REVIEW 3 major objections 5 minor 34 references
Complexity of learning matchings and half graphs via edge queries
T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Learning a hidden matching with edge queries requires n(n-1)/2 queries deterministically and Theta(n^2) even with randomization.
desk verdict Solid paper with a genuine new randomized lower bound for matchings; the deterministic proof has a repairable gap that doesn't touch the main results. 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 main objects are the matrix families: $M_n$, the $n \times n$ permutation matrices representing matchings; $C_n$, the column permutations of the lower triangular matrix $L_n$; and $H_n$, the row and column permutations of $L_n$. The randomized matching lower bound is carried by a classical adversary relation over pairs of matchings that differ by one column swap, where each query can separate only one pair in the relation, forcing $\Omega(n^2)$ queries. The half-graph results are carried by exact bijections: an edge query to a matrix in $C_n$ corresponds to a threshold comparison in sorting a permutation of $[n]$, and an edge query to a matrix in $H_n$ corresponds to a cross-list comparison in perfectly interleaved bipartite sorting. The upper bounds use binary search per column and a quicksort-style recursion whose row comparisons are implemented by a search subroutine (classical or quantum) on the XOR of two rows.
What would settle it
Run an optimal deterministic algorithm for matchings on the all-zero oracle and record the queried pairs; if at its leaf some above-diagonal entry (i,j) is unqueried while its mirror (j,i) is also unqueried yet the identity matching is the only consistent output, then Lemma 3.2's key assumption is violated and the proof would need repair.
Extended reading notes
Core claim
The paper's central claim is that the edge-query complexity of learning a hidden matching is exactly n(n-1)/2 deterministically and Theta($n^{2}$) for bounded-error randomized algorithms, with the new contribution being the randomized lower bound. It also claims that learning a column-permuted half graph is equivalent to sorting an unknown permutation with threshold comparisons, yielding tight Theta(n log n) deterministic and randomized bounds, and that learning a general half graph is equivalent to perfectly interleaved bipartite sorting, a generalization of nuts-and-bolts sorting, yielding a tight Theta(n log n) randomized bound and a tight Theta(n log n) quantum bound. Along the way it gives a simple quicksort-style randomized algorithm with O(n $log^{2}$ n) edge queries and a quantum variant that reaches O(n log n).
Load-bearing premise
The deterministic matching lower bound assumes that along the all-zero path, any query below the diagonal also comes with a query to the mirrored entry above the diagonal at no extra cost; an arbitrary deterministic algorithm is not obliged to do this, and without that assumption the proof cannot force an untouched 2x2 block.
Editorial extensions
If this is right
- For matchings, the quadratic randomized lower bound rules out any subquadratic randomized edge-query algorithm, so the naive strategy is asymptotically optimal.
- For column-permuted half graphs, the equivalence with threshold sorting means any improvement in either problem transfers immediately to the other.
- For general half graphs, the quicksort-style algorithm provides a simple $O(n \log^2 n)$ randomized solution, and its quantum variant reaches the tight $O(n \log n)$ bound.
- Quantum algorithms help for matchings, where the complexity drops from $\Theta(n^2)$ to $\Theta(n^{1.5})$, but not for half graphs, where both randomized and quantum complexities are $\Theta(n \log n)$.
Reading between the lines
- The equivalence between half-graph learning and perfectly interleaved bipartite sorting suggests that closing the deterministic gap for $H_n$ ($\Omega(n \log n)$ versus $O(n^2)$) depends on the unresolved deterministic complexity of nuts-and-bolts-style sorting.
- The paper's Appendix B certificate, which pins down a half graph with only $O(n)$ zero entries, indicates the matching lower-bound technique of following the all-zero path and exposing an untouched $2 \times 2$ block cannot be transplanted to half graphs; one could check other graph families for similarly small certificates.
- A natural next probe is to replace the one-column-swap relation in the randomized adversary proof with $k$-swap relations to see whether the $\Theta(n^2)$ matching bound is stable under stronger relations, or whether average-case matching learning can bypass it.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the edge-query complexity of reconstructing a hidden bipartite graph from three families: permutation matrices (matchings), column permutations of the lower-triangular matrix (column-permuted half graphs), and row/column permutations of the lower-triangular matrix (general half graphs). For matchings it proves D(Mn)=n(n−1)/2, R(Mn)=Θ(n²), and cites Q(Mn)=Θ(n^{1.5}); for column-permuted half graphs it proves Θ(n log n) deterministic and randomized bounds and an Ω(n)–O(n log n) quantum bound; for general half graphs it gives a quicksort-style O(n log² n) randomized algorithm and an O(n log n) quantum algorithm, with a tight Ω(n log n) quantum lower bound via prior sorting lower bounds. The paper also establishes equivalences between these learning problems and threshold sorting and perfectly interleaved bipartite sorting.
