We can apply a small perturbation δ δ r → → = δ δ r e → → r + δ δ z e → → z to the circular orbit : r → → = r 0 e → → r + δ δ r → → So, r → → ¨ ¨ + δ δ r → → ¨ ¨ = − − ∇ ∇ → → Φ Φ e f f ( r → → + δ δ r → → ) ≈ ≈ − − ∇ ∇ → → Φ Φ e f f ( r → → ) − − ∂ ∂ r 2 Φ Φ e f f ( r → → ) δ δ r − − ∂ ∂ z 2 Φ Φ e f f ( r → → ) δ δ z