Intersections of Real Symmetric Hypersurfaces

Abstract

We prove a symmetric version of B\'ezout's theorem. More precisely, we show that the symmetric orbit type of a transverse intersection of complex symmetric hypersurfaces in projective space is determined by the degrees. In the projective plane, we fully classify the possible orbit types of such intersection loci using completely elementary methods. From this classification, we obtain strong restrictions on the number of real points in the intersection of real symmetric curves. We also provide a partial classification in $\mathbb{P}^3_{\mathbb{C}}$, with a similar restriction on real points.