REVIEW 2 major objections 4 minor 34 references
Finding Possible Winners in Spatial Voting with Incomplete Information
T0 review · 2 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read The paper proves that the possible-winner problem in one-dimensional spatial voting is polynomial-time solvable for every fixed top-k scoring rule, and fixed-parameter tractable in the number of candidates in any dimension.
desk verdict The paper's headline result, polynomial-time possible winners for constant-k truncated rules in 1D, is not established: the reduction to shapes scheduling relies on Lemma 5, and Lemma 5 is false. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing device is the reduction from voter uncertainty to shapes scheduling. A shape is a vector saying how many machines a job occupies in each of k consecutive time slots; in the voting instance, time slot t is candidate c_t, and a voter-job's shape at start time t records the scores the voter could give to candidates c_t, ..., c_{t+k-1} from some feasible position. The generated job set is P-structured: every job has the same processing length, interior times share one global set of shapes, and jobs can be ordered so that deadlines and deadline shape sets are monotone. A dynamic program, built on a known equal-length jobs scheduling recursion, then decides whether the jobs fit on M* machines with all machines busy during the slot of the designated candidate. The monotonicity of a voter's top-k window as her ideal point moves rightward is what makes the instance P-structured.
What would settle it
Compare the paper's dynamic program with exhaustive search over all finitely many position segments for small one-dimensional instances under truncated Borda; any mismatch about whether a designated candidate can win would refute the claimed polynomial-time algorithm.
Extended reading notes
Core claim
The paper establishes that the one-dimensional possible-winner problem can be solved in $O(n^{1+k+3k^2}m^3)$ time for every k-truncated positional scoring rule with constant k. The proof converts each voter into a job in a new 'shapes scheduling' problem, where time slots are candidates and the shape a job occupies records the scores the voter would hand to a sliding window of k candidates; a feasible schedule with all M* machines busy at the target candidate's slot is exactly a preference completion in which that candidate wins. For d >= 2 dimensions, it proves that the same problem is fixed-parameter tractable when the parameter is the number m of candidates, for any positional scoring rule, by typing each voter according to which of finitely many voting vectors it can realize and solving a fixed-variable integer program. The same FPT conclusion holds for approval voting. For weighted voters in one dimension, the paper gives a dichotomy for two-valued rules and proves that Borda is NP-complete even when there are only four candidates.
Load-bearing premise
The reduction behind the main polynomial-time result assumes that, as a voter's ideal point moves rightward on the line, the set of candidates in her top k moves rightward monotonically without skipping; if a different tie-breaking rule or a different distance made the set skip around, the scheduling construction would no longer be valid.
Editorial extensions
If this is right
- Any fixed-top-k truncated rule in one dimension, including k-truncated Borda and common contest scoring vectors, becomes polynomial-time solvable.
- In any number of dimensions, the possible-winner problem for every positional scoring rule is fixed-parameter tractable in the number of candidates, so few-candidate elections remain solvable even in high-dimensional issue spaces.
- Approval voting, NP-complete in every dimension, is also fixed-parameter tractable in the number of candidates.
- For weighted voters on a line, a two-valued rule is tractable exactly when it approves at least half of the candidates in every candidate count; otherwise the problem is NP-complete.
- Weighted Borda is NP-complete with only four candidates on a line, in contrast to the polynomial-time unweighted one-dimensional case.
Reading between the lines
- The FPT result suggests that the practical bottleneck is the number of candidates, not the dimension; one natural test is whether the same voter-typing argument survives when distances are not Euclidean.
- The weighted dichotomy implicates the weights themselves, rather than spatial geometry alone, as the source of NP-hardness; a plausible extension is that other multi-valued scoring rules become hard in the weighted setting once the rule gives positive weight to fewer than half the candidates.
- The shapes-scheduling formulation could be reused directly for the necessary-winner problem, since the same schedule already encodes upper bounds on every candidate's score.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the possible-winner (PW) problem in spatial voting where each voter's ideal point is known only through a box of intervals in d-dimensional Euclidean space and each candidate's position is known exactly. The main claims are: (i) for any fixed k, PW in one dimension under any k-truncated positional scoring rule is polynomial-time solvable via a reduction to a new 'shapes scheduling' problem; (ii) for any dimension d and any scoring rule or approval voting, PW is fixed-parameter tractable in the number m of candidates; and (iii) a complexity classification for the weighted version in one dimension, including NP-completeness for Borda with four candidates and a dichotomy for two-valued rules. The paper introduces the shapes scheduling model, proves a structural lemma (P-structured jobs), and gives a dynamic program based on a technique of Baptiste.
