Abstract. In his study of Ricci flow, Perelman introduced a smooth-manifold invariant calledλ. We show here that, for completely elementary reasons, this invariant simply equals the Yamabe invariant, alias the sigma constant, whenever the latter is non-positive. On the other hand, the Perelman invariant just equals +∞ whenever the Yamabe invariant is positive.
Mathematics Subject Classification (2000). Primary 53C21; Secondary 58J50.Keywords. Scalar curvature, Ricci flow, conformal geometry, Perelman invariant, Yamabe problem.Let M be a smooth compact manifold of dimension n ≥ 3. Perelman's celebrated work on Ricci flow [12,13] led him to consider the functional which associates to every Riemannian metric g the least eigenvalue λ g of the elliptic operator 4∆ g + s g , where s g denotes the scalar curvature of g, and ∆ = d * d = −∇ · ∇ is the positive-spectrum Laplace-Beltrami operator associated with g. In other words, λ g can be expressed in terms of Raleigh quotients aswhere the infimum is taken over all smooth, real-valued functions u on M .One of Perelman's remarkable observations is that the scale-invariant quantity λ g V 2/n g is non-decreasing under the Ricci flow, where V g = M dµ g denotes the total volume of (M, g). This led him to consider the differential-topological invariant obtained by taking the supremum of this quantity over the space of all Riemannian metrics [13,6]: * Supported in part by NSF grant DMS-0604735.