REVIEW 2 major objections 6 minor 33 references
On the Complexity of Problems on Graphs Defined on Groups
T0 review · 2 major / 6 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Under ETH, invariant problems are never NP-complete on power graphs, yet Graph Motif defeats quasipolynomial time.
desk verdict The Graph Motif lower bound for cyclic power graphs is sound and new, but the p-group algorithm's set-vs-multiset bug makes Theorem 17 wrong as written. 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 central mechanism for the hardness results is the divisor-order characterization of cyclic power graphs: two elements of $\mathbb{Z}_N$ are adjacent in $\mathrm{Pow}(\mathbb{Z}_N)$ exactly when one's order divides the other's. The 3-SAT-to-Graph-Motif reduction sets $N$ to the product of the first $b$ primes, with $b = 2\log(2n)+1$, maps literals injectively to subsets of primes of size $(b-1)/2$ so that each literal's order is a distinct product $p_1$ times those primes, and places clause vertices in the equivalence class of the divisor element; the inequalities $\binom{2k}{k} \ge 2n$ and $c n^3 \le \phi(p_2\cdots p_{(b+1)/2})$ guarantee the map is one-to-one and the neighbourhoods are large enough. For the non-hardness results, the carrying object is canonization: an isomorphism-invariant problem reduces to its canonical form, producing a language that is quasipolynomially sparse because the number of non-isomorphic groups of order $n$ is quasipolynomial. For the recognition algorithms, the machinery is the reduced directed power graph, the quotient by closed twin classes where each class consists of generators of the same cyclic subgroup, together with a gluing process that assembles colour-isomorphisms of Sylow $p$-subgroups into a colour-isomorphism of a nilpotent group.
What would settle it
Compute, for increasing $n$ and $b = 2\log(2n)+1$, the ratio $\log \phi(p_2\cdots p_{(b+1)/2}) / \log n$: the embedding requires it to stay above 3 (to absorb the $c n^3$ clause-neighbourhood bound) while $\log N / \log n$ remains $O(\log\log n)$; if the ratio ever falls to 3 or below, Lemma 13's construction fails. Alternatively, a quasipolynomial-time algorithm for Graph Motif on power graphs of cyclic groups would directly refute Theorem 12.
Extended reading notes
Core claim
The paper proves, conditional on ETH, that the class of power graphs has a rigid complexity profile. Isomorphism-invariant graph problems over power graphs are never NP-complete, because quasipolynomial graph canonization, together with the quasipolynomial number of non-isomorphic groups of order $n$, turns any such problem into a quasipolynomially sparse language, which a classical sparse-set theorem forbids from being NP-complete under ETH. On the non-invariant side, Weighted Max-Cut is NP-complete for power graphs of cyclic groups by embedding a weighted complete graph into the clique of generators of $\mathrm{Pow}(\mathbb{Z}_{n^2})$. The main new hardness result is for Graph Motif: there is a reduction from 3-SAT that embeds any formula as an induced subgraph of $\mathrm{Pow}(\mathbb{Z}_N)$, where $N$ is the product of the first $O(\log n)$ primes, yielding instances of size $N = 2^{O(\log n \log\log n)}$; consequently no algorithm running in time $2^{O(\log^c N)}$ for constant $c$ can solve Graph Motif on cyclic power graphs unless ETH fails. In the opposite direction, Graph Motif on power graphs of $p$-groups is solvable in polynomial time by a dominating-vertex greedy algorithm, and the recognition of power graphs of abelian groups, of nilpotent groups of bounded polycyclic length, and of nilpotent groups of squarefree exponent is polynomial-time.
Load-bearing premise
The Graph Motif lower bound rests on the number-theoretic claim that a 3-SAT formula on $n$ variables embeds as an induced subgraph of $\mathrm{Pow}(\mathbb{Z}_N)$ with $N$ the product of the first $O(\log n)$ primes: specifically, that at $b = 2\log(2n)+1$ the inequalities $\binom{2k}{k} \ge 2n$ and $c n^3 \le \phi(p_2\cdots p_{(b+1)/2})$ both hold for all large $n$, so the literal map is injective and the clause vertices have room to be placed.
