REVIEW 3 major objections 6 minor 12 references
Near-optimal Approximate Discrete and Continuous Submodular Function Minimization
T0 review · 3 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Near-linear queries suffice to approximately minimize submodular functions
desk verdict Main theorem genuinely resolves the near-linear oracle complexity question for approximate SFM; the proof has a terse but repairable gap in Lemma 3.4, and the extensions are sketchier but sound. 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 Lovász extension $\hat f$ of $f$—the convex set function obtained by ordering coordinates by size—turns set minimization into convex optimization. The load-bearing mechanism is a gradient-difference data structure (Lemma 3.4): when $y-x$ is $k$-sparse, after $O(k)$ evaluations it can sample, in $\tilde{O}(1)$ further evaluations, a 1-sparse random vector $z$ with $\mathbb{E}[z]=g(y)-g(x)$ and $\mathbb{E}[\|z\|_2^2]=O(1)$. The algorithm chains these sampled differences along a binary decomposition of the iteration counter, so the stochastic subgradient at step $t$ is an estimate of $g(x_0)+\sum_j (g(x_{k_{j+1}})-g(x_{k_j}))$, and the binary structure makes the total preprocessing cost $\tilde{O}(1)$ per iteration. A continuous analogue of the extension, defined for submodular functions on $[k]^n$, plays the same role for the nonconvex minimization results.
What would settle it
Exhaustively search all submodular functions on a small universe (say $n\le 6$) for a pair of points $x,y$ with $y-x$ 1-sparse such that the coordinate-wise difference $g(y)-g(x)$, ordered by a permutation consistent with both points, changes sign more than a constant number of times. One such example would falsify Lemma 3.4 and with it the paper's $\tilde{O}(n/\epsilon^2)$ oracle bound.
Extended reading notes
Core claim
The central claim is Theorem 1: for any submodular function $f:\{0,1\}^n \to [-1,1]$ and any $\epsilon>0$, the algorithm returns a random set $S$ with $\mathbb{E}[f(S)] \le \min_{T\subseteq [n]} f(T)+\epsilon$ while making $\tilde{O}(n/\epsilon^2)$ calls to an evaluation oracle for $f$. The argument constructs a stochastic subgradient oracle for the Lovász extension whose estimates are 1-sparse with expected squared norm $\tilde{O}(1)$, so projected stochastic subgradient descent converges in $\tilde{O}(n/\epsilon^2)$ iterations. The key is avoiding the naive $\Omega(n)$ cost of forming a full subgradient: the algorithm writes each new subgradient as a sum of differences between nearby points and samples each difference from a data structure maintained at amortized cost $\tilde{O}(1)$ per iteration. The same reduction also yields an exact $\tilde{O}(sM^2)$-query algorithm when $f$ is integer valued with range $[-M,M]$ and an $s$-sparse minimizer, and a $\tilde{O}(nL^6/\epsilon^6)$-query algorithm for nonconvex continuous functions with nonpositive mixed partial derivatives and $L^\infty$-Lipschitz constant $L$.
Load-bearing premise
The entire near-linear oracle bound rests on Lemma 3.4's assertion that when two points differ in only $k$ coordinates, the difference of their subgradients splits into $O(k)$ contiguous blocks whose entries all share a sign, so each block's sum can be evaluated in one oracle call; if that sign-partition property fails, the amortized per-iteration sampling cost stops being $\tilde{O}(1)$ and the $\tilde{O}(n/\epsilon^2)$ bound collapses.
Editorial extensions
If this is right
- Oracle complexity for $\epsilon$-approximate discrete submodular function minimization drops from $\tilde{O}(n^{3/2}/\epsilon^2)$ to $\tilde{O}(n/\epsilon^2)$, matching the subgradient-oracle lower bound up to polylogarithmic factors.
- For integer-valued $f$ with range $[-M,M]$ and an $s$-sparse minimizer, exact minimization uses $\tilde{O}(sM^2)$ oracle calls; this becomes the first sublinear-time algorithm when $sM^2=o(n)$.
- For continuous $f$ on $[0,1]^n$ with all mixed partial derivatives $\le 0$ and Lipschitz constant $L$, an $\epsilon$-additive global minimizer can be found in $\tilde{O}(nL^6/\epsilon^6)$ evaluations, replacing a cubic dependence on $n$ with a nearly linear one.
- The extension to submodular functions on $[k]^n$ runs in $\tilde{O}(nk^4/\epsilon^2)$ oracle calls, so the improvement is not tied to the Boolean lattice.
