REVIEW 2 major objections 5 minor 28 references
Tracking Paths in Planar Graphs
T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read The paper proves that finding the smallest tracking set in a planar graph is NP-hard, and also gives a polynomial-time 4-approximation algorithm and a linear-time exact algorithm for bounded-clique-width graphs when a decomposition is…
desk verdict Good paper with a repairable proof gap in the 4-approximation lemma; the results stand, but the version needs a fix before citing. 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 object is the entry-exit pair: for a cycle C, an ordered pair (s',t') of vertices on C such that s connects to s', t' connects to t, the two connecting paths are disjoint, and each meets C only at its endpoint. Such a pair describes two alternative s-t paths that differ only inside C, so any tracking set must put a tracker somewhere on C outside {s',t'}. The approximation algorithm is carried by a cycle-characterization lemma from earlier work, together with a planar face-charging argument: adding faces one at a time shows each optimal tracker can account for at most two faces, giving OPT≥(|F|−1)/2; a sequence of safe reductions turns the graph into one where every face is trivially tracked by at most two trackers per face, giving the upper bound. For NP-hardness, the machinery is a gadget reduction from planar 3-satisfiability in which variable gadgets have exactly two minimum tracking sets, one per truth value, and clause gadgets are satisfied exactly when their face is tracked. For bounded clique-width, the machinery is an MSO1 formula expressing that a vertex set is a tracking set, which invokes a general linear-time algorithm for MSO1-expressible optimization problems on such graphs when a clique decomposition is supplied.
What would settle it
Run Algorithm A on all small planar graphs and compare its output size to the exact minimum tracking set found by brute force; a single instance where Algorithm A returns more than four times the optimum would disprove the 4-approximation claim. More narrowly, the stated lemma $\mathrm{ALG} \le 2(|F|-2)$ is already falsified by a triangle with s and t attached as degree-1 leaves, for which |F|=2 and the algorithm outputs one tracker.
Extended reading notes
Core claim
The paper's central claim is that Planar-Tracking, the optimization version of finding a minimum tracking set in an undirected planar graph with a source s and destination t, is NP-hard but admits a 4-approximation algorithm. The algorithm first applies safe reductions that remove vertices not on any s-t path, contract chains of degree-2 vertices, and force trackers on degree-2 vertices inside small cycles; it then outputs all remaining vertices of degree at least three except s and t. The paper proves the output is always a tracking set, proves it has size at most 2(|F|−2) on reduced graphs, and proves every tracking set has size at least (|F|−1)/2, yielding the ratio 4. The NP-hardness is shown by a polynomial reduction from planar 3-satisfiability using variable and clause gadgets whose minimum tracking sets encode truth assignments. Finally, because the property of being a tracking set is expressible in monadic second-order logic, the exact problem is solvable in linear time on graphs of bounded clique-width when a clique decomposition is given.
Load-bearing premise
The upper-bound proof of the 4-approximation assumes that after the initial reduction no vertex has degree 1, but the source s and destination t may themselves have degree 1; because of that, the lemma's stated bound $\mathrm{ALG} \le 2(|F|-2)$ is false as written, though a corrected Euler-argument that accounts for the two endpoints still yields the 4-approximation ratio.
Editorial extensions
If this is right
- Planar-Tracking is NP-complete, so unless P=NP there is no polynomial-time exact algorithm for planar instances; approximation and parameterized algorithms are the meaningful targets.
- The 4-approximation is analytically tight up to constants: there are planar instances where an optimal set has |F|/2 trackers and the algorithm outputs 2(|F|−2), so the ratio cannot be improved just by tightening the two lemmas.
- The reduction shows that tracking-set decisions are at least as hard as planar 3-satisfiability, placing the problem among other planar NP-hard optimization problems.
- For graphs of bounded clique-width, exact tracking-set optimization is fixed-parameter tractable in the clique width: cubic time without a decomposition, linear time when a bounded-width clique decomposition is given.
- The entry-exit-pair characterization means that checking whether a proposed vertex set is a tracking set can be reduced to checking every cycle, which is why the planar face structure can be exploited.
Reading between the lines
- The face-charging lower bound likely extends to graphs on surfaces of bounded genus with a constant that grows with the genus, since Euler's formula still relates vertices, edges, and faces; the paper leaves this as an open direction.