Significance. If correct, the main contribution is the new Ω(n²) randomized lower bound for learning matchings via edge queries, which matches the deterministic upper bound and is obtained by a clean application of Aaronson's adversary method. The equivalence lemmas connecting half-graph learning to sorting problems are useful observations that transfer known sorting results to the query model, and the quicksort-style algorithms are simple and instructive. Parts of the paper are carefully executed, notably the adversary calculation for R(Mn), the upper bounds, and the certificate construction in Appendix B. However, three proof gaps need to be repaired: the deterministic matching lower bound contains an unjustified symmetry assumption, the threshold-sorting equivalence has an off-by-one error, and the quantum adversary lower bound for column-permuted half graphs omits the required sets X and Y.
major comments (3)
- [Section 3.1, Lemma 3.2] The proof assumes that a query to (i,j) with i>j is accompanied by a query to (j,i) 'at no extra cost,' and also that no diagonal entries are queried on the all-0 path because all query outcomes are 0. Neither statement is valid for an arbitrary deterministic decision tree: a below-diagonal query need not have its symmetric counterpart queried, and a diagonal query would also be answered 0 on that path. The lower bound is nevertheless correct and can be repaired by a direct counting argument over unordered pairs: with fewer than n(n−1)/2 off-diagonal queries, some pair {i,j} has neither (i,j) nor (j,i) queried, and swapping columns i and j in any matching consistent with the leaf yields another consistent matching, contradicting uniqueness. As printed, however, the proof of D(Mn)=n(n−1)/2 is incomplete.
- [Section 4.1.1, Lemma 4.1] The claimed equivalence 'X[j] ≥ n − i if and only if MX(i,j)=1' is off by one. Since column j of MX is 0^{n−X[j]}1^{X[j]}, the condition MX(i,j)=1 is equivalent to X[j] > n−i (equivalently X[j] ≥ n−i+1), not X[j] ≥ n−i. Consequently, the simulation of a threshold query 'Is X[j] ≥ i?' should query entry (n−i+1,j), not (n−i,j). The lemma's conclusion is salvageable after this index correction, but the proof as stated contains a false equivalence and an out-of-range query for i=n.
- [Section 4.2, proof of Theorem 1.2 (quantum lower bound)] The application of Ambainis' adversary method (Lemma 4.3) is incomplete: the lemma requires two sets X,Y ⊆ D such that f(x)≠f(y) for every x∈X, y∈Y, but the proof defines only a relation R and verifies m,m',ℓ,ℓ' without specifying X and Y. Taking X=Y=Cn would violate the premise, since f is the identity. The proof can be fixed by taking X to be matrices in Cn whose column-weight permutation has even sign and Y to be those with odd sign, so that every adjacent-weight swap moves from one set to the other and the counts remain m=m'=n−1, ℓ=ℓ'=1. The Ω(n) lower bound is plausible, but the present derivation is not formally valid as written.
minor comments (5)
- [Section 2.2, final paragraph] The statement that all randomized and quantum upper bounds are in the 'weaker' expected-case model is not accurate for the quantum algorithm in Theorem 1.3, which relies on the bounded-error Grover subroutine from Lemma 2.2. The authors should either state that the quantum upper bounds are in the bounded-error model, or explain how the subroutines are made Las Vegas (for example, by verifying and repeating the search) so that the expected cost remains O(n log n).
- [Lemma 4.4] The proof should mention that after the search subroutine from Lemma 2.2 returns a candidate index i, the algorithm queries x_i and y_i to verify that they differ and to determine which of x≤y or x≥y holds; if the verification fails, the search is repeated. This ensures the stated expected cost and the correctness of the output.
- [Theorem 3.4] In the adversary calculation, the statement 'without loss of generality assume M1(i,j)=0' should be justified by noting that the minimum in the definition of CRA ranges over ordered pairs, so for every unordered pair of one-swap-away matrices there is an ordering in which the first matrix has a 0 at (i,j). When the first matrix has a 1 instead, θ is n/2, so the maximum of the two θ values is still at least n(n−1)/2.
- [Lemma 2.1 proof] The proof should be explicit that the constant c is chosen so that |R|^c < (2/3)|R| for all sufficiently large |R|; for small ranges the bound is subsumed by the big-Omega notation.
- [Equation (1) in Section 4.3] The 'naive upper bound' of O(Σ_j sqrt(n/j)) for the comparison sums is correct, but the text should explicitly note that Σ_{j=1}^n 1/sqrt(j) = Θ(sqrt(n)), so the comparison cost per pivot is O(n), to make the transition to the quicksort recurrence fully transparent.
