REVIEW 7 minor 41 references
Lower Bounds for Approximating the Vietoris-Rips Filtration
T0 review · 0 major / 7 minor · reviewed 2026-07-08 · glm-5.2
Pith's one-line read VR approximations must blow up without geometry
desk verdict First explicit lower bounds on c-approximation size for VR(−); clean proofs, correct arguments, deserves a serious referee. 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
homotopy interleavings
What would settle it
Construct a finitely presented c-approximation to VR(X) for the specified metric spaces that achieves linear or sub-superlinear size, which would contradict the rank lower bound of Lemma 3.1.
Extended reading notes
Core claim
The central observation is that any finitely presented c-approximation F to a filtration G must have size at least the rank of the structure map H_i(G)_r -> H_i(G)_{c^2 r}. By constructing metric spaces from graphs with high girth and large first Betti number (Turan graphs T(3n,n) for exponential bounds, generalized polygon incidence graphs and LUW graphs CD(k,p) for superlinear bounds), the author shows this rank can be made exponentially or superlinearly large, establishing that linear-size approximations are impossible for arbitrary metric spaces at any fixed approximation factor.
Load-bearing premise
The superlinear lower bounds (Theorems 3.6 and 3.9) rely on Lemma 3.5, which uses a result from Adamaszek (2013) stating that if the 1-skeleton of VR(X)_1 has girth at least 3j+1, then the inclusion VR(X)_1 -> VR(X)_j is a homotopy equivalence. If that girth-to-stability result had hidden conditions, the superlinear bounds would fail. The exponential bound (Theorem 3.2) does not depend on this lemma.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves lower bounds on the size of approximations to the Vietoris-Rips filtration VR(−) for arbitrary finite metric spaces, in the framework of homotopy interleavings. The central tool is Lemma 3.1, which lower-bounds the size of any finitely presented c-approximation by the rank of a structure map H_i(G)_r → H_i(G)_{c²r}. Two main results are established: (1) for c ∈ [1, √2), exponential lower bounds via Turán graphs T(3n, n) (Theorem 3.2), and (2) for any fixed c ≥ 1, superlinear lower bounds via incidence graphs of generalized polygons (Theorem 3.6, for c < √6) and Lazebnik–Ustimenko–Woldar graphs (Theorem 3.9, for all c ≥ 1). Both results extend to the intrinsic Čech filtration (Corollaries 3.10, 3.11) and to any bifiltration containing VR(−) as a 1-parameter slice (Corollaries 3.13, 3.14).
Significance. This paper makes a substantial contribution by providing the first explicit lower bounds on the size of c-approximations to VR(−) for arbitrary metric spaces. The exponential bound (Theorem 3.2) cleanly complements Sheehy's linear-size (1+ε)-approximations for bounded doubling dimension, showing that the geometric assumption is necessary. The superlinear bound (Theorem 3.9) is particularly notable: it shows that no fixed approximation factor yields linear-size approximations for arbitrary metric spaces, closing a natural question in the area. The extensions to the intrinsic Čech filtration and to bifiltrations (function-Rips, degree-Rips, subdivision-Rips) broaden the impact considerably. The proofs are clean and rely on standard, well-established tools (Künneth for joins, Alexander duality, Adamaszek's girth-to-stability result). The reliance on [1, Prop 2.2] for Lemma 3.5 is well-grounded: the girth conditions are verified correctly in all cases, and the exponential bound (Theorem 3.2) is independent of this lemma. The Turán graph construction for the exponential bound is well-motivated by the Beers–Botnan extremal result.
minor comments (7)
- §3.1, proof of Theorem 3.2: The claim that VR(X_n)_r = Cl(G_n) for r ∈ [1,2) should specify that this holds because G_n is the 1-skeleton at scale 1 and the metric is twice the shortest path metric, so no new edges appear until scale 2. This is implicit but making it explicit would aid the reader.
- §3.1, proof of Theorem 3.6: The table listing the three cases uses q^4, q^6, q^12 for dim H_1(G_q), but the text states |X_q| = Θ(q^3), Θ(q^5), Θ(q^11). The exponents for ϵ(c) follow from these, but a brief sentence explaining the computation (e.g., 'since |X_q| = Θ(q^n) and dim H_1 = Θ(q^{2n-2}), we get ϵ = (2n-2)/n - 1 = (n-2)/n') would make the derivation more transparent.
- §3.1, equation (1): The formula d(c) := k(c) − ⌊(k(c)+2)/4⌋ + 1 could use a brief explanation of its origin, or a forward reference to where it appears in [30], as its role is not immediately clear.
- §3.2, proof of Corollary 3.10: The ball computation B(x,2) = {x} ∪ (X_n ∖ P_j) is correct but the subsequent argument that ⋂_{x∈σ} B(x,2) ≠ ∅ iff |σ ∩ P_j| ≤ 1 for some j could be stated more carefully; the current phrasing conflates 'there exists j' with 'for a specific j.'
