Revisiting Diameter in Directed Graphs
Pith reviewed 2026-06-27 19:04 UTC · model grok-4.3
The pith
Approximating reachability diameter in directed graphs requires cubic time under fine-grained assumptions, separating weighted and unweighted cases.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Under fine-grained assumptions there is no O(n^{ω-ε}) algorithm giving any approximation of ReachDiam in weighted directed graphs, and no algorithm achieving better than 2-approximation in unweighted graphs. Additive approximation algorithms exist for unweighted graphs, while constant-factor approximations are possible for DAGs of bounded width and graphs of bounded treewidth; the same techniques yield exact hopsets with hopbound 2 on bounded-treewidth graphs.
What carries the argument
Fine-grained reductions from APSP-like problems to ReachDiam, together with dynamic-programming techniques on tree decompositions for bounded-treewidth instances.
If this is right
- ReachDiam is computationally harder than several other directed-graph distance problems under the same assumptions.
- Weighted and unweighted versions of the problem behave differently, unlike Min-Diameter or classical diameter.
- Approximating ReachDiam is technically related to constructing shortcut sets and hopsets.
- Bounded-treewidth graphs admit exact 2-hop hopsets as a byproduct.
Where Pith is reading between the lines
- Similar hardness may apply to other reachability-based aggregate measures on directed graphs.
- Practical algorithms for ReachDiam will likely need to exploit bounded-width or low-treewidth structure or accept additive error.
- The reduction techniques could be adapted to study diameter variants on other restricted graph families.
Load-bearing premise
The standard fine-grained hypotheses that rule out slightly sub-cubic algorithms for APSP-like problems hold.
What would settle it
An algorithm running in O(n^{ω-ε}) time that returns any constant-factor approximation of ReachDiam on weighted directed graphs, or a (2-ε)-approximation on unweighted directed graphs.
Figures
read the original abstract
The reachability diameter ($\mathrm{ReachDiam}$) of a directed graph is the maximum distance over all pairs $u,v$ where $v$ is reachable from $u$. This notion is present in the definition of shortcut sets, and the name was recently coined in that context by Haeupler, Jiang, and Saranurak [SOSA 2026]. While this is a very natural notion of diameter in directed graphs, and especially DAGs, it is so far not computationally explored. Other definitions of diameter in directed graphs are either trivial (infinite) in graphs that are not strongly connected (e.g., the classical definition) or are non-trivial only in highly restrictive graph classes (e.g., Min-Diameter). We initiate the problem of computing the (approximate) reachability diameter from a fine-grained complexity point of view. Under certain fine-grained assumptions, we prove that there is no algorithm in time $\mathcal{O}(n^{\omega - \varepsilon}$) that gives any approximation of $\mathrm{ReachDiam}$ in weighted graphs. Similarly, there is no algorithm with better than $2$-approximation for unweighted graphs in this time. To supplement this, we provide algorithmic upper bounds that lead to additive approximation of $\mathrm{ReachDiam}$ for unweighted graphs. Hence, we establish a strong separation between the weighted and unweighted cases, which makes this type of diameter different in nature than other known notions. Considering the hardness in general weighted graphs, we also study special graph classes and get small constant approximations for DAGs with bounded width or graphs with bounded treewidth. Interestingly, our techniques also lead to exact hopsets with hopbound $2$ for bounded treewidth graphs. This and some of our upper bounds for general graphs show technical connections between approximating $\mathrm{ReachDiam}$ and computing shortcut sets and hopsets.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces ReachDiam (maximum distance over reachable pairs) as a natural diameter notion for directed graphs and DAGs. From a fine-grained viewpoint, it proves conditional lower bounds showing no O(n^{ω−ε}) algorithm exists for any approximation of ReachDiam in weighted graphs, and no better than 2-approximation in unweighted graphs. Complementary upper bounds yield additive approximations for unweighted graphs in general, constant-factor approximations for bounded-width DAGs and bounded-treewidth graphs, and exact hopsets with hopbound 2 for bounded-treewidth graphs, establishing a weighted/unweighted separation and technical links to shortcut sets and hopsets.
