Comparison results for semilinear elliptic equations using a new symmetrization method

In this paper, we prove some pointwise comparison results between the solutions of some second-order semilinear elliptic equations in a domain $Omega$ of $R^n$ and the solutions of some radially symmetric equations in the equimeasurable ball $Omega^*$. The coefficients of the symmetrized equations in~$Omega^*$ satisfy similar constraints as the original ones in~$Omega$. We consider both the case of equations with linear growth in the gradient and the case of equations with at most quadratic growth in the gradient. Lastly, we show some improved quantified comparisons when the original domain is not a ball. The method is based on a symmetrization of the second-order terms.