Significance. If the results were correct, the paper would be significant: it would resolve the tractability of the possible-winner problem for one-dimensional spatial voting with interval uncertainty for the broad and practically relevant class of k-truncated scoring rules with constant k, and it would provide the first FPT algorithms parameterized by the number of candidates for multidimensional spatial voting. The reduction-to-scheduling framework is a potentially useful new idea, and the weighted-case classification addresses an open direction. However, the central positive result of Section 3 rests on a structural lemma that is false, so the main theorem is not established by the proofs given.
major comments (2)
- [Section 3.2, Lemma 5] Lemma 5 is false. The proof orders jobs by nondecreasing upper bounds u_j and considers a shape f(E) that lies in F_{d_j}^{(j)} but not in F_{d_j'}^{(j')}. It treats only the case where the later interval lies to the left of the segment E, leading to u_j < e_1 and a contradiction. The opposite case, where the later interval lies to the right of E, is never ruled out. This case actually occurs. Take candidates at 0, 10, 20, 30 and the 2-truncated rule (2,1). Voter v1 with interval [10.1,14.9] has r_1=2, d_1=4 and F_2^{(1)}={(2,1)}. Voter v2 with interval [15.1,19.9] also has r_2=2, d_2=4, but F_2^{(2)}={(1,2)}. These two endpoint sets are incomparable, so no total order can satisfy Definition 3's condition that equal deadlines imply inclusion of the endpoint shape sets. Thus the jobs generated by the reduction are not necessarily P-structured. Since Lemma 6 (the swap argument) and Lemma 7 (the DP) both rely on the existence of this order and on the inclusion property to swap jobs with equal deadlines, the dynamic program in Section 3.3 cannot be applied to the instances produced by the reduction. Consequently, Theorem 1, the paper's main positive result, is not proved.
- [Section 4, integer program for Theorem 4] The integer program given for the FPT algorithm does not constrain the variables x^z_tau when z is not in the type tau. The text says that a voter of type tau may vote exactly according to the vectors in tau, yet variables x^z_tau are introduced for every z in Z and no constraint forces x^z_tau=0 for z not in tau. As written, the ILP can assign voters to voting vectors that are not allowed by their type, producing solutions that do not correspond to any profile completion. The constraint sum_z x^z_tau = n_tau does not repair this. This needs an explicit restriction (for example, defining x^z_tau only for z in tau, or adding x^z_tau=0 for z not in tau) before Theorem 4 is established.
minor comments (4)
- [Section 3.3, Lemma 6] The statement of Lemma 6 refers to a job j'' that is never defined in the proof; the proof uses j and j'. This makes the lemma difficult to parse and should be corrected.
- [Section 3.2 and Definition 3] The notation F_{d_j}^{(j)} is used for the set of shapes at start time d_j-k, but the reader is left to infer this. The paper should define this explicitly, since the subscript d_j does not match the start-time convention used elsewhere.
- [Section 3.3, Lemma 7] The recurrence in Lemma 7 uses an undefined term b(S_j', f(j')). It presumably represents the contribution to the objective when job j' is scheduled in shape f(j'), but it is never defined in the text.
- [Throughout] There are several typographical and grammatical errors, including 'V oting' in the title, 'traceable' for 'tractable' in Section 5, and 'P +n + 2' in the proof of Theorem 6. These should be corrected.
Circularity Check
No significant circularity: the new results are derived from explicit reductions to scheduling and integer programming, not from reusing their own conclusions.
full rationale
The central claims are self-contained or use external algorithmic machinery. For Theorem 1, the chain is explicit: Lemma 1 bounds the reachable candidate window, Lemma 2 constructs shape sets from midpoint segments, Lemma 3 gives a bijection between voter positions and scheduled jobs, Lemma 4 translates possible-winner existence into an all-machines-busy schedule condition, and Lemmas 5-7 plus the DP yield the polynomial bound. The if-and-only-if statements are factual equivalences of the reduction; they do not assume the target result. For Theorem 4 and Theorem 5, the algorithms use type-based ILP formulations and fixed-dimension LP/IP solvers (Lenstra; Reis and Rothvoss; Grigor'ev and Vorobjov), which are external, and the parameter m bounds the number of variables. The weighted hardness results in Theorems 6 and 7 are reductions from PARTITION with explicit score computations. The self-citation to Imber et al. [22] supplies the model and previous baselines, but the new polynomial-time and FPT proofs do not depend on [22] for their load-bearing steps. A possible mathematical gap in Lemma 5's ordering argument would be a correctness concern, not circularity, because it would not make any output equal to an input by construction.
