Abstract. Simple, or Kleinian, singularities are classified by Dynkin diagrams of type ADE. Let g be the corresponding finitedimensional Lie algebra, and W its Weyl group. The set of ginvariants in the basic representation of the affine Kac-Moody algebraĝ is known as a W-algebra and is a subalgebra of the Heisenberg vertex algebra F . Using period integrals, we construct an analytic continuation of the twisted representation of F . Our construction yields a global object, which may be called a W -twisted representation of F . Our main result is that the total descendant potential of the singularity, introduced by Givental, is a highest weight vector for the W-algebra.1. Introduction 1.1. Motivation from Gromov-Witten theory. Recall that the Gromov -Witten (GW) invariants of a projective manifold X consist of correlators (1.1) τ k 1 (v 1 ), . . . , τ kn (v n ) g,n,dwhere v 1 , . . . , v n ∈ H * (X; C) are cohomology classes and the enumerative meaning of the correlator is the following. Let C 1 , . . . , C n be n cycles in X in a sufficiently generic position that are Poincaré dual to v 1 , . . . , v n , respectively. Then the GW invariant (1.1) counts the number of genus-g, degree-d holomorphic curves in X that are tangent (in an appropriate sense) to the cycles C i with multiplicities k i . For the precise definition we refer to [103,75,8,83]. After A. Givental [57], we organize the GW invariants in a generating series D X called the total descendant potential of X and defined as follows. Choose a basis {v i }
N i=1of the vector (super)space H = H * (X; C) and let t k = τ k 1 (t k 1 ), . . . , τ kn (t kn ) g,n,d , where t = (t 0 , t 1 , . . . ) = (t i k ) and the definition of the correlator is extended multi-linearly in its arguments. The function D X is interpreted as a formal power series in the variables t i k with coefficients formal Laurent series in whose coefficients are elements of the Novikov ring C [Q].When X is a point and hence d = 0, the potential D pt (also known as the partition function of pure gravity) is a generating function for certain intersection numbers on the Deligne-Mumford moduli space of Riemann surfaces M g,n . It was conjectured by Witten [103] and proved by Kontsevich [74] that D pt is a tau-function for the KdV hierarchy of soliton equations. (We refer to [19,102] for excellent introductions to soliton equations.) In addition, D pt satisfies one more constraint called the string equation, which together with the KdV hierarchy determines uniquely D pt (see [103]). It was observed in [22,54,69] that the taufunction of KdV satisfying the string equation is characterized as the unique solution of L n D pt = 0 for n ≥ −1, where L n are certain differential operators representing the Virasoro algebra. This means that D pt is a highest-weight vector for the Virasoro algebra and in addition satisfies the string equation L −1 D pt = 0.One of the fundamental open questions in Gromov-Witten theory is the Virasoro conjecture suggested by S. Katz and the physicists Eguchi, Hori, Xiong, and...