Strategy-proof interval-social choice correspondences over extended single-peaked domains

We consider a social choice model where voters have single-peaked preferences over a finite and ordered set of alternatives that are aggregated to produce contiguous sets or intervals of a fixed cardinality. This is applicable in situations where the alternatives can be arranged in a line (e.g. plots of land) and a contiguous subset of these is required (e.g. a hospital or a school). We define interval-social choice correspondences (I-SCCs) on profiles of single-peaked preferences which select intervals. We extend single-peaked preferences to intervals using responsiveness. We show that generalized median-interval (GMI) rules are the only strategy-proof, anonymous and interval efficient I-SCCs. These rules are interval versions of generalized median voter rules which consist of the median, min and max rules. We show that responsiveness over intervals is necessary for the strategy-proofness of the GMI rule if preferences over alternatives are single-peaked.