Isolated Singularities of Polyharmonic Operator in Even Dimension

TL;DR

This paper investigates isolated singularities of solutions to a biharmonic equation in four dimensions, establishing conditions for existence, regularity, and the influence of nonlinear growth on singularity behavior.

Contribution

It extends previous work by analyzing singular solutions of the polyharmonic operator in even dimensions, especially in four dimensions, with new growth conditions and regularity results.

Findings

Identifies growth conditions of nonlinearity f that determine singularity coefficients.

Establishes regularity results for solutions with exponential growth bounds.

Extends higher-dimensional barrier function analysis to the four-dimensional case.

Abstract

We consider the equation $\Delta^2 u=g(x,u) \geq 0$ in the sense of distribution in $\Omega'=\Omega\setminus \{0\} $ where $u$ and $ -\Delta u\geq 0.$ Then it is known that $u$ solves $\Delta^2 u=g(x,u)+\alpha \delta_0-\beta \Delta \delta_0,$ for some non-negative constants $\alpha$ and $ \beta.$ In this paper we study the existence of singular solutions to $\Delta^2 u= a(x) f(u)+\alpha \delta_0-\beta \Delta \delta_0$ in a domain $\Omega\subset \mathbb{R}^4,$ $ a$ is a non-negative measurable function in some Lebesgue space. If $\Delta^2 u=a(x)f(u)$ in $\Omega',$ then we find the growth of the nonlinearity $f$ that determines $\alpha$ and $\beta$ to be $0.$ In case when $\alpha=\beta =0,$ we will establish regularity results when $f(t)\leq C e^{\gamma t},$ for some $C, \gamma>0.$ This paper extends the work of Soranzo (1997) where the author finds the barrier function in higher dimensions $(N\geq 5)$ with a specific weight function $a(x)=|x|^\sigma.$ Later we discuss its analogous generalization for the polyharmonic operator.