REVIEW 2 major objections 4 minor 24 references
This paper adapts stochastic gradient descent to solve tensor linear systems when data are missing under uniform, column-block, or frontal-slice models, and proves convergence rates.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-04 10:56 UTC pith:MMJVVPUA
load-bearing objection Useful general-C framework for tensor SGD with missing data, but the constant-step convergence proof rests on a false co-coercivity lemma and the experiments don't match the stated assumptions. the 2 major comments →
Stochastic Gradient Descent for Incomplete Tensor Linear Systems
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central discovery is that the difficulty of missing tensor data can be reduced to choosing a 0/1 tensor C such that the expected value of the masked Gram tensor A~*_{i::} * A~_{i::} decomposes as p C∘(A*_{i::} A_{i::}) plus p² (1-C)∘(...). When this holds for every row, the update direction g(X) = (1/p²)(A~*_{i::}*(A~_{i::} X - p B_{i::})) - ((1-p)/p²) C∘(A~*_{i::} A~_{i::}) X is an unbiased estimator of the gradient ∇F(X) = (1/m) A* (A X - B). The paper shows three concrete models fit this condition: uniform entry missingness with C as the frontal diagonal, column-block missingness with C block-diagonal in every frontal slice, and frontal-slice missingness with C supported only on the z
What carries the argument
The load-bearing object is the correction tensor C, a 0/1 tensor that marks which products in the masked Gram tensor A~*_{i::} * A~_{i::} keep the coefficient p rather than p² when a random row's missing entries are averaged. Equations (3) and (4) are the unbiasedness conditions: E[C∘(A~*_{i::} A~_{i::})] = p C∘(A*_{i::} A_{i::}) and E[(1-C)∘(A~*_{i::} A~_{i::})] = p²(1-C)∘(A*_{i::} A_{i::}). The t-product, defined through block-circulant matrices, lets the whole argument run with tensor notions of transpose, norm, and gradient in close analogy to the matrix case.
Load-bearing premise
Everything for the varying-step result rests on a 0/1 correction tensor C satisfying (3)-(4) for every row; the constant-step result additionally rests on the inequality ∥g(X)-g(Y)∥² ≤ L⟨X-Y,g(X)-g(Y)⟩, which is guaranteed only when g is the gradient of a convex function, and the paper's signed-quadratic construction of the potential f does not ensure convexity.
What would settle it
With n=1, A=[1,-1] as the single data row, p=0.5, B=0, and the uniform-model C (the identity), the potential f for the update g is ½ Xᵀ H X with H = (1/p²)(Aᵀ A - (1-p)I), whose eigenvalues are -2 and 6. For X=0 and Y=(1,1), the co-coercivity inequality reads 8 ≤ -24 and fails. This directly checks the constant-step proof's key inequality for a valid instance of the method; if the theorem is to survive, it needs a route that does not rely on convexity of f.
If this is right
- For any of the three missingness models, running Algorithm 1 with step size C/√t drives expected objective error to zero at the same O((2+log t)/√t) rate as the uniform case.
- A fixed step size α < 1/L_g contracts the expected squared iterate error by factor r = 1 - 2αµ(1-αL_g) each iteration, down to a convergence horizon αG*/(µ(1-αL_g)); a practical schedule is fast fixed-step descent followed by decaying steps.
- The frontal-slice missing model has no matrix analogue, so the tensor setting genuinely needs the new analysis rather than a direct translation of matrix results.
- The framework is not tied to one noise model: if another missingness pattern admits a 0/1 correction tensor satisfying (3) and (4), the same convergence theorems apply.
Where Pith is reading between the lines
- The correction-tensor template suggests a recipe for other missingness structures: partition entries into groups that vanish together, and set C to mark pairs in the same group; the same proof should carry through as long as one 0/1 C works simultaneously for every row.
- The frontal-slice model highlights that t-product Gram matrices mix all frontal slices; missingness in one slice can contaminate the estimated Gram entries of another slice, so correction must be designed at the level of the block-circulant structure, not entrywise.
- A testable practical prediction from the bounds is that the convergence-horizon radius scales roughly as 1/(p³ m) in the row norms for small p, so heavier missingness should visibly slow the fixed-step phase; the paper's synthetic plots appear consistent with this, and a controlled experiment could verify it.
- The fixed-step theorem's proof route requires the update map to satisfy a co-coercivity inequality that is guaranteed only for gradients of convex functions; if non-convex potentials arise, either an added condition or an alternative argument is needed for that theorem.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes mSGDT, an SGD-based iterative method for solving tensor linear systems A*X=B under the t-product when the coefficient tensor A is only partially observed. The method uses a modified update direction g(X) with a correction term involving a 0/1 tensor C, designed so that E[g(X)] = ∇F(X) whenever C satisfies conditions (3) and (4). The paper states two convergence results: Theorem 1 gives an O(log t / sqrt(t)) bound under a decaying step size, and Theorem 2 gives a linear contraction rate to a convergence horizon under a constant step size. It constructs explicit C tensors for three missing-data models—uniform, column-block, and frontal-slice—and reports synthetic and video experiments. The framework generalizes earlier work by the same authors from the matrix case and from the uniform-missing tensor case.
