1. L. Bartholdi, R. I. Grigorchuk and Z. Šunik, Branch roups, in Handbook of Algebra, Vol. 3, North-Holland, Amsterdam, 2003, pp. 989–1112.
2. L. Bartholdi, O. Siegenthaler and P. Zalesskii, The congruence subgroup problem for branch groups, Israel Journal of Mathematics 187 (2012), 419–450.
3. H. Bass, M. Lazard and J.-P. Serre, Sous-groupes d’indice fini dans SL(n,Z), Bulletin of the American Mathematical Society 70 (1964), 385–392.
4. K.-U. Bux, M. V. Ershov and A. S. Rapinchuk, The congruence subgroup property for Aut F2: a group-theoretic proof of Asada’s theorem, Groups, Geometry, and Dynamics 5 (2011), 327–353.
5. A. Garrido and J. S. Wilson, On subgroups of finite index in branch groups, Journal of Algebra 397 (2014), 32–38.