  We perform a one-loop calculation of the vacuum energy of a tachyon field in
anti de-Sitter space with boundary conditions corresponding to the presence of
a double-trace operator in the dual field theory. Such an operator can lead to
a renormalization group flow between two different conformal field theories
related to each other by a Legengre transformation in the large N limit. The
calculation of the one-loop vacuum energy enables us to verify the holographic
c-theorem one step beyond the classical supergravity approximation.
