Lexicographic choice functions

Abstract

We investigate a generalisation of the coherent choice functions considered by Seidenfeld et al. [35], by sticking to the convexity axiom but imposing no Archimedeanity condition. We define our choice functions on vector spaces of options, which allows us to incorporate as special cases both Seidenfeld et al.'s [35] choice functions on horse lotteries and also pairwise choice—which is equivalent to sets of desirable gambles [29]—, and to investigate their connections.We show that choice functions based on sets of desirable options (gambles) satisfy Seidenfeld's convexity axiom only for very particular types of sets of desirable options, which are exactly those that are representable by lexicographic probability systems that have no non-trivial Savage-null events. We call them lexicographic choice functions. Finally, we prove that these choice functions can be used to determine the most conservative convex choice function associated with a given binary relation.