Equation Icon  ( ) δn = ∂NABC--∇ XA ∇ XB ∇ XC δXD νλτ ∂XD ν λ τ ( A B C A B C A B C ) +NABC ∇ νδX ∇λX ∇ τX + ∇ νX ∇ λδX ∇ τX + ∇ νX ∇ λX ∇ τδX . (128 )