to C 2 . • If n is even, there are (m−1)/2 extra components consisting of partially reducible representations, each isomorphic to {(u, v) ∈ C 2 |v = 0, v = u 2 }. • 1 12 (n−1)(n−2)(m−1)(m−2) componens of dimension 4, consisting of irreducible representations, all isomorphic to each other, and which are described explicitly in Remark 8.5.• 1 2 (n − 1)(m − 1)(n + m − 4) components consisting of irreducible representations, each isomorphic to (C * ) 2 − {x + y = 1}.Moreover m, n can be recovered from the above information.We also describe geometrically how these components fit into the whole of the SL(3, C)-character variety, that is what is the closure of each of the strata in X 3 . In ...