REVIEW 1 major objections 5 minor 49 references
Detecting weighted hidden cliques
T0 review · 1 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read A weighted analogue of the hidden clique problem is statistically detectable once the planted set has logarithmically many vertices, with polynomial-time spectral tests when $k=\Omega(\sqrt{n})$.
desk verdict A useful information-theoretic extension of planted clique with a mostly sound KL/chi-square threshold, but the advertised polynomial-time test for all singular pairs rests on a false interval-separation claim and a backwards binomial bound. 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
Three objects carry the argument. The likelihood ratio is $L(X)=\frac{1}{\binom{n}{k}}\sum_{|S|=k}\prod_{e\in E(S)}\frac{q(X_e)}{p(X_e)}$; its second moment under the null reduces to a sum over subset pairs in which the overlap size $i$ contributes $\rho^{\binom{i}{2}}$, and this is what yields the logarithmic threshold in the lower-bound direction. The scan statistic maximizing $\prod_{e\in E(S)} q(X_e)/p(X_e)$ over all $k$-subsets and comparing with $n^k$ yields the detectability bound in the upper direction. The spectral test transforms each edge weight $x$ into the indicator of the set $A=\{x:p(x)>q(x)\}$; because $\mathbb{E}_0[\phi(X)]-\mathbb{E}_1[\phi(X)]=d_{TV}(P,Q)$, the centred maximum eigenvalue separates the two hypotheses at the $\sqrt{n}$ scale, and the same transform works with only a set where the densities are ordered instead of the full densities.
What would settle it
Take $P$ uniform on $[0,1]$ and $Q$ a countable mixture of point masses on the rationals: $Q$ is not absolutely continuous with respect to $P$, yet every interval has positive $P$-mass, so the interval-scan test in Theorem 3.7 loses its key guarantee that outside edges almost surely avoid the chosen interval; checking whether any polynomial-time test still achieves vanishing risk for $k_n\to\infty$ on this pair would settle whether the claimed conclusion holds beyond the proof.
Extended reading notes
Core claim
The paper claims that the weighted hidden clique problem has a logarithmically growing information-theoretic threshold for essentially every distinct pair of distributions. The optimal likelihood-ratio test has risk controlled by the second moment $\mathbb{E}_0[L(X)^2]$, which evaluates to an average of $\rho^{\binom{|S\cap T|}{2}}$ over pairs of $k$-subsets, where $\rho=\chi^2(Q\Vert P)+1$; this produces an indistinguishability bound near $k_n \approx 2\log_\rho n$ and, through a scan test, a detection guarantee near $k_n \ge (2+\epsilon)\log n / D_{\mathrm{KL}}(Q\Vert P)$. When $Q$ has mass on a set that $P$ assigns probability zero, a simple test looking for any edge weight in that set makes the risk at most $(1-Q(A))^{\binom{k}{2}}$, so $k_n\to\infty$ suffices. The paper also establishes that polynomial-time spectral tests succeed once $k_n=\Omega(\sqrt{n})$, both with full knowledge of the densities and, when the means differ and the supports are bounded, with knowledge of only the means.
Load-bearing premise
The proof of the unknown-distribution result assumes that whenever $Q$ is not absolutely continuous with respect to $P$, some interval has zero $P$-mass and positive $Q$-mass; singular pairs built from countable point masses can violate this, so the stated claim is not established for those pairs.
Editorial extensions
If this is right
- For any distinct $P$ and $Q$, $k_n\ge c\log n$ makes the optimal risk vanish, so the weighted problem inherits the classical logarithmic information-theoretic threshold.
- If $Q$ is not absolutely continuous with respect to $P$, detection requires only $k_n\to\infty$; with unknown distributions a polynomial-time interval scan achieves this whenever the singular set can be witnessed by an interval.
- The spectral test based on the indicator transform decides with risk at most $\delta$ whenever $k_n > 4\sqrt{(\log 9)n + \log(4/\delta)}/d_{TV}(P,Q)$, using only the ordering of the densities.
- When the densities are unknown but the means are known and differ, the same spectral test works at $k_n = \Omega(\sqrt{n}/|\mu_Q-\mu_P|)$ for bounded supports.
- The classical hidden clique is the special case $P=\mathrm{Bernoulli}(p)$, $Q=\delta_1$; substituting gives the familiar $k_n\sim 2\log_{1/p} n$ detection threshold.
