Well I have the metric of a sphere given as: ds2= α (dθ2+ sin2θ dφ2 ) And I'm asked to prove that the constant α is non-absorbable by proving there is no solution to this set of equations: Δξgμν = ∂gμν/∂α where: Δξgμν = gμν;λ ξλ + gλν ξλ;μ + gμλ ξλ;ν Finally, I'm asked to provide a second way to prove that α is non-absorbable. I don't understand what it means by non-absorbable at all. Do you know where am I supposed to find what it is? From books I have Weinberg's "Gravitation and Cosmology Principles and appls of GTR". In addition, any hint would be appreciated... Thank you..