Equation Icon  ( ) ( ) Δ ˜μ = δ˜μ + ξi∇ ˜μ = ∂μ˜ δn + ξi∇ μ˜= − ∂˜μ- ∇ (nξi) + ξi∇ ˜μ. (236 ) i ∂n i ∂n i i