Distribution solutions of a static dispersion Schrödinger equation

In this paper, we study qualitative properties of distribution solutions of a fourth order equation $$-\Delta u(x)+a^2\Delta^2u(x)=u^q(x), \quad u(x)>0 \ \ in \ \ \mathbb{R}^3,$$ where $a>0$ and $q>0$. It is the static equation of a mixed dispersion Schrodinger equation, and also the Euler-Lagrange equation satisfied by extremal functions of an embedding inequality. We obtain some Liouville theorems and the corresponding related critical exponents, which imply the best constant of the embedding inequality cannot be attainable. We also obtain some regularity results (involving differentiability, integrability, radial symmetry) and asymptotics at infinity of distribution solutions. Here an equivalent integral equation with the Coulomb potential $|x|^{-1}(1-e^{-|x|/a})$ plays a key role. In addition, we also use the Pohozaev identity in integral form to obtain the Liouville theorem of this integral equation. Such the Pohozaev identity still works to handle the Allen-Cahn-type integral equation.