Geometric Holography, the Renormalization Group and the c-Theorem
  In this paper the whole geometrical set-up giving a conformally invariant holographic projection of a diffeomorphism invariant bulk theory is clarified. By studying the renormalization group flow along null geodesic congruences a holographic version of Zamolodchikov's c-theorem is proven. 
Enrique Alvarez and Cesar Gomez