Equation Icon  ∇2 Φ = 4πG ρ + ∇. [f′(|∇ (Φ − ˆΦ )|2∕a2)∇ (Φ − Φˆ)] and ∇2Φˆ = 4πG ρˆ+ ∇. [f ′(|∇ (Φ − ˆΦ )|2∕a2)& 0 0