Assumptions & free parameters
free parameters (2)
- k =
constant parameter of the rule
- m (number of candidates) =
parameter of FPT algorithms
assumptions (4)
- domain assumption Euclidean distances induce single-peaked-like monotone candidate windows in one dimension (Lemma 1).
- standard math Integer programming with fixed number of variables can be solved in FPT time.
- standard math Solving systems of polynomial inequalities of bounded degree in fixed dimension takes polynomial time.
- domain assumption The type-counting approach of Kimelfeld, Kolaitis, Tibi [24] is applicable to spatial voting.
invented entities (1)
-
Shapes scheduling problem
Cite this review
Pith. "Pith review of Finding Possible Winners in Spatial Voting with Incomplete Information." pith.science (2026). https://pith.science/paper/ZZNLM4OB
@misc{pith2026250512451,
author = {Pith},
title = {Pith review of: Finding Possible Winners in Spatial Voting with Incomplete Information},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZZNLM4OB}},
note = {Machine review of arXiv:2505.12451}
}
abstract
We consider a spatial voting model where both candidates and voters are positioned in the $d$-dimensional Euclidean space, and each voter ranks candidates based on their proximity to the voter's ideal point. We focus on the scenario where the given information about the locations of the voters' ideal points is incomplete; for each dimension, only an interval of possible values is known. In this context, we investigate the computational complexity of determining the possible winners under positional scoring rules. Our results show that the possible winner problem in one dimension is solvable in polynomial time for all $k$-truncated voting rules with constant $k$. Moreover, for some scoring rules for which the possible winner problem is NP-complete, such as approval voting for any dimension or $k$-approval for $d \geq 2$ dimensions, we give an FPT algorithm parameterized by the number of candidates. Finally, we classify tractable and intractable settings of the weighted possible winner problem in one dimension, and resolve the computational complexity of the weighted case for all two-valued positional scoring rules when $d=1$.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[1]
C. Al ´os-Ferrer and G. D. Grani´c. Political space representations with approval data. Electoral Studies, 39:56–71, Jan. 2015
work page 2015
- [2]
-
[3]
J. J. Bartholdi, C. A. Tovey, and M. A. Trick. The computational difficulty of manipulating an election. Social choice and welfare, 6:227–241, 1989
work page 1989
-
[4]
D. Baumeister, P. Faliszewski, J. Lang, and J. Rothe. Campaigns for lazy voters: truncated ballots. In AAMAS, pages 577–584, 2012
work page 2012
-
[5]
D. Baumeister, M. Roos, and J. Rothe. Computational complexity of two variants of the possible winner problem. In The 10th International Conference on Autonomous Agents and Multiagent Systems-Volume 2, pages 853–860, 2011
work page 2011
-
[6]
D. Baumeister, M. Roos, J. Rothe, L. Schend, and L. Xia. The possible winner problem with uncertain weights. In ECAI 2012, pages 133–138. IOS Press, 2012
work page 2012
-
[7]
D. Baumeister and J. Rothe. Taking the final step to a full dichotomy of the possible winner problem in pure scoring rules. Information Processing Letters, 112(5):186–190, 2012. 21
work page 2012
-
[8]
N. Betzler and B. Dorn. Towards a dichotomy for the possible winner problem in elections based on scoring rules. Journal of Computer and System Sciences, 76(8):812–836, 2010
work page 2010
Show all 34 references
-
[9]
D. Black. On the rationale of group decision-making. Journal of political economy, 56(1):23–34, 1948
1948
-
[10]
Bogomolnaia and J.-F
A. Bogomolnaia and J.-F. Laslier. Euclidean preferences. Journal of Mathematical Economics , 43(2):87–98, 2007
2007
-
[11]
Brandt, M
F. Brandt, M. Brill, E. Hemaspaandra, and L. A. Hemaspaandra. Bypassing combinatorial protections: Polynomial-time algorithms for single-peaked electorates. Journal of Artificial Intelligence Research, 53:439– 496, 2015
2015
-
[12]
Brandt, V
