Holography, Probe Branes and Isoperimetric Inequalities
  In many instances of holographic correspondences between a d dimensional boundary theory and a d+1 dimensional bulk, a direct argument in the boundary theory implies that there must exist a simple and precise relation between the Euclidean on-shell action of a (d-1)-brane probing the bulk geometry and the Euclidean gravitational bulk action. This relation is crucial for the consistency of holography, yet it is non-trivial from the bulk perspective. In particular, we show that it relies on a nice isoperimetric inequality that must be satisfied in a large class of Poincar\'e-Einstein spaces. Remarkably, this inequality follows from theorems by Lee and Wang. 
Frank Ferrari (U.L. Bruxelles and Intl. Solvay Inst.) and Antonin   Rovai (LMU Munich and MIT)
hep-th math-ph math.MP