Kernel estimates for elliptic operators with unbounded diffusion, drift and potential terms

Abstract

In this paper we prove that the heat kernel $k$ associated to the operator $A:= (1+|x|^α)Δ+b|x|^{α-1}\frac{x}{|x|}\cdot\nabla -|x|^β$ satisfies $$ k(t,x,y) \leq c_1e^{λ_0 t+ c_2t^{-γ}}\left(\frac{1+|y|^α}{1+|x|^α}\right)^{\frac{b}{2α}} \frac{(|x||y|)^{-\frac{N-1}{2}-\frac{1}{4}(β-α)}}{1+|y|^α} e^{-\frac{\sqrt{2}}{β-α+2}\left(|x|^{\frac{β-α+2}{2}}+ |y|^{\frac{β-α+2}{2}}\right)} $$ for $t>0,\,|x|,\,|y|\ge 1$, where $b\in\mathbb{R}$, $c_1,\,c_2$ are positive constants, $λ_0$ is the largest eigenvalue of the operator $A$, and $γ=\frac{β-α+2}{β+α-2}$, in the case where $N>2,\,α>2$ and $β>α-2$. The proof is based on the relationship between the log-Sobolev inequality and the ultracontractivity of a suitable semigroup in a weighted space.

15 pages