We study complete Riemannian
manifolds satisfying the equation
by studying the associated PDE Δff + mμexp(2f∕m) = 0 for μ ≤ 0. By developing
a gradient estimate for f, we show there are no nonconstant solutions. We then
apply this to show that there are no nontrivial Ricci flat warped products
with fibers that have nonpositive Einstein constant. We also show that for
nontrivial steady gradient Ricci solitons, the quantity R + |∇f|2 is a positive
constant.