Fujita exponent for the global-in-time solutions to a semilinear heat equation with non-homogeneous weights

We consider a non-homogeneous parabolic equation with degenerate coefficients of the form \(u_t-L_{\omega } u=u^p\) $u_t-L_{\omega }u=u^p$ , where \(L_{\omega }=\omega ^{-1}\mathrm div(\omega \nabla )\) $L_{\omega }={\omega }^{-1}\mathrm{d}iv(\omega \mathrm{\nabla })$ . This paper establishes the existence/non-existence of global-in-time mild solutions based on a critical exponent, known as the Fujita exponent. Similar topics for a semilinear heat equation with degenerate coefficients are treated in Fujishima (Calc Var Partial Differ Equ 58:25, 2019). They considered an equation \(u_t-\textrm{div}(\omega \nabla u) =u^p\) $u_t-\text{div}(\omega \mathrm{\nabla }u)=u^p$ , which is not self-adjoint, with two types of homogeneous weights: \(\omega (x) = |x_1|^a\) $\omega (x)=|x_1{|}^a$ and \(\omega (x) = |x|^b\) $\omega (x)={|x|}^b$ where \(a,b>0\) $a,b>0$ . In this paper we consider the case of a self-adjoint operator, and extend to more general weights that meet certain restrictions such as being in the Muckenhoupt class \(A_2\) $A_2$ , non-decreasing, and where the limits \(\alpha :=\lim _{|x'|\rightarrow \infty }(\log \omega (x))/(\log |x'|)\) $\alpha :={lim}_{|x^{\prime }|\to \infty }(log\omega (x))/(log|x^{\prime }|)$ and \(\beta :=\lim _{|x'|\rightarrow 0}(\log \omega (x))/(\log |x'|)\) $\beta :={lim}_{|x^{\prime }|\to 0}(log\omega (x))/(log|x^{\prime }|)$ exist, where \(x' = (x_1, \dots , x_n)\) $x^{\prime }=(x_1,\cdots ,x_n)$ and \(1\le n\le N\) $1\le n\le N$ . The main result establishes that the Fujita exponent is given by \(p_F = 1+2/(N+\alpha )\) $p_F=1+2/(N+\alpha )$ . This means that the asymptotic behavior of the weight at infinity affects global existence of solutions and the one at the origin does not.