Optimal kernel estimates for a Schr\"odinger type operator

In the paper the principal result obtained is the estimate for the heat kernel associated to the Schr\"odinger type operator $(1+|x|^\alpha)\Delta-|x|^\beta$ $$k(t,x,y)\leq Ct^{-\frac{\theta}{2}}\frac {\varphi(x)\varphi(y)}{1+|x|^\alpha},$$ where $\varphi=(1+|x|^\alpha)^{\frac{2-\theta}{4}+\frac{1}{\alpha}\frac{\theta-N}{2}}$, $\theta\geq N$ and $0<t<1$, provided that $N>2$, $\alpha> 2$ and $\beta>\alpha-2$. This estimate improves a similar estimate in \cite {can-rhan-tac2} with respect to the dependence on spatial component.