Theory
G(z,z̄) = Σ_O λ²_O g_{Δ,l}(z,z̄)
s-channel OPE: z→0, O₁O₂ fuse. t-channel: z→1, O₁O₄ fuse. The same 4-point function must be reproduced.
crossing: z ↔ 1-z
Σ_O λ²_O F_{Δ,l}(z) = 0
Conformal blocks g_{Δ,l}(z,z̄) are exactly known functions. The crossing equation is an infinite set of constraints on the CFT data {Δ_O, λ_O}. This is the basis of the conformal bootstrap.