- The degree-1 endpoint oversight in the upper-bound proof is repairable: a corrected Euler count that charges the endpoints separately still preserves the 4-approximation ratio, so the theorem's conclusion is probably safe even though the stated lemma is not.
- Because the MSO1 formulation is the only graph-structure requirement, exact linear-time algorithms should also hold for any graph class of bounded clique-width if one is willing to pay cubic preprocessing, and on small networks brute force can validate the approximation ratio in practice.
- The gadget reduction suggests a natural testable extension: one can attempt to strengthen the inapproximability ratio beyond the factor 4 by composing the planar 3-SAT reduction with gap amplification, which the paper does not do.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the tracking-paths problem on planar graphs, in which one seeks a minimum set of vertices whose intersections with every s-t path are pairwise distinct sequences. The authors claim three main results: (1) a 4-approximation algorithm for Planar-Tracking based on reductions and an Euler-formula counting argument, (2) NP-hardness of Planar-Tracking via a reduction from Planar-3-SAT, and (3) solvability of the general problem in linear time on graphs of bounded clique-width when a clique decomposition is provided, by an MSO1 formulation and Courcelle's theorem. The NP-hardness reduction is detailed and plausible, and the clique-width application is a standard, correct use of Courcelle's theorem. The main issue is in the approximation section: Lemma 14's stated upper bound is false as written because it neglects degree-1 terminals, although the argument can be repaired to yield the same 4-approximation ratio.
Significance. If the approximation proof is repaired, the paper makes a solid contribution: it establishes that Planar-Tracking is NP-hard and admits a constant-factor approximation, closing a natural gap left by earlier work on general graphs. The bounded-clique-width result is a clean application of Courcelle's theorem and is interesting as a theoretical algorithmic result. The NP-hardness construction is detailed and appears sound. The 4-approximation is the weakest link: the key counting lemma is false in the form stated, but the failure is local and the corrected bound ALG <= 2|F|-2 still combines with Lemma 6's lower bound OPT >= (|F|-1)/2 to give the claimed approximation ratio. Thus the central claim is defensible but the manuscript as written contains a load-bearing error that must be fixed.
major comments (2)
- [Section 3.2, Lemma 14] Lemma 14 states ALG <= 2(|F|-2), but the proof explicitly assumes that after Reduction 1 there are no vertices of degree 1. This is false because the source s and destination t may themselves have degree 1. A concrete counterexample is a triangle with pendant edges from s to one triangle vertex and from t to another: after Reduction 1 the graph has |F|=2, and Algorithm A places one tracker (on the remaining degree-2 triangle vertex via Reduction 3, or on a degree-3 vertex after further suppression), so ALG=1, violating the claimed bound 2(|F|-2)=0. The Euler-formula argument omits the contribution of degree-1 vertices from the degree sum. Including them yields ALG <= 2|F|-2 rather than 2(|F|-2). Since Lemma 6 gives OPT >= (|F|-1)/2, the corrected bound still implies the 4-approximation in Theorem 3, but the lemma and its proof must be revised before Theorem 3 is valid as written.
- [Section 3.2, Reduction 2 and Appendix A, Lemma 24] Reduction 2 as stated removes an arbitrary one of two adjacent degree-2 vertices and does not exclude s and t. If a terminal has degree 2 and is adjacent to another degree-2 vertex, the algorithm can delete the source or destination, after which the output of Algorithm A is not a tracking set for the original s-t instance. The proof of Lemma 24 implicitly assumes the removed vertex is not a terminal when it argues that moving a tracker from u to u' preserves the tracking property; this argument fails when u=s or v=t. The reduction rule and its safety proof must explicitly forbid removing s and t (or otherwise handle terminal vertices), and the effect of this restriction on the degree-counting argument in Lemma 14 should be rechecked.
minor comments (5)
- [Abstract and Introduction] The attribution of the origin of the tracking-paths problem is inconsistent: the abstract says it was first introduced in [3], while the introduction cites both [4] and [3]. These references should be reconciled.
- [Section 1, problem definition] The problem is stated as an optimization problem ('What is the smallest tracking set?'), but Corollary 18 claims NP-completeness. The corresponding decision version (does there exist a tracking set of size at most k?) should be stated explicitly.
