STABILITY CONDITIONS AND THE A2 QUIVER TOM BRIDGELAND, YU QIU AND TOM SUTHERLAND Abstract. For each integer n > 2 we describe the space of stability conditions on the derived category of the n-dimensional Ginzburg algebra associated to the A2 quiver. The form of our results points to a close relationship between these spaces and the Frobenius- Saito structure on the unfolding space of the A2 singularity. 1. Introduction In this paper we study spaces of stability conditions [2] on the sequence of CYn trian- gulated categories Dn associated to the A2 quiver. Our main result is Theorem 1.1 below. There are several striking features. Firstly, we obtain uniform results for all n > 2: the space of stability conditions quotiented by the action of the spherical twists is independent of n, although the identification maps are highly non-trivial. Secondly, there is a close link between our spaces of stability conditions and the Frobenius-Saito structure on the unfolding space of the A2 singularity: in fact this structure is precisely what encodes the identifications between our stability spaces for various n. A third interesting feature is that the space of stability conditions on the derived category of the path algebra of the A2 quiver arises as a kind of limit of the spaces for the categories Dn as n ! 1. 1.1. Statement of results. For each integer n > 2 we let Dn = DCYn (A2) denote the bounded derived category of the CYn complex Ginzburg algebra associated to the A2 quiver. It is a triangulated category of finite type over C, and is characterised by the following two properties: (a) It is CYn, i.e. for any pair of objects A; B 2 Dn there are natural isomorphisms ∗ ∼ ∗ _ HomDn (A; B) = HomDn (B; A[n]) : (1) (b) It is generated by two spherical objects S1;S2 satisfying ∗ HomDn (S1;S2) = C[−1]: (2) We denote by D1 the bounded derived category of the complex path algebra of the A2 quiver. It is again a C-linear triangulated category, and is characterised by the property 1 STABILITY CONDITIONS AND THE A2 QUIVER 2 that it is generated by two exceptional objects S1;S2, which satisfy (2) and ∗ HomD1 (S2;S1) = 0: The notation D1 is convenient: the point being that as n increases, the Serre dual to the extension S1 ! S2[1] occurs in higher and higher degrees, until when n = 1 it doesn't occur at all. For each 2 6 n 6 1 we denote by Stab(Dn) the space of stability conditions on the category Dn. We define Stab∗(Dn) ⊂ Stab(Dn) to be the connected component containing stability conditions in which the objects S1 and S2 are stable of equal phase. Let Aut(Dn) denote the group of exact C-linear autoequivalences of the category Dn, considered up to isomorphism of functors. We define Aut∗(Dn) to be the subquotient consisting of autoe- quivalences which preserve the connected component Stab∗(Dn), modulo those which act trivially on it. When n < 1 the objects Si are spherical and hence define Seidel-Thomas twist functors TwSi 2 Aut(Dn). These autoequivalences preserve the connected component Stab∗(Dn), and we denote by Sph∗(Dn) ⊂ Aut∗(Dn) the subgroup they generate. The simple complex Lie algebra associated to the A2 quiver is g = sl3(C). Its Cartan subalgebra can be described explicitly as 3 X h = f(u1; u2; u3) 2 C : ui = 0g: i The complement of the root hyperplanes is reg h = f(u1; u2; u3) 2 h : i 6= j =) ui 6= ujg: There is an obvious action of the Weyl group W = S3 permuting the ui which is free on hreg. The quotient h=W is isomorphic to C2, and has natural co-ordinates (a; b) obtained by writing 3 p(x) = (x − u1)(x − u2)(x − u3) = x + ax + b: The image of the root hyperplanes ui = uj is the discriminant 2 3 2 ∆ = f(a; b) 2 C : 4a + 27b = 0g: We can now state the main result of this paper. Theorem 1.1. (a) For 2 6 n < 1 there is an isomorphism of complex manifolds ∼ reg Stab∗(Dn)= Sph∗(Dn) = h =W: STABILITY CONDITIONS AND THE A2 QUIVER 3 2 Under this isomorphism the central charge map Stab∗(Dn) ! C induces the multi- valued map hreg=W ! C2 given by Z p(x)(n−2)=2 dx γi for an appropriate basis of paths γi connecting the zeroes of the polynomial p(x). (b) For n = 1 there is an isomorphism of complex manifolds ∼ Stab(D1) = h=W: 2 Under this isomorphism the central charge map Stab(D1) ! C corresponds to the map h=W ! C2 given by Z ep(x) dx δi for an appropriate basis of paths δi which approach 1 in both directions along rays for which Re(x3) ! −∞. Theorem 1.1 gives a precise link with the Frobenius-Saito structure on the unfolding 3 space of the A2 singularity x = 0. The corresponding Frobenius manifold is precisely M = h=W . The maps appearing in part (a) of our result are then the twisted period maps of M with parameter ν = (n − 2)=2 (see Equation (5.11) of [6]). The map in part (b) is given by the deformed flat co-ordinates of M with parameter ~ = 1 (see [5, Theorem 2.3]). 