Significance. The paper's general C-condition framework is a genuine contribution: it isolates the algebraic condition needed for an unbiased stochastic gradient and applies it to missing-data models beyond the uniform one, including a tensor-specific frontal-slice model. Proposition 1 and the verification of conditions (3) and (4) for the three models are correct. Theorem 1, conditional on Assumptions 1 and 2, follows from a standard SGD lemma, and the explicit G bound in the appendix is a useful concrete estimate. The writing is clear, the proofs are detailed, and the authors provide reproducible code. However, the stronger advertised result, Theorem 2, is not established as written because its proof uses a co-coercivity inequality that requires convexity of a function that Lemma 8 constructs with indefinite Hessian. There is also an unresolved issue with the without-replacement row sampling permitted by Assumption 1. These are load-bearing for the constant-step linear-rate claim, which motivates the two-stage step-size schedule used in the experiments.
major comments (2)
- [Section 3, Lemma 2 and proof of Theorem 2, step (c)] Lemma 2 is co-coercivity, which is valid only when f is convex. Lemma 8 constructs f as a signed sum of quadratic terms f(X) = Σ α_j/2 ||u_j^* * X||^2 with α_j ∈ {±1}; such an f is not convex in general, and the construction itself does not guarantee nonnegative α_j. A concrete counterexample occurs already under the uniform missing model: take n = 1, row A_i = [1, -1], p = 0.5, both entries observed, and B = 0. Then g(X) = MX with M = (1/p^2)(A_i^T A_i - (1-p)I) = [[2, -4], [-4, 2]], which has eigenvalues 6 and -2. For v = [1, 1], ||g(v) - g(0)||^2 = 8 while ⟨v, g(v) - g(0)⟩ = -4, so Lemma 2's inequality would require 8 ≤ L(-4), impossible for any L > 0. Therefore the replacement in step (c), which uses Lemma 2 to bound E_{t-1}||g(X_{t-1}) - g(X*)||^2, is unjustified. Since this step produces the contraction factor r = 1 - 2αμ(1 - αL_g), the constant-step linear-rate bound and the conve
- [Assumption 1 and proofs of Theorems 1 and 2] The first alternative in Assumption 1—'every row A_i:: is chosen at most once during Algorithm 1'—does not support the unbiasedness property used in the proofs. If rows are sampled without replacement, then conditionally on the previous iterations the next row is uniform over the remaining rows, not over all m rows. Consequently E_{t-1}[g(X_{t-1})] ≠ ∇F(X_{t-1}) in general, which is exactly the equality used in step (a) of the proof of Theorem 2 and is also needed for the application of Lemma 1 in Theorem 1. The second alternative (mask redrawn each iteration) combined with i.i.d. uniform row sampling would suffice, but the without-replacement branch does not. Since the experiments in Section 4.1 explicitly sample rows without replacement, the gap is directly relevant to the reported validation. The authors should either restrict the analysis to i.i.d. row sampling with replacement, or p
minor comments (4)
- [Throughout] The symbol C is used both for the 0/1 correction tensor and for the step-size constant in Theorem 1 (α_t = C/√t). This is confusing, especially since Theorem 2 uses L_g with a related notation. Rename the step-size constant, e.g., η.
- [Section 1.2] Typo: 'the the first two models' should be 'the first two models'.
- [Proof of Theorem 2, step (b)] There is a missing parenthesis in the displayed bound: 'E_{t-1}[||g(X_{t-1} - g(X*)||^2]' should read 'E_{t-1}[||g(X_{t-1}) - g(X*)||^2]'.
- [Algorithm 1 and Assumption 1] The sampling mechanism should be stated precisely. Algorithm 1 says 'Choose row index i ∈ [m] uniformly at random,' which conventionally means with replacement, but Assumption 1 and the experiments allow without replacement. Clarify which sampling model is used and how the proofs apply to it.
Circularity Check
No significant circularity: conditions (3)-(4) are verified per model and convergence follows from standard SGD lemmas; self-citations are context, not load-bearing.
full rationale
The key reduction is the unbiased-gradient identity (5): given Assumptions 1-2 and a 0/1 tensor C satisfying (3)-(4), E[g(X)] = ∇F(X) by direct expectation computation. This is an ex ante verification, not a fitted prediction: C is not chosen to match the convergence bound, and the bound follows from the standard SGD lemma [20]. Proposition 1 independently constructs C for the uniform, column-block, and frontal-slice missing-data models by direct computation of E_D[(a~_j^* a~_k)_{xy}], yielding (6)-(7) and hence (3)-(4). The new models are not defined in terms of the target theorems, and the uniform-model case is re-proved in Proposition 1. The paper cites the same authors' prior works [1] and [11] as context and proof templates (e.g., 'These lemmas closely follow the proof steps done in [1]' and 'Lemma 7 is a restatement of a lemma appearing in [1]'), but the central convergence claims do not reduce to those citations. The possible failure of Lemma 2's co-coercivity when the auxiliary f in Lemma 8 is nonconvex is a correctness issue, not circularity, since it does not smuggle the desired conclusion into the inputs. Accordingly, the circularity burden is very low.
Axiom & Free-Parameter Ledger
free parameters (1)
- experiment step-size scaling factor =
5000
axioms (7)
- domain assumption Assumption 1: either each row is chosen at most once, or the mask is redrawn each iteration
- domain assumption Assumption 2: every entry of A occurs with marginal probability p (E[˜A]=pE[A])
- domain assumption Existence of a binary (and for Theorem 2, Hermitian) tensor C satisfying (3) and (4) for all rows
- standard math Lemma 1 (Shamir-Zhang): standard SGD convergence for convex F with unbiased, bounded-variance gradients
- ad hoc to paper Lemma 2 co-coercivity inequality applied to the random g
- domain assumption F is µ-strongly convex, with µ=σ_min²/m from the T-SVD of A
- domain assumption W is a bounded closed convex set containing X⋆
read the original abstract
Solving large tensor linear systems poses significant challenges due to the high volume of data stored, and it only becomes more challenging when some of the data is missing. Recently, Ma et al. showed that this problem can be tackled using a stochastic gradient descent-based method, assuming that the missing data follows a uniform missing pattern. We adapt the technique by modifying the update direction, showing that the method is applicable under other missing data models. We prove convergence results and experimentally verify these results on synthetic data.
Reference graph
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