Significance. If the results hold, the work initiates fine-grained study of ReachDiam and demonstrates a clear algorithmic separation between weighted and unweighted cases that distinguishes it from other diameter measures. The positive results on special graph classes and the hopset connections are concrete contributions that could influence research on shortcut sets and sub-cubic algorithms.
major comments (2)
- [Abstract / Introduction] The abstract invokes 'certain fine-grained assumptions' for the lower bounds without naming them (e.g., the precise APSP or 3SUM hypothesis); the introduction or § on hardness should state the exact hypothesis and confirm that the reduction rules out all approximations (weighted) or (2−ε)-approximations (unweighted) while preserving the time bound O(n^{ω−ε}).
- [Upper bounds section] The claimed separation between weighted and unweighted cases rests on the upper bounds for additive approximation in the unweighted case; the relevant section should include a precise statement of the additive error (e.g., +O(1) or +O(log n)) and the running time to allow direct comparison with the 2-approx hardness.
minor comments (2)
- [Abstract] The citation [SOSA 2026] for the coining of ReachDiam should be checked for accuracy (conference year or arXiv reference) and added to the bibliography.
- [Preliminaries] Notation for ReachDiam and related quantities (e.g., weighted vs. unweighted distances) should be introduced consistently in the preliminaries to avoid ambiguity when moving between the hardness and algorithmic sections.
Simulated Author's Rebuttal
We thank the referee for the positive recommendation of minor revision and the helpful comments on clarity. We address each major comment below and will update the manuscript accordingly.
read point-by-point responses
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Referee: [Abstract / Introduction] The abstract invokes 'certain fine-grained assumptions' for the lower bounds without naming them (e.g., the precise APSP or 3SUM hypothesis); the introduction or § on hardness should state the exact hypothesis and confirm that the reduction rules out all approximations (weighted) or (2−ε)-approximations (unweighted) while preserving the time bound O(n^{ω−ε}).
Authors: We agree that naming the hypotheses improves precision. In the revision we will explicitly identify the fine-grained assumptions (e.g., the APSP hypothesis) in the abstract, introduction, and hardness section, and we will add a sentence confirming that the reductions rule out every approximation factor for weighted graphs and every (2−ε)-approximation for unweighted graphs while preserving the O(n^{ω−ε}) time bound. revision: yes
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Referee: [Upper bounds section] The claimed separation between weighted and unweighted cases rests on the upper bounds for additive approximation in the unweighted case; the relevant section should include a precise statement of the additive error (e.g., +O(1) or +O(log n)) and the running time to allow direct comparison with the 2-approx hardness.
Authors: We agree that an explicit statement is needed for direct comparison. We will revise the upper-bounds section to state the precise additive error term and the running time of the algorithm, allowing immediate juxtaposition with the 2-approximation hardness result. revision: yes
Circularity Check
No significant circularity identified
full rationale
The paper establishes conditional lower bounds for approximating ReachDiam under external fine-grained hypotheses (standard APSP-style assumptions ruling out O(n^{ω-ε}) time) and supplies matching algorithmic upper bounds for additive approximations and special graph classes. These hypotheses are invoked as given external inputs rather than derived internally; the name ReachDiam is attributed to an independent prior citation (Haeupler et al., SOSA 2026) with no author overlap. No self-definitional equations, fitted inputs renamed as predictions, load-bearing self-citations, or ansatz smuggling appear. The derivation chain remains self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Certain fine-grained complexity assumptions (e.g., no slightly sub-cubic APSP algorithm) hold.
Forward citations
Cited by 1 Pith paper
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Structural parameterizations of Geodetic Set on directed (acyclic) graphs
Directed Geodetic Set admits a 2^{O(vcn)} kernel on digraphs and a (kΔ)^{O(rdiam)} kernel, with ETH-tight runtimes, and remains W[2]-hard parameterized by k on degree-3 DAGs.
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