  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.
