A generalised comparison principle for the Monge–Ampère equation and the pressure in 2D fluid flows

We extend the generalised comparison principle for the Monge–Ampère equation due to Rauch & Taylor (1977) [15] to nonconvex domains. From the generalised comparison principle, we deduce bounds (from above and below) on solutions to the Monge–Ampère equation with sign-changing right-hand side. As a consequence, if the right-hand side is nonpositive (and does not vanish almost everywhere), then the equation equipped with a constant boundary condition has no solutions. In particular, due to a connection between the two-dimensional Navier–Stokes equations and the Monge–Ampère equation, the pressure p in 2D Navier–Stokes equations on a bounded domain cannot satisfy $\mathrm{\Delta }p\le 0$ in Ω unless $\mathrm{\Delta }p\equiv 0$ (at any fixed time). As a result, at any time $t>0$ there exists $z\in \mathrm{\Omega }$ such that $\mathrm{\Delta }p(z,t)=0$.     

Intersection Dimension and Maximum Degree

We show that the intersection dimension of graphs with respect to several hereditary properties can be bounded as a function of the maximum degree. As an interesting special case, we show that the circular dimension of a graph with maximum degree Δ is at most $O(\mathrm{\Delta }\frac{\log \mathrm{\Delta }}{\log \log \mathrm{\Delta })}$. We also obtain bounds in terms of treewidth.     

Schrödinger operators with negative potentials and Lane–Emden densities

We consider the Schrödinger operator $?\mathrm{\Delta }+V$ for negative potentials V, on open sets with positive first eigenvalue of the Dirichlet–Laplacian. We show that the spectrum of $?\mathrm{\Delta }+V$ is positive, provided that V is greater than a negative multiple of the logarithmic gradient of the solution to the Lane–Emden equation $?\mathrm{\Delta }u=u^{q?1}$ (for some $1\le q<2$). In this case, the ground state energy of $?\mathrm{\Delta }+V$ is greater than the first eigenvalue of the Dirichlet–Laplacian, up to an explicit multiplicative factor. This is achieved by means of suitable Hardy-type inequalities, that we prove in this paper.