2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/117818In his study of Ricci flow, Perelman introduced a smooth-manifold invariant called lambda-bar. 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 + infinity whenever the Yamabe invariant is positive.LaTeX2e, 7 pages. To appear in Arch. Math. Revised version improves result to also cover positive caseDifferential GeometryFunctional Analysis53C21, 58J50text