F. Brandt, V . Conitzer, U. Endriss, J. Lang, and A. D. Procaccia. Handbook of computational social choice . Cambridge University Press, 2016
2016
-
[13]
Chakraborty and P
V . Chakraborty and P. G. Kolaitis. Classifying the complexity of the possible winner problem on partial chains. In AAMAS’21: Proceedings of the 20th International Conference on Autonomous Agents and MultiAgent Systems , 2021
2021
-
[14]
Conitzer and T
V . Conitzer and T. Sandholm. Complexity of manipulating elections with few candidates. In AAAI/IAAI, pages 314–319, 2002
2002
-
[15]
Conitzer, T
V . Conitzer, T. Sandholm, and J. Lang. When are elections with few candidates hard to manipulate? Journal of the ACM (JACM), 54(3):14–es, 2007
2007
-
[16]
Cygan, F
M. Cygan, F. V . Fomin, Ł. Kowalik, D. Lokshtanov, D. Marx, M. Pilipczuk, M. Pilipczuk, and S. Saurabh. Parameterized algorithms, volume 5. 2015
2015
-
[17]
Do ˘gan and A
O. Do ˘gan and A. E. Giritligil. Implementing the borda outcome via truncated scoring rules: a computational study. Public Choice, 159:83–98, 2014
2014
-
[18]
R. G. Downey, M. R. Fellows, et al. Fundamentals of parameterized complexity, volume 4. 2013
2013
-
[19]
Elkind, M
E. Elkind, M. Lackner, and D. Peters. Preference restrictions in computational social choice: A survey. arXiv preprint arXiv:2205.09092, 2022
2022 arXiv
-
[20]
Faliszewski, E
P. Faliszewski, E. Hemaspaandra, L. A. Hemaspaandra, and J. Rothe. The shield that never was: Societies with single-peaked preferences are more open to manipulation and control. In Proceedings of the 12th Conference on Theoretical Aspects of Rationality and Knowledge, pages 11...
2009
-
[21]
D. Y . Grigor’ev and N. N. V orobjov Jr. Solving systems of polynomial inequalities in subexponential time. Journal of symbolic computation, 5(1-2):37–64, 1988
1988
-
[22]
Imber, J
A. Imber, J. Israel, M. Brill, H. Shachnai, and B. Kimelfeld. Spatial voting with incomplete voter information. In Proceedings of the AAAI Conference on Artificial Intelligence, volume 38, pages 9790–9797, 2024
2024
-
[23]
B. Kenig. The complexity of the possible winner problem with partitioned preferences. In Proceedings of the 18th International Conference on Autonomous Agents and MultiAgent Systems, pages 2051–2053, 2019
2019
-
[24]
Kimelfeld, P
B. Kimelfeld, P. G. Kolaitis, and M. Tibi. Query evaluation in election databases. In Proceedings of the 38th ACM SIGMOD-SIGACT-SIGAI Symposium on Principles of Database Systems, pages 32–46, 2019
2019
-
[25]
Konczak and J
K. Konczak and J. Lang. V oting procedures with incomplete preferences. In Proc. IJCAI-05 Multidisciplinary Workshop on Advances in Preference Handling, volume 20, 2005
2005
-
[26]
H. W. Lenstra Jr. Integer programming with a fixed number of variables. Mathematics of operations research, 8(4):538–548, 1983
1983
-
[27]
H. Moulin. Generalized condorcet-winners for single peaked and single-plateau preferences. Social Choice and Welfare, 1(2):127–147, 1984. 22
1984
-
[28]
Niedermeier
R. Niedermeier. Invitation to fixed-parameter algorithms. Habilitationschrift, University of T¨ubingen, 19, 2002
2002
-
[29]
M. S. Pini, F. Rossi, K. B. Venable, and T. Walsh. Incompleteness and incomparability in preference aggregation: Complexity results. Artificial Intelligence, 175(7-8):1272–1289, 2011
2011
-
[30]
Reis and T
V . Reis and T. Rothvoss. The subspace flatness conjecture and faster integer programming. In 2023 IEEE 64th Annual Symposium on Foundations of Computer Science (FOCS 2023), pages 974–988, 2023
2023
-
[31]
Stockemer, A
D. Stockemer, A. Blais, F. Kostelka, and C. Chhim. V oting in the eurovision song contest. Politics, 38(4):428– 442, 2018
2018
-
[32]
Terzopoulou and U
Z. Terzopoulou and U. Endriss. The borda class: An axiomatic study of the borda rule on top-truncated prefer- ences. Journal of Mathematical Economics, 92:31–40, 2021
2021
-
[33]
T. Walsh. Uncertainty in preference elicitation and aggregation. In AAAI, volume 7, pages 3–8, 2007
2007
-
[34]
Y . Yang. On the complexity of borda control in single-peaked elections. In Proceedings of the 16th Conference on Autonomous Agents and MultiAgent Systems, pages 1178–1186, 2017. 23
2017
Reviewed August 15, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.