Reading between the lines
- The binary-counter amortization used here suggests a general recipe: any projected subgradient method whose iterates move by sparse steps and whose objective admits cheap interval-sum queries can enjoy the same near-linear amortized oracle cost; coordinate descent is a natural next test case.
- The sparse-minimizer theorem has a consequence the authors leave implicit: when the optimum is small and the integer range is bounded, exact SFM need not inspect all $n$ elements, which points toward streaming or local-search versions of cut and matroid-rank minimization.
- One testable reading of the continuous theorem is as a discretization lemma: functions with pairwise nonpositive mixed partials and Lipschitz constant $L$ can be minimized on a grid of step $\epsilon/(2L)$; applying that to $g(\sum_i c_i x_i)$ with concave $g$ gives a near-linear primitive for a class of nonconvex resource-allocation problems.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims a randomized algorithm for ε-approximate submodular function minimization on {0,1}^n with range [-1,1] using ~O(n/ε^2) evaluation-oracle calls, improving on prior ~O(n^{5/3}/ε^2) and ~O(n^{3/2}/ε^2) algorithms. It further gives an ~O(sM^2)-call algorithm for integer-valued SFM with an s-sparse minimizer, an ~O(nk^4/ε^2)-call algorithm for submodular functions on [k]^n, and an ~O(nL^6/ε^6)-call algorithm for continuous twice-differentiable functions with nonpositive mixed partials. The method runs projected stochastic subgradient descent on the Lovász extension and uses a data structure that samples sparse unbiased estimators of gradient differences; the binary decomposition of the iteration counter is used to amortize preprocessing costs.
Significance. If the main theorem is correct, it resolves the open problem of near-linear oracle complexity for approximate SFM posed by CLSW17 and HRRS19, and it is optimal up to the ε dependence given the subgradient-oracle lower bound. The sparse pseudopolynomial result gives the first sublinear-time SFM algorithm in the sparse regime, and the continuous extension substantially improves Bach's cubic dependence on n. The paper's high-level architecture is sound: Lemma 3.1 and Lemma 3.3 provide the needed norm and interval-sum tools, and the amortization of preprocessing costs over the trajectory is a genuine contribution. The main caveat is that the proof of the key data-structure lemma is incomplete as written, so the central claim currently rests on a gap that must be repaired.
major comments (3)
- [Appendix B, proof of Lemma 3.4] The proof asserts that because Px and Py differ by k deletions and insertions, there are subsets I1,...,I_{2k} that are intervals in both permutations and carry a common sign for g(y)-g(x). This is not demonstrated: Lemma 3.2 supplies a sign only for coordinates outside the support of d=y-x, while the changed coordinates can have arbitrary signs, and a common interval containing changed coordinates need not be uniform in sign. The proposed alternative of taking maximal common subsequences of unchanged coordinates also needs an argument that those blocks are contiguous in both full permutations, which is not automatic. Since the O(k) preprocessing bound and the O(1) second-moment guarantee of the lemma both depend on this partition, the proof must supply an explicit construction of the intervals and a proof of the uniform-sign property.
- [Algorithm 1, lines 3, 7, 8, 12; Appendix B] Algorithm 1 uses g(x) as if it were a single-valued function, but g(x) is defined by a consistent permutation and is not unique at ties; at x0=0 every permutation is consistent. Without a deterministic tie-breaking rule, the same point can receive different subgradients in different calls to Process and Sample, and the telescoping identity used to form g⋆i can fail. The paper should fix a deterministic consistent-permutation rule, for example breaking ties by coordinate index, and prove that all invocations use the same rule.
- [Section 4, proof of Theorem 7 and Theorem 2] Section 4 derives the exact-minimizer guarantee of Theorem 2 from Theorem 7, which only proves E[ˆf(x)] < min_T f(T) + 1. The step 'It is direct to see' omits the amplification argument that repeated runs, combined with Lemma 2.2 to convert each point to a set, give an exact minimizer with high probability. This should be stated because Theorem 2's contribution is exactness in the sparse regime.
minor comments (6)
- [Section 1.2] There is a typo in 'weekl y polynomial' in the first sentence; it should read 'weakly polynomial.'
- [Theorem 2 statement] The notation 'argminS⊆{0,1}nf(S)' should be written as 'argmin_{S⊆[n]} f(S)' for clarity.
- [Algorithm 4 header] The parameter ε is not used in the stated guarantee of SparseSFM, which returns a point with E[ˆf(x)] < min_T f(T) + 1; either rename the parameter or clarify that the guarantee is additive error 1.
- [Figure 1] The caption and diagram for the decomposition of [0,11] are hard to parse; adding explicit segment values such as (0,8), (8,10), (10,11) would improve readability.
