We introduce a notion of quasilinear parabolic equations over metric measure spaces. Under sharp structural conditions, we prove that local weak solutions are locally bounded and satisfy the parabolic Harnack inequality. Applications include the parabolic maximum principle and pointwise estimates for weak solutions.
Let $ngeq 3$, $0le m<frac{n-2}{n}$, $rho_1>0$, $beta>beta_0^{(m)}=frac{mrho_1}{n-2-nm}$, $alpha_m=frac{2beta+rho_1}{1-m}$ and $alpha=2beta+rho_1$. For any $lambda>0$, we prove the uniqueness of radially symmetric solution $v^{(m)}$ of $La(v^m/m)+alpha_m v+beta xcdot abla v=0$, $v>0$, in $R^nsetminus{0}$ which satisfies $lim_{|x|to 0}|x|^{frac{alpha_m}{beta}}v^{(m)}(x)=lambda^{-frac{rho_1}{(1-m)beta}}$ and obtain higher order estimates of $v^{(m)}$ near the blow-up point $x=0$. We prove that as $mto 0^+$, $v^{(m)}$ converges uniformly in $C^2(K)$ for any compact subset $K$ of $R^nsetminus{0}$ to the solution $v$ of $Lalog v+alpha v+beta xcdot abla v=0$, $v>0$, in $R^nbs{0}$, which satisfies $lim_{|x|to 0}|x|^{frac{alpha}{beta}}v(x)=lambda^{-frac{rho_1}{beta}}$. We also prove that if the solution $u^{(m)}$ of $u_t=Delta (u^m/m)$, $u>0$, in $(R^nsetminus{0})times (0,T)$ which blows up near ${0}times (0,T)$ at the rate $|x|^{-frac{alpha_m}{beta}}$ satisfies some mild growth condition on $(R^nsetminus{0})times (0,T)$, then as $mto 0^+$, $u^{(m)}$ converges uniformly in $C^{2+theta,1+frac{theta}{2}}(K)$ for some constant $thetain (0,1)$ and any compact subset $K$ of $(R^nsetminus{0})times (0,T)$ to the solution of $u_t=Lalog u$, $u>0$, in $(R^nsetminus{0})times (0,T)$. As a consequence of the proof we obtain existence of a unique radially symmetric solution $v^{(0)}$ of $La log v+alpha v+beta xcdot abla v=0$, $v>0$, in $R^nsetminus{0}$, which satisfies $lim_{|x|to 0}|x|^{frac{alpha}{beta}}v(x)=lambda^{-frac{rho_1}{beta}}$.