Extending Utility Functions on Arbitrary Sets

We consider the problem of extending a function $f_P^{}$ defined on a subset $P$ of an arbitrary set $X$ to $X$ strictly monotonically with respect to a preorder $\succcurlyeq$ defined on $X$, without imposing continuity constraints. We show that whenever $\succcurlyeq$ has a utility representation, $f_P^{}$ is extendable if and only if it is gap-safe increasing. A class of extensions involving an arbitrary utility representation of $\succcurlyeq$ is proposed and investigated. Connections to related topological results are discussed. The condition of extendability and the form of the extension are simplified when $P$ is a Pareto set.