On Extendable Derivations

Abstract

There are derivations $f\:mathR\to\mathR$ which are almost continuous in the sense of Stallings but not extendable. Every derivation $f\colon \mathR\to \mathR$ can be expressed as the sum of two extendable derivations, as the discrete limit of a sequence of extendable derivations and as the limit of a transfinite sequence of extendable derivations. Analogous results hold for additive functions.