Editorial extensions
If this is right
- Under ETH, no isomorphism-invariant graph problem—Hamiltonian Path, Partition into Cliques, Feedback Vertex Set, Subgraph Isomorphism, graph colouring, and the like—is NP-complete on power graphs, commuting graphs, enhanced power graphs, directed power graphs, or bounded-degree Cayley graphs; the same holds for isomorphism-invariant group problems given by Cayley tables.
- Weighted Max-Cut is NP-complete on power graphs even when the underlying group is cyclic, since the generator clique of $\mathrm{Pow}(\mathbb{Z}_{n^2})$ contains any weighted complete graph on $n$ vertices.
- Graph Motif on power graphs of cyclic groups cannot be solved in time $2^{O(\log^c n)}$ for any constant $c$ under ETH, and the same conclusion follows from the weaker assumption $\mathrm{EXP} \ne \mathrm{NEXP}$.
- Graph Motif on power graphs of cyclic groups is solvable in $2^{n^{O(1/\log\log n)}}$ time via neighbourhood diversity, so under ETH the problem sits strictly between quasipolynomial and subexponential time.
- Graph Motif on power graphs of $p$-groups lies in P, and the recognition problem for power graphs of abelian groups, nilpotent groups of bounded polycyclic length, and nilpotent groups of squarefree exponent is solvable in polynomial time.
Reading between the lines
- The same canonization-plus-sparsity template would apply to any graph class with quasipolynomially many isomorphism types on $n$ vertices; power graphs are one instance, and the argument suggests a general criterion for when invariant problems evade NP-completeness under ETH.
- The $2^{O(\log n \log\log n)}$-size embedding indicates that the number of divisors of $N$, equivalently the neighbourhood diversity of $\mathrm{Pow}(\mathbb{Z}_N)$, is the parameter that really controls Graph Motif on cyclic power graphs: problems parameterized by neighbourhood diversity are FPT, while the ETH bound shows this parameter can be close to its maximum.
- Because the $p$-group algorithm checks the colour set of a dominating vertex's closed neighbourhood, a natural verification is whether that certificate remains sufficient when the motif contains repeated colours, which the formal multiset definition of a motif allows.
- The paper's own $\mathrm{EXP} \ne \mathrm{NEXP}$ variants of the main theorems suggest the lower bound is not an artifact of ETH's particular formulation; a stronger testable version would ask whether the quasipolynomial barrier persists for any reduction whose image size is $n^{O(\log\log n)}$.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the computational complexity of graph problems restricted to graphs defined on groups, with emphasis on power graphs. The main contributions are: (i) a proof that any isomorphism-invariant graph problem over power graphs, and also over commuting graphs, enhanced power graphs, directed power graphs, and bounded-degree Cayley graphs, cannot be NP-complete unless ETH fails, via canonization and sparsity (Theorem 4); an analogous statement for isomorphism-invariant group problems given by Cayley tables (Theorem 9); (ii) NP-completeness of Weighted Max-Cut on power graphs of cyclic groups (Theorem 11); (iii) an ETH-based lower bound showing Graph Motif on power graphs of cyclic groups cannot be solved in quasipolynomial time, proved by a reduction from 3-SAT using an embedding into Pow(Z_N) with N a product of the first O(log n) primes (Theorem 12 and Lemma 13); (iv) a claimed polynomial-time algorithm for Graph Motif on power graphs of p-groups (Theorem 17); and (v) polynomial-time recognition algorithms for power graphs of abelian groups, nilpotent groups of bounded polycyclic length, and nilpotent groups of squarefree exponent (Theorem 22 and Section 5).
