In four dimensions, the fundamental solution to Laplace's equation is | x − − y | − − 2 for any fixed y . Superposing N + 1 of these gives N -soliton solutions of the form ρ ρ ( x ) = ∑ ∑ p = 1 N λ λ p | x − − x p | 2 . All solutions of instanton number 1 or 2 are of this form, but for larger instanton number there are solutions not of this form.