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Abstract

We study a class of elliptic inequalities which arise in the study of blow-up rate estimates for parabolic problems, and obtain a sharp existence/nonexistence result. Namely, for any $p\ge 1$, we show that the inequality $\Delta u+ u^p \leq \varepsilon$ in $\mathbb R ^n$ with $u(0)=1$ admits a radial, positive nonincreasing solution for all $\varepsilon \gt 0$ if and only if $n\ge 2$. This solves a problem left open in [Souplet & Tayachi, Colloq. Math. 88 (2001)]. The result stands in contrast with the classical case $\varepsilon =0$.