Inspired by an example of Gueritaud-Kassel [Geom. Topol. 2017], we construct a family of infinitely generated discontinuous groups $\Gamma$ for the 3-dimensional anti-de Sitter space $\mathrm{ AdS}^{ 3}$. These groups are not necessarily sharp (a kind of "strong" properly discontinuous condition introduced by Kassel and Kobayashi [Adv. Math. 2016]), and we give its criterion. Moreover, we find upper and lower bounds of the counting $N_{ \Gamma}(R)$ of a $\Gamma$-orbit contained in a pseudo-ball $B(R)$ as the radius $R$ tends to infinity. We then find a non-sharp discontinuous group $\Gamma$ for which there exist infinitely many $L^2$-eigenvalues of the Laplacian on the noncompact anti-de Sitter manifold $\Gamma\backslash\mathrm{ AdS}^{ 3}$, by applying the method established by Kassel-Kobayashi. We also prove that for any increasing function $f$, there exists a discontinuous group $\Gamma$ for $\mathrm{ AdS}^{ 3}$ such that the counting $N_{ \Gamma}(R)$ of a $\Gamma$-orbit is larger than $f(R)$ for sufficiently large $R$.
Categories: math.GR math.DS