Suppose ψ ψ : D → → R 3 is piecewise smooth at the neighborhood of D , with Σ Σ = ψ ψ ( D ) . If Γ Γ is the space curve defined by Γ Γ ( t ) = ψ ψ ( γ γ ( t ) ) then we call Γ Γ the boundary of Σ Σ , written ∂ ∂ Σ Σ .