SPENSER, The Faerie Queene, Bk. 7
Let xi be the coordinates of a point P and r its distance from the origin. Then
Now put k = i and apply the summation convention; since
so that 1/r is a solution of Laplace’s equation, ∇2ϕ = 0, except at the origin.
It follows at once that if ξi are the coordinates of another point Q, and
(summed), then ...