1.2. Related work. Just as we were finishing this paper, A. Ikeda posted the paper [10] which also proves Theorem 1.1 (a), and indeed generalizes it to the case of the Ak quiver for all k > 1. The methods we use here are quite different however, and also yield (b), so we feel that this paper is also worth publishing. As explained above, two of the most interesting features of Theorem 1.1 are the fact that the space Stab∗(Dn)= Sph∗(Dn) is independent of n < 1, and that this space embeds in Stab(D1). At the level of exchange graphs such results were observed for arbitrary acyclic quivers by one of us with A.D. King [14]. The n < 1 case of Theorem 1.1 was first considered by R.P. Thomas in [23]: he obtained the n = 2 case and discussed the relationship with Fukaya categories and homological mirror symmetry. The n = 2 case was also proved in [3] and generalised to arbitrary ADE Dynkin diagrams. The n = 3 case of Theorem 1.1 was proved in [21], and was STABILITY CONDITIONS AND THE A2 QUIVER 4 extended to all Dynkin quivers of A and D type in [4]. The first statement of part (a), ∼ reg that Stab∗(D) = h =W , was proved for all n < 1 in [18]. The case n = 1 of Theorem 1.1 was first considered by King [13] who proved that ∼ 2 Stab(D1) = C . This result was obtained by several other researchers since then, and a proof was written down in [18]. The more precise statement of Theorem 1.1 (b) was conjectured by A. Takahashi [22]. Since the first version of this paper was posted, several generalizations and extensions of Theorem 1.1 have appeared. The following seem particularly noteworthy. In [9] a general result relating stability conditons on Fukaya categories of surfaces to spaces of quadratic differentials with exponential singularities is proved. This includes Theorem 1.1 (b) as a very special case. In [11] a new notion of q-stability conditions is introduced, in terms of which one can make sense of the statement of Theorem 1.1 when the Calabi-Yau dimension n > 2 is replaced with an arbitrary s 2 C with Re(s) > 2. This allows one to see all twisted periods of the Frobenius manifold via central charges. In [15], an analogue of Theorem 1.1 for the Kronecker quiver is proved. In this case the relevant Frobenius manifold is the quantum cohomology of P1. The case of quivers of type affine An was considered in [24], and a general discussion. A general framework for such results, involving Fukaya categories of surfaces, quadratic differentials and Hurwitz spaces is explained in [12]. Acknowledgements. We thank Alastair King and Caitlin McAuley for many useful con- versations on the topic of this paper. 2. Autoequivalences and t-structures In this section we describe the principal components of the exchange graphs of the categories Dn = DCYn (A2) and study the action of the group of reachable autoequivalences. We start by recalling some general definitions concerning tilting (see [4, Section 7] for more details). 2.1. Let D be a triangulated category. We shall be concerned with bounded t-structures on D. Any such t-structure is determined by its heart A ⊂ D, which is a full abelian subcategory. We use the term heart to mean the heart of a bounded t-structure. A heart will be called finite-length if it is artinian and noetherian as an abelian category. STABILITY CONDITIONS AND THE A2 QUIVER 5 We say that a pair of hearts (A1; A2) in D is a tilting pair if the equivalent conditions A2 ⊂ hA1; A1[−1]i; A1 ⊂ hA2[1]; A2i are satisfied. Here the angular brackets signify the extension-closure operation We also say that A1 is a left tilt of A2, and that A2 is a right tilt of A1. Note that (A1; A2) is a tilting pair precisely if so is (A2[1]; A1). If (A1; A2) is a tilting pair in D, then the subcategories T = A1 \A2[1]; F = A1 \A2 form a torsion pair (T ; F) ⊂ A1. Conversely, if (T ; F) ⊂ A1 is a torsion pair, then the subcategory A2 = hF; T [−1]i is a heart, and the pair (A1; A2) is a tilting pair.