This is the first book of the Mishneh Torah of the Rambam, Rabbi Moshe ben Maimon. Laws of Torah Study.
This chapter explains whether a sage, the leader (Nasi), or the second in command (Av Beit Din) is excommunicated; what a Nidwi or a Jerem is, how they are permitted, and all the laws pertaining to both forms of excommunication: excommunication in a dream, excommunication while awake, and whether it occurs face-to-face or not. It also addresses whether a sage can permit his own excommunication and if a period of time is required between the excommunication and the excommunication itself. The custom of the early pious was not to excommunicate anyone.