A comparison theorem for the Levi equation (Q1327584)

Summary: We prove a strong comparison principle for the solution of the Levi equation \[ \begin{multlined} \sum_{i=1}^ n \bigl( (1+u^ 2_ t) (u_{x_ i x_ i}+ u_{y_ i y_ i})+ (u^ 2_{x_ i}+ u^ 2_{y_ i}) u_{tt}+ 2(u_{y_ i} -u_{x_ i} u_ t) u_{x_ j t}- 2(u_{x_ i}+ u_{y_ i} u_ t) u_{y_ i t} \bigr)+\\ +k(x,y,t) (1+| Du|^ 2 )^{3/2} =0, \end{multlined} \] applying the Bony propagation principle.