Circularity Check
No significant circularity: the paper's bounds are established by explicit algorithms, adversary/counting lower bounds, and independently sourced external theorems, with no fitted parameters or self-referential reductions.
full rationale
The derivation chain is self-contained in the circularity-relevant sense. Theorem 1.1's deterministic bound is supported by an explicit greedy algorithm (Lemma 3.1) and a lower-bound argument via the all-0 leaf (Lemma 3.2). The randomized Θ(n^2) bound in Theorem 3.4 is proved directly using Aaronson's adversary theorem with a column-swap relation; the θ computation is arithmetic on the permutation-matrix domain, not a fitted parameter. The equivalences in Lemmas 4.1 and 4.2 are explicit bijections with query-by-query simulations, so transferring bounds between sorting and half-graph learning is not definitional circularity. Upper bounds in Theorems 1.2 and 1.3 are concrete algorithms with recurrences; lower bounds use counting (Lemma 2.1), Ambainis's adversary method (Lemma 4.3), and imported comparison lower bounds (HNS02, AGL+21, GJ22/GJ24), none of which are self-citations. No parameter is fitted to a subset of data and then renamed a prediction. The paper's own caveats—e.g., the unverified KMS98 claim in Section 1.1.3, the admitted failure to adapt HNS02 for Q(Cn) in Section 1.1.2, and Appendix B's O(n)-certificate observation—are correctness or provenance caveats, not circular steps. The only genuine defect is in Lemma 3.2's sentence 'Whenever A queries a pair on this path of the form (i,j) with i>j, we assume that it has also queried the pair (j,i) at no extra cost': this is an invalid proof step for a known deterministic bound, but it is a correctness issue rather than a circular reduction, and it does not affect the paper's central new randomized lower bound. Therefore the circularity score is 0.
Assumptions & free parameters
assumptions (7)
- standard math Aaronson's adversary method [Aar04, Theorem 5] gives R(f)=Ω(CRA(f)) for randomized query complexity.
- standard math Ambainis' adversary method [Amb02] quantum lower bound applies to the Cn relation with ℓ=ℓ'=1.
- domain assumption Høyer-Neerbek-Shi [HNS02, Theorem 2] quantum comparison-sorting lower bound Ω(n log n) applies to the same-permutation row/column half-graph subfamily.
- standard math Boyer et al. [BBHT98] quantum search finds a 1 in O(√(n/t)) expected queries when t entries are 1.
- domain assumption van Apeldoorn et al. [AGL+21] proved Q(Mn)=Θ(n^{1.5}) for learning matchings.
- domain assumption Goswami-Jacob [GJ22,GJ24b] give an O(n log n) randomized algorithm for perfectly interleaved bipartite sorting.
- domain assumption The hidden matrix in Hn is a row/column permutation of a lower triangular matrix, so rows are pairwise entrywise comparable and have distinct Hamming weights.
Cite this review
Pith. "Pith review of Complexity of learning matchings and half graphs via edge queries." pith.science (2026). https://pith.science/paper/A4CJQJOY
@misc{pith2026250703151,
author = {Pith},
title = {Pith review of: Complexity of learning matchings and half graphs via edge queries},
year = {2026},
howpublished = {\url{https://pith.science/paper/A4CJQJOY}},
note = {Machine review of arXiv:2507.03151}
}
abstract
The problem of learning or reconstructing an unknown graph from a known family via partial-information queries arises as a mathematical model in various contexts. The most basic type of access to the graph is via \emph{edge queries}, where an algorithm may query the presence/absence of an edge between a pair of vertices of its choosing, at unit cost. While more powerful query models have been extensively studied in the context of graph reconstruction, the basic model of edge queries seems to have not attracted as much attention. In this paper we study the edge query complexity of learning a hidden bipartite graph, or equivalently its bipartite adjacency matrix, in the classical as well as quantum settings. We focus on learning matchings and half graphs, which are graphs whose bipartite adjacency matrices are a row/column permutation of the identity matrix and the lower triangular matrix with all entries on and below the principal diagonal being 1, respectively. \begin{itemize} \item For matchings of size $n$, we show a tight deterministic bound of $n(n-1)/2$ and an asymptotically tight randomized bound of $\Theta(n^2)$. A quantum bound of $\Theta(n^{1.5})$ was shown in a recent work of van Apeldoorn et al.~[ICALP'21]. \item For half graphs whose bipartite adjacency matrix is a column-permutation of the $n \times n$ lower triangular matrix, we give tight $\Theta(n \log n)$ bounds in both deterministic and randomized settings, and an $\Omega(n)$ quantum lower bound. \item For general half graphs, we observe that the problem is equivalent to a natural generalization of the famous nuts-and-bolts problem, leading to a tight $\Theta(n \log n)$ randomized bound. We also present a simple quicksort-style method that instantiates to a $O(n \log^2 n)$ randomized algorithm and a tight $O(n \log n)$ quantum algorithm. \end{itemize}
Figures
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