- [Figure 3(b)] Figure 3(b) is labeled as a tight example for ALG = 2(|F|-2). If Lemma 14 is corrected to ALG <= 2|F|-2, the tight-example claim and caption should be updated to match the corrected bound.
- [Section 3.2, Claim 7] Claim 7 states that a vertex on an entry-exit pair has degree greater than 2, with proof 'Trivial by Definition 1.' This is not true for s and t: a terminal of degree 2 can lie on a cycle and serve as an entry or exit vertex. The claim should be restricted to non-terminal vertices, which is all that the later reduction proofs actually need.
- [Section 5] The introduction says Tracking can be solved in cubic time for bounded clique-width, but Theorem 22 and the surrounding text state polynomial time (via Oum's cubic-time decomposition construction) and linear time only when the decomposition is given. The wording should be made consistent and precise.
Circularity Check
No significant circularity: the derivation chain is anchored to independent external results (Banik et al., Lichtenstein, Courcelle) and introduces no self-referential definitions or fitted predictions.
full rationale
The paper's claims are built on independent external results rather than on its own conclusions. The tracking-set characterization (Lemma 2), the safety of Reduction 1 (Lemma 4), and the tracking-set/feedback-vertex-set and kernel facts are cited from Banik et al. [3], with Reductions 9-11 explicitly extending an independent lemma from that same source. The NP-hardness reduction uses Lichtenstein's NP-completeness of Planar-3-SAT and Knuth-Raghunathan's rectilinear drawing theorem. The bounded-clique-width result applies Courcelle's theorem to an MSO1 formula whose correctness is argued directly (Remarks 20-21) and whose underlying equivalence (Lemma 19) is derived from Lemma 2, not from the theorem being proved. No quantity is fit to data, no parameter is renamed as a prediction, and no uniqueness theorem from the present authors is invoked to force a choice. The paper does not cite any prior work by its own authors in a load-bearing way. The identified flaw in Lemma 14 — that after Reduction 1 the endpoints s and t may have degree 1, so the stated bound ALG ≤ 2(|F|−2) can fail on a triangle with pendant s and t — is a correctness gap in the proof, not a circularity: the lemma's inequality is derived from Euler's formula and degree counting, not from the approximation statement or from an input-dependent fit. The derivation chain is therefore self-contained with respect to the circularity criteria, and no circular step can be exhibited.
Assumptions & free parameters
assumptions (6)
- domain assumption Banik et al. Lemma 2: a vertex set T is a tracking set iff every simple cycle has no untracked entry-exit pair
- standard math Planar-3-SAT is NP-complete, and planar rectilinear drawings with variables on a line exist (Lichtenstein; Knuth and Raghunathan)
- standard math Courcelle's theorem: MSO1-expressible optimization problems are linear-time on bounded clique-width graphs given a clique decomposition
- standard math Euler's formula and the face-edge inequality 2m >= 3f for simple planar graphs
- domain assumption Reduction 1 (removing edges and vertices not on any s-t path) is safe and polynomial-time (Banik et al.)
- standard math Oum's cubic-time algorithm approximates rank-width, giving a bounded-width clique decomposition in polynomial time
Cite this review
Pith. "Pith review of Tracking Paths in Planar Graphs." pith.science (2026). https://pith.science/paper/S4GJUEMD
@misc{pith2026190805445,
author = {Pith},
title = {Pith review of: Tracking Paths in Planar Graphs},
year = {2026},
howpublished = {\url{https://pith.science/paper/S4GJUEMD}},
note = {Machine review of arXiv:1908.05445}
}
abstract
We consider the NP-complete problem of tracking paths in a graph, first introduced by Banik et. al. [3]. Given an undirected graph with a source $s$ and a destination $t$, find the smallest subset of vertices whose intersection with any $s-t$ path results in a unique sequence. In this paper, we show that this problem remains NP-complete when the graph is planar and we give a 4-approximation algorithm in this setting. We also show, via Courcelle's theorem, that it can be solved in linear time for graphs of bounded-clique width, when its clique decomposition is given in advance.
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Reviewed August 14, 2026 · model on record in the stance chip above.
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