Equation Icon  ⟨ ⟩ 1 ∮ a ⟨ ⟩ 1 ∮ a F¯a0 (t,s) = -- ¯Fa0(t,s,ω )d𝒜, F¯ab (t,s) = -- ¯Fab(t,s,ω )d𝒜, (18.25 ) 𝒜 S(t,s) 𝒜 S(t,s)