Conversely, if the radius r is less than a, we may factor out 1 a and expand in powers of ( r / a ) < 1 , once again using Legendre polynomials Φ Φ ( r ) = q 4 π π ε ε a ∑ ∑ k = 0 ∞ ∞ ( r a ) k P k ( cos ⁡ ⁡ θ θ ) ≡ ≡ 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 ≡ ≡ q a k + 1 contain everything specific to a given charge distribution; the other parts depend only on the coordinates of the observation point P.