Liouville theorems for parabolic systems with homogeneous nonlinearities and gradient structure

Liouville theorems for scaling invariant nonlinear parabolic equations and systems (saying that the equation or system does not possess nontrivial entire solutions) guarantee optimal universal estimates of solutions of related initial and initial-boundary value problems. Assume that $p>1$ is subcritical in the Sobolev sense. In the case of nonnegative solutions and the system $$U_t-\Delta U=F(U)\quad\hbox{in}\quad {\mathbb R}^n\times{\mathbb R},$$ where $U=(u_1,\dots,u_N)$, $F=\nabla G$ is $p$-homogeneous and satisfies the positivity assumptions $G(U)>0$ for $U\neq0$ and $\xi\cdot F(U)>0$ for some $\xi\in{\mathbb R}^N$ and all $U\geq0$, $U\ne 0$, it has recently been shown in [P. Quittner, Duke Math. J. 170 (2021), 1113-1136] that the parabolic Liouville theorem is true whenever the corresponding elliptic Liouville theorem for the system $-\Delta U=F(U)$ is true. By modifying the arguments in that proof we show that the same result remains true without the positivity assumptions on $G$ and $F$, and that the class of solutions can also be enlarged to contain (some or all) sign-changing solutions. In particular, in the scalar case $N=1$ and $F(u)=|u|^{p-1}u$, our results cover the main result in [T. Bartsch, P. Polacik and P. Quittner, J. European Math. Soc. 13 (2011), 219-247]. We also prove a parabolic Liouville theorem for solutions in ${\mathbb R}^n_+\times{\mathbb R}$ satisfying homogeneous Dirichlet boundary conditions on $\partial{\mathbb R}^n_+\times{\mathbb R}$ since such theorem is also needed if one wants to prove universal estimates of solutions of related systems in $\Omega\times(0,T)$, where $\Omega\subset{\mathbb R}^n$ is a smooth domain. Finally, we use our Liouville theorems to prove universal estimates for particular parabolic systems.