“…where U = (u, ρ) := (U 1 , U 2 , U 3 , U 4 ), AU = (∆u, ∆ρ), B(U ) := −P(u • ∇U ), G(U ) = P(ρe 3 , 0) = P(U 4 e 3 , 0), σ(U ) = (Pσ (1) (U ), σ (2) (U )). Notice that when U 0 dx = 0, ∇ • b = 0, and σ satisfies condition (2.11) below, one can integrate system (2.4) in T 3 to obtain that U dx = 0 for all t ≥ 0.…”