О возможности имплементации такой функции коллективного выбора, как объединение минимальных внешнеустойчивых множеств, и о других ее полезных свойствах и возможностях применения
A set of related majority rule-based social choice correspondences are considered: the union of minimal Р-dominating sets MPD (Duggan 2011, Subochev 2016) the union of weakly stable sets MWS (Aleskerov & Kurbanov 1999), the union of minimal P-externally stable sets MPES (Wuffl et al. 1989, Subochev 2008) and the union of minimal R-externally stable sets MRES (Aleskerov & Subochev 2009, 2013). These tournament solutions have not attracted much attention so far. However, the analysis of their properties suggests that MPES and MRES can be useful as instruments of choice, for instance when it is necessary to aggregate rankings. Their implementation is also possible under certain conditions.
The results presented are the following.
1) In a general case of a topological space of alternatives, a sufficient and necessary condition has been provided for an alternative to belong to a minimal P-dominating set. This characteristic condition is related to some version of the covering relation. It has been established that the union of minimal P-dominating sets and the uncovered set are logically nested neither in a general case, nor in finite tournaments. The characterization obtained provides a sufficient condition of nonemptiness of MPES and MRES in a general case of a topological space of alternatives.
2) It has been found that MPES and MRES both satisfy the following axioms:
a) monotonicity with respect to changes in social preferences (P-monotonicity),
b) the generalized Nash independence of irrelevant alternatives,
c) the idempotence,
d) the Aizerman-Aleskerov property,
e) the independence of social preferences for irrelevant alternatives (the independence of losers),
but they do not satisfy the extension axiom (Sen’s property g). It has also been demonstrated that MPD satisfies neither of these axioms, and MWS satisfies P-monotonicity only.
3) It has been found that MPES and MRES both satisfy Sanver monotonicity (a.k.a. cover monotonicity). Thus, despite they are not Maskin monotonic, these social choice correspondences can be implemented in a nonstandard setting, where actors have (extended) preferences for sets of alternatives. It has also been demonstrated that MPD and MWS do not satisfy Sanver monotonicity.