The three non-trivial four-point functions are of the type ⟨ ⟨ σ σ 4 ⟩ ⟩ , ⟨ ⟨ σ σ 2 ϵ ϵ 2 ⟩ ⟩ , ⟨ ⟨ ϵ ϵ 4 ⟩ ⟩ . For a four-point function ⟨ ∏ ∏ i = 1 4 V i ( z i ) ⟩ , let F j ( s ) and F j ( t ) be the s- and t-channel Virasoro conformal blocks, which respectively correspond to the contributions of V j ( z 2 ) (and its descendants) in the operator product expansion V 1 ( z 1 ) V 2 ( z 2 ) , and of V j ( z 4 ) (and its descendants) in the operator product expansion V 1 ( z 1 ) V 4 ( z 4 ) . Let x = z 12 z 34 z 13 z 24 be the cross-ratio.