Topological Properties of the Real Numbers Object in a Topos

In his presentation at the categories Session at Oberwolfach in 1973, Tierney defined the continuous reals for a topos with a natural numbers object (he called them Dedekind reals). Mulvey studied the algebraic properties of the object of continuous reals and proved that the construction gave the sheaf of germs of continuous functions from X to R in the spatial topos Sh( X). This paper presents the results of the study of the topological properties of the continuous reals with an emphasis on similarities with classical mathematics and applications to familiar concepts rephrased in topos terms.