As with axial multipole moments, we may also consider the case when the radius r of the observation point is less than the radius r ′ of the charge. In that case, we may write Φ Φ ( r ) = q 4 π π ε ε r ′ ∑ ∑ ℓ ℓ = 0 ∞ ∞ ( r r ′ ) ℓ ℓ ( 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 ′ ) which can be written as Φ Φ ( r ) = 1 4 π π ε ε ∑ ∑ ℓ ℓ = 0 ∞ ∞ ∑ ∑ m = − − ℓ ℓ ℓ ℓ I ℓ ℓ m r ℓ ℓ 4 π π 2 ℓ ℓ + 1 Y ℓ ℓ m ( θ θ , ϕ ϕ ) where the interior spherical multipole moments are defined as the complex conjugate of irregular solid harmonics I ℓ ℓ m = d e f q ( r ′ ) ℓ ℓ + 1 4 π π 2 ℓ ℓ + 1 Y ℓ ℓ m ∗ ∗ ( θ θ ′ , ϕ ϕ ′ )