# A pointwise inequality for a biharmonic equation with negative exponent and related problems

## TL;DR

This paper establishes a new pointwise inequality for solutions of a biharmonic equation with negative exponent, introduces a maximum principle method for high-growth solutions, and applies it to related elliptic and parabolic systems.

## Contribution

It introduces a novel maximum principle technique for high-growth solutions and derives inequalities and comparison properties for biharmonic and Lane-Emden systems with negative exponents.

## Key findings

- Established a pointwise inequality for solutions of ^2 u = -u^{-q} in  R^n.
- Proved comparison properties for Lane-Emden systems with mixed sign exponents.
- Extended results to parabolic Lane-Emden type systems.

## Abstract

Inspired by a recent pointwise differential inequality for positive bounded solutions of the fourth-order H\'enon equation $\Delta^2 u = |x|^a u^p$ in ${\mathbb R}^n$ with $a \geqslant 0$, $p > 1$, $n \geqslant 5$ due to Fazly, Wei, and Xu [ Anal. PDE., 8(2015) 1541--1563], first for some positive constants $\alpha$ and $\beta$ we establish the following pointwise inequality \[ \Delta u \geqslant \alpha u^{-\frac{q-1}2} + \beta u^{-1} |\nabla u|^2 \] in ${\mathbb R}^n$ with $n \geqslant 3$ for positive $C^4$-solutions of the fourth-order equation \[ \Delta^2u=-u^{-q} \quad \text{ in } \mathbb R^n \] where $q > 1$. Next, we prove a comparison property for Lane--Emden system with exponents of mixed sign. Finally, we give an analogue result for parabolic models by establishing a comparison property for parabolic system of Lane--Emden type. To obtain all these results, a new argument of maximum principle is introduced, which allows us to deal with solutions with high growth at infinity. We expect to see more applications of this new method to other problems in different contexts.