Asymptotic large time behavior of singular solutions of the fast diffusion equation

We study the asymptotic large time behavior of singular solutions of the fast diffusion equation $u_t=Δu^m$ in $({\mathbb R}^n\setminus\{0\})\times(0,\infty)$ in the subcritical case $0<m<\frac{n-2}{n}$, $n\ge3$. Firstly, we prove the existence of singular solution $u$ of the above equation that is trapped in between self-similar solutions of the form of $t^{-α} f_i(t^{-β}x)$, $i=1,2$, with initial value $u_0$ satisfying $A_1|x|^{-γ}\le u_0\le A_2|x|^{-γ}$ for some constants $A_2>A_1>0$ and $\frac{2}{1-m}<γ<\frac{n-2}{m}$, where $β:=\frac{1}{2-γ(1-m)}$, $α:=\frac{2β-1}{1-m},$ and the self-similar profile $f_i$ satisfies the elliptic equation $$ Δf^m+αf+βx\cdot \nabla f=0\quad \mbox{in ${\mathbb R}^n\setminus\{0\}$} $$ with $\lim_{|x|\to0}|x|^{\frac{ α}{ β}}f_i(x)=A_i$ and $\lim_{|x|\to\infty}|x|^{\frac{n-2}{m}}{f_i}(x)= D_{A_i}$ for some constants $D_{A_i}>0$. When $\frac{2}{1-m}<γ<n$, under an integrability condition on the initial value $u_0$ of the singular solution $u$, we prove that the rescaled function $\tilde u(y,τ):= t^{\,α} u(t^{\,β} y,t),\quad{ τ:=\log t},$ converges to some self-similar profile $f$ as $τ\to\infty$.

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