Significance. If the main claims hold, the paper gives a fairly complete ETH-based picture for power graphs: invariant problems are not NP-complete, Weighted Max-Cut is NP-complete, Graph Motif requires super-quasipolynomial time even on cyclic power graphs, and recognition for abelian and some nilpotent classes is polynomial. The reduction in Lemma 13 is intricate and the divisibility argument is internally consistent; the use of Mahaney's theorem with Babai canonization is elegant. The paper also deserves credit for explicitly identifying (Appendix A.2) the false-positive problem in applying the earlier power-graph isomorphism algorithm of Das et al. to recognition. However, two load-bearing gaps remain: the p-group Graph Motif algorithm mishandles multiset motifs, and the reduction in Lemma 21 partitions by the prime factors of the wrong integer. Both are local and fixable, but they currently invalidate the statements of Theorem 17 and the proof of Theorem 22.
major comments (2)
- [§4, Theorem 17 (p. 11–12)] The algorithm tests the condition 'M is a subset of {col(v)} ∪ col(N_C(v))' using set inclusion, but M is defined as a multiset of colours (p. 7). This produces false positives when M has repeated colours. For example, in Pow(Z_3 × Z_3) take two elements a,b of order 3 that generate the same subgroup, colour a = R and b = B, and let M = {R,R}. After deleting vertices whose colours are not in M, the remaining graph is the single vertex a; a is dominating in its component and the closed-neighbourhood colour set is {R}, so the set-inclusion test passes and the algorithm answers Yes, although no connected induced subgraph has colour multiset {R,R}. The fix is to test multiset containment, counting multiplicities in the closed neighbourhood of a dominating vertex; with that replacement the dominating-vertex argument goes through. This gap does not affect Theorem 12, but it means the polynomial-time p-group claim is not established as written.
- [§5, Lemma 21 (p. 13–14)] The proof factors the number n of vertices of the input reduced directed graph and then partitions the graph into subgraphs induced by vertices whose out-degree is a power of one of the prime factors of n. The relevant primes, however, are the primes dividing the order of the group, and these need not divide n. A concrete counterexample is the reduced directed power graph of Z_3 × Z_3: it has 5 vertices (the identity class and four classes of order 3), so n = 5, but all out-degrees are 1 or 3; the algorithm would look for a subgraph on vertices with out-degree a power of 5, find none, and reject a valid abelian 3-group. The proof should instead compute the group order (or the set of primes appearing among the vertex colours) and partition by those primes. As written, this flaw undermines the proof of Lemma 21 and hence of Theorem 22, although it is local and fixable.
minor comments (6)
- [§3, proof of Theorem 4] The proof concludes that L is not NP-hard under ETH from the quasipolynomial reducibility of L to the sparse language L'. Since Theorem 5 is stated for NP-completeness under polynomial-time reductions, please state explicitly the sparse-hardship variant for quasipolynomial reductions (or justify that the standard Mahaney argument adapts).
- [§5, p. 13, reduced graph definition] In the bullet list defining the reduced graph, 'For each closed twin-class τ of R(D), we create a vertex in D' should read '... of D, we create a vertex in R(D)'.
- [§5, Lemma 21 proof, p. 14] The proof ends with an incomplete instruction: 'Then we run the Gluing process on inputs D; R(DPow(G); R(DPow(Gi)), Di, fi, for all 1 ≤ i ≤ k; .' Please complete the sentence and state explicitly that Anil returns Yes if and only if the Gluing process succeeds.
- [§4, Corollary 16, p. 11] The displayed running time '2nc/ log logn' is ambiguous; it should read 2^{n^{c/\log\log n}}.
- [§2, p. 4] The Euler totient formula for ϕ(p1^{α1}...pk^{αk}) is missing a product symbol; as printed it reads as a single term.
- [§4, Lemma 13, p. 8] The notation P_{(b-1)/2}({2,3,...,b}) for subsets of indices is introduced abruptly; a sentence explaining that these are index subsets whose corresponding primes form the order of h would improve readability.
