Parkkonen, Jouni, and Frédéric Paulin. “A Classification of $\mathbb{C}$-Fuchsian Subgroups of Picard Modular Groups”. Mathematica Scandinavica, vol. 121, no. 1, Sept. 2017, pp. 57-74, https://doi.org/10.7146/math.scand.a-26128.