The two cases can be subsumed in a single expression if r < and r > are defined to be the lesser and greater, respectively, of the two radii r and r ′ ; the potential of a point charge then takes the form, which is sometimes referred to as Laplace expansion Φ Φ ( r ) = q 4 π π ε ε ∑ ∑ ℓ ℓ = 0 ∞ ∞ r < ℓ ℓ r > ℓ ℓ + 1 ( 4 π π 2 ℓ ℓ + 1 ) ∑ ∑ m = − − ℓ ℓ ℓ ℓ Y ℓ ℓ m ( θ θ , ϕ ϕ ) Y ℓ ℓ m ∗ ∗ ( θ θ ′ , ϕ ϕ ′ )