Absorption by 3-branes and the AdS-CFT correspondence

In the first part of this paper I discuss two somewhat different supergravity approaches to calculating correlation functions in strongly-coupled Yang-Mills theory. The older approach relates two-point functions to cross sections for absorption of certain incident quanta by 3-branes. In this approach the normalization of operators corresponding to the incident particles is fixed unambiguously by the D3-brane DBI action. By calculating the absorption cross sections of all partial waves of the dilaton we find corresponding two-point functions at strong 't Hooft coupling and show that they are identical to the weak-coupling results. The newer approach to correlation functions relates them to boundary conditions in AdS space. Using this method we show that for a certain range of negative mass-squared there are two possible operator dimensions corresponding to a given scalar field in AdS, and indicate how to calculate correlation functions for either of these choices. In the second part of the paper I discuss an example of AdS-CFT duality which arises in the context of type 0 string theory. The CFT on N coincident electric and magnetic D3-branes is argued to be stable for sufficiently weak 't Hooft coupling. It is suggested that its transition to instability at a critical coupling is related to the singularity of planar diagrams.