A fundamental solution of Laplace's equation satisfies Δ Δ u = u x x + u y y + u z z = − − δ δ ( x − − x ′ , y − − y ′ , z − − z ′ ) , where the Dirac delta function δ denotes a unit source concentrated at the point (x ′, y ′, z ′). No function has this property: in fact it is a distribution rather than a function; but it can be thought of as a limit of functions whose integrals over space are unity, and whose support (the region where the function is non-zero) shrinks to a point (see weak solution). It is common to take a different sign convention for this equation than one typically does when defining fundamental solutions. This choice of sign is often convenient to work with because −Δ is a positive operator. The definition of the fundamental solution thus implies that, if the Laplacian of u is integrated over any volume that encloses the source point, then ∭ ∭ V ∇ ∇ ⋅ ⋅ ∇ ∇ u d V = − − 1.