A conformal field theory in flat Euclidean space R d is thus defined by its spectrum { ( Δ Δ p , ρ ρ p ) } and OPE coefficients (or three-point structure constants) { C p p ′ p ″ } , satisfying the constraint that all four-point functions are crossing-symmetric. From the spectrum and OPE coefficients (collectively referred to as the CFT data), correlation functions of arbitrary order can be computed.