Reading between the lines
- Editorial inference: the interval-positivity condition in Theorem 3.7 is not implied by non-absolute continuity; $P$ uniform on $[0,1]$ and $Q$ a countable sum of point masses on the rationals form a singular pair with no $P$-null interval of positive $Q$-mass, so the conclusion is not proved for such pairs even if it is true.
- Editorial inference: the gap between the KL-based detectability threshold and the $\chi^2$-based indistinguishability threshold suggests that a sharp phase transition, if it exists, is governed by an intermediate divergence; computing the risk of the likelihood-ratio test near $k_n \sim \log n$ for pairs like $N(0,1)$ versus $N(1,1)$ would locate the crossover.
- Editorial inference: the spectral transform is exactly the test built from total-variation separation, so the required $k_n=\Omega(\sqrt{n})$ is plausibly the price of discarding the likelihood structure; simulating the spectral gap for $k_n$ between $\log n$ and $\sqrt{n}$ would quantify the computational gap for weighted graphs.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper studies a weighted generalization of the hidden clique problem. Under the null hypothesis all edge weights of the complete graph are i.i.d. from P; under the alternative, a random set of k vertices has all internal edge weights drawn from Q. The paper gives statistical limits and tests in two scenarios. When P and Q are known, Theorem 3.1 shows that if Q is not absolutely continuous with respect to P then the optimal risk tends to 0 whenever k tends to infinity; Theorem 3.4 gives divergence-based bounds when Q is absolutely continuous with respect to P; Theorem 3.6 gives a spectral test for k=Omega(sqrt(n)). In the partial-information scenario, Theorem 3.7 claims a polynomial-time interval-scan test for unknown singular Q whenever k tends to infinity, and Theorem 3.8 gives a spectral test using only the means and support when k=Omega(sqrt(n)). The proofs of Theorems 3.1, 3.2, 3.3, 3.4, 3.6, and 3.8 are mostly standard second-moment and concentration arguments. The main gap is Theorem 3.7, whose key geometric assertion about singular measures is false.
Significance. The weighted hidden clique problem is a natural and timely generalization, and the divergence-based thresholds in Theorem 3.4 are clean and potentially useful. The spectral tests are explicit and apply in both information scenarios. If Theorem 3.7's claim could be established, the paper would make a strong contribution by showing that unknown singular alternatives are always polynomially detectable at any k tending to infinity. However, the central advertised result for unknown singular Q is currently unsupported: the proposed interval-scan test provably fails for continuous singular measures such as the Cantor distribution against Lebesgue measure. The remaining results appear sound and are likely publishable after substantial revision.
major comments (1)
- [Section 4.6, Theorem 3.7] The proof begins with the key assertion that if Q is not absolutely continuous with respect to P, then there exists an interval I with P(I)=0 and Q(I)>0. This is false for continuous singular measures: take P=Uniform[0,1] and Q the standard Cantor distribution. Then Q is singular with respect to P, but every interval with Q(I)>0 also has P(I)>0. For such a pair, the event E used to define the test cannot occur under H1: any interval with positive Q-mass has positive P-mass, so outside edges of the planted clique fall in I with positive probability, making the condition sum over e not in E(S) of 1{X_e in I}=0 fail. The Type II error of the proposed test therefore tends to 1 rather than 0 when k tends to infinity. Consequently Theorem 3.7 and the corresponding claim in the abstract are not established; a different test or a different argument is needed for singular continuous Q.
minor comments (5)
- [Section 3.1, Theorem 3.4] The statement should explicitly assume P is not equal to Q, or that rho = chi^2(Q||P)+1 > 1. Otherwise, for P=Q the conditions have D_KL=0 and chi^2=0, and part (a) would assert R(T*)->0 for k at least (2+epsilon) log n / 0, which is vacuous and false because H0 and H1 are identical.
- [Section 4.4, proof of Theorem 3.4(a)] The display after the inequality 'P1(Tscan <= n^k) <= P1(...)' is garbled; it contains stray factors such as '{k choose 2} k^2/2' and should be rewritten as P1( (1/{k choose 2}) * sum_{e in E(S*)} log(q(X_e)/p(X_e)) <= 2 log n / k ), so that the strong-law step is clear.
