Conversely, if the radius r is smaller than the smallest | ζ ζ | for which λ λ ( ζ ζ ) is significant (denoted ζ ζ min ), the electric potential may be written Φ Φ ( r ) = 1 4 π π ε ε ∑ ∑ k = 0 ∞ ∞ I k r k P k ( cos ⁡ ⁡ θ θ ) where the interior axial multipole moments I k are defined I k ≡ ≡ ∫ ∫ d ζ ζ λ λ ( ζ ζ ) ζ ζ k + 1