A note on the analyticity of AdS scalar exchange graphs in the crossed   channel
  We discuss the analytic properties of AdS scalar exchange graphs in the crossed channel. We show that the possible non-analytic terms drop out by virtue of non-trivial properties of generalized hypergeometric functions. The absence of non-analytic terms is a necessary condition for the existence of an operator product expansion for CFT amplitudes obtained from AdS/CFT correspondence. 
L. Hoffmann, A. C. Petkou and W. Ruehl