Equation Icon  [( ) ] 2 j j 2 j ∂˜μ j ∂tξi + 2v ∇j ∂tξi + (v ∇j ) ξi + ∇i δΦ + ξ ∇i ∇j (Φ + ˜μ) − ∇i ∂n- ∇j(n ξ ) = 0. (237 )