- [Section 4.7, proof of Theorem 3.8] The cardinality of the epsilon-net on the unit sphere should be 9^n, not 9n as written. With the printed '9n' the union bound in equation (5) does not yield the claimed delta/2; the later substitution t = 4(b-a) sqrt((log 9)n + log(4/delta)) explicitly requires the 9^n bound.
- [Section 4.6, Theorem 3.7] The polynomial-time implementation of the interval scan could be clarified: for a fixed interval I, the condition that all edges with weights in I lie inside some set S of size k depends only on the set V_I of vertices incident to those edges and the number of such edges. The check reduces to whether |V_I| <= k and the number of edges with weights in I is at least k, not to an explicit search over all k-subsets.
- [General] There are minor typographical issues, such as 'scantest' in Section 3.1, 'definea' in the introduction, and 'coloumns' in the proof of Theorem 3.8, which should be corrected in a revision.
Circularity Check
No significant circularity; the central results are derived from first principles and standard divergence and concentration inequalities.
full rationale
The derivation chain is self-contained. Theorem 3.4(a) is proved directly from the likelihood-ratio scan statistic via a union bound and the strong law of large numbers; Theorem 3.4(b) is a moment calculation of E0[L(X)^2] that invokes a standard threshold from Bollobás's random graph text, not the authors' own results. Theorem 3.6 is obtained by applying Theorem 3.8 to the indicator-transformed matrix Z, and Theorem 3.8 is proved from first principles using an epsilon-net argument and Hoeffding's inequality. There are no fitted parameters renamed as predictions, no self-definitional identities, and no load-bearing self-citations: citations to Lugosi and Devroye-Gyorfi-Lugosi are to textbook facts about epsilon nets and risk-divergence identities. The notable weakness in Theorem 3.7—the claim that if Q is not absolutely continuous with respect to P then some interval has P-mass zero and positive Q-mass, which fails for continuous singular distributions such as Cantor versus uniform—is a mathematical correctness gap, not a circularity: the test and its analysis are not equivalent to the theorem's assumptions by construction. Therefore no circular step is present and the circularity score is 0.
Assumptions & free parameters
assumptions (4)
- domain assumption Under the alternative hypothesis, the set of k clique vertices is chosen uniformly at random and independently of the edge weights.
- domain assumption For Theorems 3.4 and 3.6, P and Q have densities with respect to a common dominating measure and Q is absolutely continuous with respect to P.
- standard math Standard probabilistic tools: strong law of large numbers, Hoeffding's inequality, epsilon-net bound for the unit sphere with net size at most 9^n, and the second-moment convergence bound for the planted-clique likelihood ratio from Bollobas.
- standard math Proposition A.3 relating the optimal risk to E0[sqrt(L)], and divergence inequalities such as D_KL <= log(1 + chi-squared).
Cite this review
Pith. "Pith review of Detecting weighted hidden cliques." pith.science (2026). https://pith.science/paper/YIUOCOAC
@misc{pith2026250621543,
author = {Pith},
title = {Pith review of: Detecting weighted hidden cliques},
year = {2026},
howpublished = {\url{https://pith.science/paper/YIUOCOAC}},
note = {Machine review of arXiv:2506.21543}
}
abstract
We study a generalization of the classical hidden clique problem to graphs with real-valued edge weights. Formally, we define a hypothesis testing problem. Under the null hypothesis, edges of a complete graph on $n$ vertices are associated with independent and identically distributed edge weights from a distribution $P$. Under the alternate hypothesis, $k$ vertices are chosen at random and the edge weights between them are drawn from a distribution $Q$, while the remaining are sampled from $P$. The goal is to decide, upon observing the edge weights, which of the two hypotheses they were generated from. We investigate the problem under two different scenarios: (1) when $P$ and $Q$ are completely known, and (2) when there is only partial information of $P$ and $Q$. In the first scenario, we obtain statistical limits on $k$ when the two hypotheses are distinguishable, and when they are not. Additionally, in each of the scenarios, we provide bounds on the minimal risk of the hypothesis testing problem when $Q$ is not absolutely continuous with respect to $P$. We also provide computationally efficient spectral tests that can distinguish the two hypotheses as long as $k=\Omega(\sqrt{n})$ in both the scenarios.
Reference graph
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