Coalitional Manipulation of Set-Valued Median Voting

We study coalitional manipulation of interval-valued median correspondences on the single-peaked domain. Under the pessimistic extension, group strategy-proofness rules out any genuinely set-valued choice. Under the optimistic and best--worst extensions, by contrast, group strategy-proofness characterizes target set correspondences. Moreover, coalitions of at most two voters suffice for both the impossibility and characterization results. Thus, coalitional robustness sharply restricts, but need not eliminate, set-valued flexibility.