Cohomogeneity one actions on the three-dimensional Einstein universe

Abstract: The aim of this paper is to classify the cohomogeneity one conformal actions on the 3-dimensional Einstein universe $Ein^{1;2}$ , up to orbit equivalence. In a paper [21], we studied the unique (up to conjugacy) irreducible action of $PSL(2; \mathbb{R})$ on $Ein^{1;2}$ and we showed that the action is of cohomogeneity one. In the present paper, we determine all the subgroups of $Conf(Ein^{1;2})$, up to conjugacy, acting reducibly and with cohomogeneity one on $Ein^{1;2}$. We show that any cohomogeneity one reducible action on $Ein^{1;2}$ induces a fixed point in the 4-dimensional projective space $\mathbb{R} \mathbb{P}^4$. Also, we describe all the codimension one induced orbits by these actions