- §2.4: The definition of c-interleaving via the category I_c is standard but slightly non-standard in that it uses [0,∞) × {0,1} rather than the more common R × {0,1}. A remark noting that this is equivalent for filtrations indexed by [0,∞) would help readers familiar with the [8] formulation.
- Figure 2: The graph G_3 is labeled but the caption could note that this is T(9,3), connecting it to the Turán graph terminology used in Remark 3.3.
- References [25] and [31]: Both are dated 2026, which appears to be a forward-dating issue; the authors should verify these are correct.
Simulated Author's Rebuttal
We thank the referee for a careful reading and for the positive assessment of the paper's contributions. The referee's report recommends minor revision but does not raise any specific major or minor comments requiring changes to the manuscript. We are grateful for the referee's thorough summary of the results and the accurate characterization of the paper's significance. We have reviewed the manuscript in light of the referee's remarks and confirm that the girth conditions, the reliance on [1, Proposition 2.2], and the independence of Theorem 3.2 from Lemma 3.5 are all correctly verified as the referee notes. No revisions to the mathematical content are needed. We will conduct a final proofreading pass to address any typographical issues before the final version.
Circularity Check
No circularity found; derivation is self-contained with appropriate external citations
full rationale
The paper's derivation chain is clean and self-contained. Lemma 3.1 (the central tool) is proved in full via a straightforward diagram chase through the interleaving category I_c; the proof does not depend on any unverified self-citation. The size definition is attributed to [32] (Lesnick-McCabe) but is a standard notion recovering simplex counts for filtrations, and the paper re-derives what it needs. Theorem 3.2 (exponential bound) uses only Lemma 3.1 plus the observation that VR(X_n)_1 = VR(X_n)_{c²} = A_n when c² ∈ [1,2) and distances are even integers—a genuine mathematical fact, not a definitional identity. Lemma 3.5, the other load-bearing ingredient for the superlinear bounds (Theorems 3.6 and 3.9), cites [1, Proposition 2.2] by Adamaszek (Israel J. Math, 2013), who is not an author of this paper; this is an external, peer-reviewed result. The graph-theoretic ingredients (Turán graphs from [5], generalized polygons from [35], LUW graphs from [30]) are all externally sourced. The Čech extensions (Corollaries 3.10–3.11) use standard Alexander duality and the classical VR/Čech interleaving. The bifiltration extension (Lemma 3.12) is a direct restriction argument proved in full. No step reduces to its inputs by construction, and no self-citation is load-bearing in a circular sense.
Assumptions & free parameters
assumptions (7)
- standard math Künneth formula for reduced homology of simplicial joins (Lemma 2.1, cited from [36])
- standard math Alexander duality for simplicial complexes (Lemma 2.2, cited from [7])
- domain assumption Girth ≥ 3j+1 implies VR(X)_1 ↪ VR(X)_j is a homotopy equivalence (Lemma 3.5, cited from [1, Prop 2.2])
- domain assumption Existence and properties of generalized polygons (Proposition 3.4, cited from [35])
- domain assumption Properties of LUW graphs CD(k,p): connected, bipartite, p-regular, girth ≥ k+5, |V| ≤ 2p^{d(c)} (Lemma 3.8, cited from [30, Theorem 3.2])
- standard math Feit–Higman theorem: finite generalized n-gons exist only for n ∈ {2,3,4,6,8} (Theorem 3.7, cited from [26])
- standard math Existence and uniqueness of minimal presentations for persistence modules over products of totally ordered sets (Section 2.2, cited from [33])
Cite this review
Pith. "Pith review of Lower Bounds for Approximating the Vietoris-Rips Filtration." pith.science (2026). https://pith.science/paper/MWULPXBH
@misc{pith2026260706524,
author = {Pith},
title = {Pith review of: Lower Bounds for Approximating the Vietoris-Rips Filtration},
year = {2026},
howpublished = {\url{https://pith.science/paper/MWULPXBH}},
note = {Machine review of arXiv:2607.06524}
}
abstract
The Vietoris-Rips filtration $\mathcal{VR}(-)$ is a standard tool for analyzing the shape of data within topological data analysis. Beginning with seminal work of Sheehy, a substantial amount of research has centered on constructing linear-size sparse approximations to $\mathcal{VR}(-)$ and related filtrations for metric spaces of bounded doubling dimension. We show that this geometric assumption is necessary in a precise sense. Working in the framework of homotopy interleavings, we show that for any fixed $c \in [1, \sqrt{2})$, there exists a family of finite metric spaces for which any finitely presented $c$-approximation to $\mathcal{VR}(-)$ has exponential size. We also show that for any fixed $c \geq 1$, there exists a family of finite metric spaces for which any finitely presented $c$-approximation to $\mathcal{VR}(-)$ has superlinear size, yielding an obstruction to linear-size approximations for any fixed approximation factor. Both results extend to the intrinsic \v{C}ech filtration and to any bifiltration containing $\mathcal{VR}(-)$ as a $1$-parameter slice, including the function-Rips, degree-Rips, and subdivision-Rips bifiltrations.
Figures
Reference graph
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