The Discrete Objects in the Effective Topos
J. M. E. Hyland, E. Powell Robinson, Giuseppe Rosolini · Proceedings of the London Mathematical Society · 1990
The original aim of this paper was to give a rather quick and undemanding proof that the effective topos contains two non-trivial small (i.e. internal) full subcategories which are closed under all small limits in the topos (and hence in particular are internally complete). The interest in such subcategories arises from