A lattice-theoretic approach to arbitrary real functions on frames

Abstract

In this paper we model discontinuous extended real functions in pointfree topology following a lattice-theoretic approach, in such a way that, if $L$ is a subfit frame, arbitrary extended real functions on $L$ are the elements of the Dedekind-MacNeille completion of the poset of all extended semicontinuous functions on $L$. This approach mimicks the situation one has with a $T_1$-space $X$, where the lattice $\overline{\mathrm{F}}(X)$ of arbitrary extended real functions on $X$ is the smallest complete lattice containing both extended upper and lower semicontinuous functions on $X$. Then, we identify real-valued functions by lattice-theoretic means. By construction, we obtain definitions of discontinuous functions that are conservative for $T_1$-spaces. We also analyze semicontinuity and introduce definitions which are conservative for $T_D$-spaces.