  By carefully analyzing the radial part of the wave-equation for a scalar
field in AdS, we show that for a particular range of boundary conditions on the
scalar field, the radial spectrum contains a bound state. Using the AdS/CFT
correspondence, we interpret this peculiar phenomenon as being dual to an
unstable double trace deformation of the boundary conformal field theory. We
thus show how the bulk theory holographically detects whether a boundary
perturbation is stable.