- [Appendix B, proof of Lemma 4.4] The phrase 'use the proof of Lemma 3.2 above' should read 'use the proof of Lemma 3.4,' and the word 'coordiantes' is misspelled. More importantly, the reduction to points x′, y′ should explicitly note that g depends only on the consistent permutation, so the data structure for x′, y′ yields the same g(y)-g(x).
- [Section 5.3, proof of Theorem 3] The sentence 'We can also verify that f′ is submodular' would benefit from a one-line verification, since the discrete submodularity of f′ follows from the continuous mixed-partial condition by integrating over grid cells.
Circularity Check
No significant circularity; the main result is a new algorithm built on prior published lemmas, with no fitted parameter or prediction that reduces to an input.
full rationale
The paper's central claim, Theorem 1, is a new oracle-complexity bound for approximate submodular function minimization obtained by projected stochastic subgradient descent on the Lovasz extension. The derivation chain is self-contained modulo standard, independently published ingredients: the stochastic subgradient descent template is attributed to Bubeck (Theorem 5), the Lovasz extension facts are classical (Lovasz, Fujishige), and the structural lemmas on subgradients (Lemmas 3.1, 3.2, 3.3) are cited to CLSW17, a prior STOC paper with independent published proofs. Those lemmas are not restatements of the target result; they are used as tools to build a new data structure and sampling procedure. Lemma 3.4, the key technical step, is proved in Appendix B from these lemmas; although the assertion that g(y)-g(x) partitions into O(k) constant-sign intervals is terse and arguably under-justified, this is a proof-presentation gap that is repairable, not a circular reduction. The continuous-domain result (Theorem 3) reduces to the discrete result through a direct discretization argument, again with no fitted parameters. No step in the paper defines an input in terms of an output, fits a parameter to data and then calls the result a prediction, or imports a uniqueness theorem from the authors' own prior work to force a choice. Self-citations occur (CLSW17, LSW15), but they are normal references to prior published algorithms and lemmas that are not equivalent to the new claims. Overall, the paper's derivation is not circular.
Assumptions & free parameters
assumptions (5)
- standard math The Lovász extension properties: convexity, consistency on {0,1}^n, min preservation, and the subgradient formula (Theorem 4).
- standard math Projected stochastic subgradient descent guarantee (Theorem 5) providing E[f(avg x_i)] ≤ min f + R B sqrt(2/T).
- standard math The subgradient L1 norm bound ||g(x)||_1 ≤ 3M for f with range [-M,M] (Lemma 3.1), and the interval sum formula (Lemma 3.3).
- domain assumption For the sparse algorithm, moving a positive (resp. negative) marginal coordinate to the far left (resp. right) of the permutation preserves its nonzeroness and sign (Section 4, proof of Lemma 4.1).
- domain assumption The discretized function f'(x) = f(x/k) on [k]^n is submodular when f has all mixed partials ≤ 0 (Section 5.3, proof of Theorem 3).
Cite this review
Pith. "Pith review of Near-optimal Approximate Discrete and Continuous Submodular Function Minimization." pith.science (2026). https://pith.science/paper/TX2GXFPI
@misc{pith2026190900171,
author = {Pith},
title = {Pith review of: Near-optimal Approximate Discrete and Continuous Submodular Function Minimization},
year = {2026},
howpublished = {\url{https://pith.science/paper/TX2GXFPI}},
note = {Machine review of arXiv:1909.00171}
}
abstract
In this paper we provide improved running times and oracle complexities for approximately minimizing a submodular function. Our main result is a randomized algorithm, which given any submodular function defined on $n$-elements with range $[-1, 1]$, computes an $\epsilon$-additive approximate minimizer in $\tilde{O}(n/\epsilon^2)$ oracle evaluations with high probability. This improves over the $\tilde{O}(n^{5/3}/\epsilon^2)$ oracle evaluation algorithm of Chakrabarty \etal~(STOC 2017) and the $\tilde{O}(n^{3/2}/\epsilon^2)$ oracle evaluation algorithm of Hamoudi \etal. Further, we leverage a generalization of this result to obtain efficient algorithms for minimizing a broad class of nonconvex functions. For any function $f$ with domain $[0, 1]^n$ that satisfies $\frac{\partial^2f}{\partial x_i \partial x_j} \le 0$ for all $i \neq j$ and is $L$-Lipschitz with respect to the $L^\infty$-norm we give an algorithm that computes an $\epsilon$-additive approximate minimizer with $\tilde{O}(n \cdot \mathrm{poly}(L/\epsilon))$ function evaluation with high probability.
Reference graph
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