(η-ζ)⁢(ζ-ξ)⁢(ξ-η)⁢∇2=(ζ-η)⁢f⁡(ξ)⁢f′⁡(ξ)⁢∂∂⁡ξ+(ξ-ζ)⁢f⁡(η)⁢f′⁡(η)⁢∂∂⁡η+(η-ξ)⁢f⁡(ζ)⁢f′⁡(ζ)⁢∂∂⁡ζ,