Circularity Check
No significant circularity: the main hardness results are explicit reductions, and the recognition algorithms reuse a published prior-work subroutine as an external black box rather than as a self-defined conclusion.
full rationale
The derivation chain for the main lower bounds is self-contained. Theorem 4 is a direct application of Babai canonization plus the standard fact that quasipolynomially sparse languages are not NP-complete under ETH; it does not assume the non-NP-completeness it concludes. Lemma 13 is an explicit many-one reduction from 3-SAT to Graph Motif on Pow(Z_N), with correctness shown by the divisibility characterization (Remark 1) and by explicit inequalities (C(2k,k) >= 2n and the phi-product bound); the target hardness is not fed in as an input. Weighted Max-Cut hardness is an explicit embedding of K_n into the generator clique of Pow(Z_{n^2}). The Section 5 recognition algorithms do import the p-group directed-power-graph isomorphism test from the authors' earlier FSTTCS paper [10]; this is self-citation, but [10] is a published parameter-free algorithm with stated assumptions, and the present paper supplies the independent reduction/gluing and enumeration steps, so the citation does not make the argument circular. The manuscript itself flags in Appendix A.2 that [10] is a promise algorithm that can false-positive, which is a correctness caveat, not a self-referential step. Theorem 9's proof is only sketched as 'similar,' an omission but not a circular reduction. The only in-scope concern I would record separately is Theorem 17: the greedy test uses set inclusion 'If M is a subset of {col(v)} ∪ col(N_C(v))' although M is defined as a multiset, so the proof as written can accept colour-multiset violations; this is a correctness gap, not a circular derivation, and it does not affect Theorem 12 or the other hardness results.
Assumptions & free parameters
assumptions (8)
- domain assumption Exponential Time Hypothesis: 3-SAT cannot be solved in 2^{o(n)} time.
- standard math Babai's quasipolynomial-time graph canonization and graph isomorphism algorithm [5].
- standard math Mahaney-type sparse-set theorem: under ETH no quasipolynomially sparse language is NP-complete.
- standard math Quasipolynomial upper bound 2^{O(log^3 n)} on the number of non-isomorphic groups of order n [27].
- standard math Ganian's FPT algorithm for Graph Motif parameterized by neighbourhood diversity [17].
- standard math Structure facts about power graphs: complete iff cyclic prime-power order [9], automorphism formula for Pow(Z_n) [14], and the order-divisibility characterization for cyclic groups.
- standard math Prior isomorphism algorithm for directed power graphs of p-groups and nilpotent groups by Das, Ghosh and Kumar [10], including the reduced-graph characterization and p-group linear-time isomorphism.
- ad hoc to paper A motif occurrence in a p-group power graph component can be certified by checking whether the motif's colour set is contained in the closed neighbourhood of a dominating vertex.
Cite this review
Pith. "Pith review of On the Complexity of Problems on Graphs Defined on Groups." pith.science (2026). https://pith.science/paper/TMFAV7V7
@misc{pith2026250705860,
author = {Pith},
title = {Pith review of: On the Complexity of Problems on Graphs Defined on Groups},
year = {2026},
howpublished = {\url{https://pith.science/paper/TMFAV7V7}},
note = {Machine review of arXiv:2507.05860}
}
read the original abstract
We study the complexity of graph problems on graphs defined on groups, especially power graphs. We observe that an isomorphism invariant problem, such as Hamiltonian Path, Partition into Cliques, Feedback Vertex Set, Subgraph Isomorphism, cannot be NP-complete for power graphs, commuting graphs, enhanced power graphs, directed power graphs, and bounded-degree Cayley graphs, assuming the Exponential Time Hypothesis (ETH). An analogous result holds for isomorphism invariant group problems: no such problem can be NP-complete unless ETH is false. We show that the Weighted Max-Cut problem is NP-complete in power graphs. We also show that, unless ETH is false, the Graph Motif problem cannot be solved in quasipolynomial time on power graphs, even for power graphs of cyclic groups. We study the recognition problem of power graphs when the adjacency matrix or list is given as input and show that for abelian groups and some classes of nilpotent groups, it is solvable in polynomial time.
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