A note on lifespan estimates for higher-order parabolic equations
We investigate the lifespan of solutions to the higher-order semilinear parabolic equation $$u_t+(-Δ)^m u=|u|^p, \quad x \in \mathbb{R}^n, t>0 $$ with initial data. We focus on the precise asymptotic behavior of the lifespan of nontrivial solutions. By combining the test function method and semigroup estimates, we derive both upper and lower bounds for the lifespan of solutions $$T_{\varepsilon} \simeq \left\{\begin{array}{l}\varepsilon^{-\left(\frac{1}{p-1}-\frac{n}{2m}\right)^{-1}}, \,\, 1<p<p_{\text {Fuj}}, \\ \exp\left(\varepsilon^{-(p-1)}\right), \,\, p=p_{\text {Fuj}},\end{array}\right.$$ where $p_{Fuj}=1+\frac{2m}{n}$ is the critical exponent of Fujita. These estimates refine and extend the earlier results of Caristi-Mitidieri [J. Math. Anal. Appl., 279:2 (2003), 710-722] and Sun [Electron. J. Differential Equations, 17 (2010)], who obtained only upper bounds under slowly decaying initial data assumptions. In our setting, the above condition on the initial data is replaced by the assumption $L^1\cap L^\infty$, which sharpens the results of the aforementioned works.