In this paper, one of our aims is to investigate the instability of the distribution of the critical point set of a solution u to a semilinear equation with Dirichlet boundary condition in the planar annular domains. Precisely, we prove that in an eccentric circle annular domain, or a petal-like domain, or an annular domain where the interior and exterior boundaries are equally scaled ellipses contains only finitely many points rather than a Jordan curve. This result indicates that the critical point set is unstable when any boundary of planar concentric circle annular domain Ω has some small deformation or minor perturbation. Based on studying the distribution of the nodal sets and , we prove that the solution u on each symmetric axis has exactly two critical points under some conditions. Meanwhile, we further obtain that only has two critical points in an eccentric circle annular domain, has four critical points in an exterior petal-like domain with the exterior boundary is an ellipse, and the maximum points are distributed on the long symmetric semi-axis and the saddle points on the short symmetric semi-axis. Moreover, we describe the geometric location of critical points of the solution u by the moving plane method.
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