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Global Heat Kernel Estimates for $Δ+Δ^{α/2}$ in Half-space-like domains

Suppose that $d\ge 1$ and $α\in (0, 2)$. In this paper, by using probabilistic methods, we establish sharp two-sided pointwise estimates for the Dirichlet heat kernels of $\{Δ+ a^αΔ^{α/2}; \ a\in (0, 1]\}$ on half-space-like $C^{1, 1}$ domains in ${\mathbb R}^d$ for all time $t>0$. The large time estimates for half-space-like domains are very different from those for bounded domains. Our estimates are uniform in $a \in (0, 1]$ in the sense that the constants in the estimates are independent of $a\in (0, 1]$. Thus it yields the Dirichlet heat kernel estimates for Brownian motion in half-space-like domains by taking $a\to 0$. Integrating the heat kernel estimates in time $t$, we obtain uniform sharp two-sided estimates for the Green functions of $\{Δ+ a^αΔ^{α/2}; \ a\in (0, 1]\}$ in half-space-like $C^{1, 1}$ domains in ${\mathbb R}^d$.