In 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.Comment: LaTeX2e, 7 pages. To appear in Arch. Math. Revised version improves
result to also cover positive cas