Nonnegative solutions with a nontrivial nodal set for elliptic equations   on smooth symmetric domains
  We consider a semilinear elliptic equation on a smooth bounded domain $\Om$ in $\R^2$, assuming that both the domain and the equation are invariant under reflections about one of the coordinate axes, say the y-axis. It is known that nonnegative solutions of the Dirichlet problem for such equations are symmetric about the axis, and, if strictly positive, they are also decreasing in $x$ for $x>0$. Our goal is to exhibit examples of equations which admit nonnegative, nonzero solutions for which the second property fails; necessarily, such solutions have a nontrivial nodal set in $\Om$. Previously, such examples were known for nonsmooth domains only. 
Peter Polacik and Susanna Terracini
math.AP