## Representation formulae for solutions to some classes of higher order systems and related Liouville theorems.(English)Zbl 1186.35026

The authors studied some representation formulae and related Liouville theorems for solutions to some classes of higher order differential equations and systems on Euclidean space.
Let $$m \geq 1$$ be an integer and $$N > 2m$$. Let $$\mu$$ be a positive Radon measure on $${\mathbb R}^N$$ and $$l \in {\mathbb R}$$. Then the authors proved that if $$u$$ is a solution of $$(-\Delta)^m u = \mu$$ on $${\mathbb R}^N$$ in the distribution sense and for a.e. $$x \in {\mathbb R}^N$$,
$\liminf_{R\to \infty}\frac{1}{R^N}\int_{R\leq |x-y|\leq 2R} |u(y) - l| \,dy = 0,\tag{1}$
then $$u \in L^l_{\text{loc}}({\mathbb R}^N)$$ and
$u(x) = l + C \int_{{\mathbb R}^N} \frac{d\mu(y)}{|x-y|^{N-2m}}\quad \text{a.e. } x\in {\mathbb R}^N,\tag{2}$
where $$C$$ is a positive constant depending only on $$m$$ and $$N$$. The converse is also true. As a corollary, one can replace the positive Radon measure $$\mu$$ by a function $$h \in L^l_{\text{loc}}({\mathbb R}^N)$$. Namely if $$u$$ is a distributional solution of $$(-\Delta)^m u \geq h$$ and (1) holds, then $$u$$ has of the form (2) when $$d\mu$$ is replaced by $$h$$.
As a consequence of the representation formula (2), the authors obtained some Liouville theorems. For instance, let $$p >1$$ and let $$u \in L^1_{\text{loc}}({\mathbb R}^N)$$ be a distributional solution of $$(-\Delta)^m u \geq 0$$ with $$N > 2m$$. If $$u \in L^p({\mathbb R}^N)$$ with $$(N-2m)p \leq N$$ or $$u \in L^p_\omega({\mathbb R}^N)$$, the weak-$$L^p$$ space, with $$(N-2m)p < N$$, then $$u \equiv 0$$ a.e. on $${\mathbb R}^N$$.
Next, the authors obtained some representation formulae and related Liouville theorems for polyharmonic systems related with the Hardy-Littlewood-Sobolev system (HLS for brevity). Using their representation formulae for polyharmonic systems, the authors proved a non-existence result for non-negative radial solutions of the HLS systems of integral equations.

### MSC:

 35C15 Integral representations of solutions to PDEs 31C10 Pluriharmonic and plurisubharmonic functions 45G15 Systems of nonlinear integral equations 35B53 Liouville theorems and Phragmén-Lindelöf theorems in context of PDEs 35J30 Higher-order elliptic equations
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