Samia Challal. “Regularity of the Free Boundary in $div(a(x) \nabla u(x,y) )= -(h(x)\gamma(u))_x$ With $h^\prime (x)<0$”. International Journal of Mathematics And Its Applications, vol. 9, no. 4, Dec. 2021, pp. 1-11, http://ijmaa.in/index.php/ijmaa/article/view/23.