∑ ∑ k = 1 N F k ⋅ ⋅ r k = ∑ ∑ k = 2 N ∑ ∑ j = 1 k − − 1 F j k ⋅ ⋅ ( r k − − r j ) = − − ∑ ∑ k = 2 N ∑ ∑ j = 1 k − − 1 d V j k d r j k | r k − − r j | 2 r j k = − − ∑ ∑ k = 2 N ∑ ∑ j = 1 k − − 1 d V j k d r j k r j k .