REVIEW 4 major objections 6 minor 2 cited by
Block-Biased Mamba for Long-Range Sequence Processing
T0 review · 4 major / 6 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read Mamba's S6 unit fails on long-range tasks for three structural reasons, and the paper's B2S6 unit fixes all three, beating S4 and S4D on Long-Range Arena while matching Mamba's language-modeling performance.
desk verdict Strong theory, promising architecture, but the headline empirical claim is unproven because the S6 baseline is causal while B2S6 is bidirectional. 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 machinery is the S6 recurrence $x^{(i)}_k = A^{(i)}_k x^{(i)}_{k-1} + B^{(i)}_k u^{(i)}_k$, $y_k = C_k x_k$, with $A^{(i)}_k=\exp(\Delta^{(i)}_k A)$, $\Delta^{(i)}_k=\mathrm{softplus}(w^\top u_k+b^{(i)})$, $B^{(i)}_k=A^{-1}(A^{(i)}_k-I)Bu_k$, and $C_k=u_k^\top C$. Its expressiveness defect is that $B$ and $C$ are shared across channels, making the final output a quadratic function of the input that cannot distinguish certain pairs of sequences. B2S6 modifies this by splitting $u_k$ into $h$ blocks of size $p=d/h$ and giving each block its own $B_{\mathrm{weight}}$, $B_{\mathrm{bias}}$, and $C$, so each block sees only its subvector but the collection of blocks behaves like a wider unit; the bias term $B^{(i)}_{\mathrm{bias}}$ is input-independent and channel-specific, providing additional effective width.
What would settle it
Take the exact Mamba/S6 configuration with bidirectional processing, complex parameters, and the same learning-rate schedule used for B2S6, and run it on all six Long-Range Arena tasks; if a properly tuned bidirectional S6 reaches or passes the 87.71 average that B2S6 reports, the paper's core claim that S6 fails on these tasks is refuted. A second check is to train single-layer S6 and S4D models on a continuous target with growing width: if S6's error falls with width in the same way as S4D's, then the non-universality theorem's practical significance fails.
Extended reading notes
Core claim
The paper's central claim is that a single-layer S6 unit is not a universal approximator of continuous sequence-to-point functions, whereas a single-layer S4D unit is, and that this difference is caused by S6 sharing the matrices $B$ and $C$ across all channels. Beyond expressiveness, the paper proves that S6's input-dependent $\Delta$ imposes an exponentially large relative-gradient bias when any input vector grows in magnitude, and that the gradients of the S6 output with respect to $\Delta$ parameters are $\Omega(c^3)$ times S4D's as input magnitude $c\to\infty$ and grow relative to S4D as sequence length $L$ increases. The proposed B2S6 restores universal approximation either with the block structure alone or with the channel-specific bias alone, softens the bias from exponential to polynomial rates, and, together with a reduced learning rate on the $\Delta$ parameters, trains stably on long sequences. Empirically, B2S6 achieves an average Long-Range Arena accuracy of 87.71, making it, by the paper's account, the first selective SSM to reach state-of-the-art performance on this benchmark, while language-modeling perplexity on SlimPajama-6B stays close to S6's.
Load-bearing premise
The load-bearing premise is that the S6 baseline was trained comparably to B2S6, with similar bidirectional structure, normalization, and schedule, and the stability theorem's $w=0$ assumption means the training-instability diagnosis does not cover the input-dependent selection mechanism itself.
Editorial extensions
If this is right
- Replacing only the recurrent unit in Mamba with B2S6 lifts average Long-Range Arena accuracy from 66.59 to 87.71, so the architecture outside the S6 unit is not the bottleneck.
- Because a single S6 unit is not a universal approximator, any model built only from S6 units needs many layers or unshared channel parameters to fit continuous sequence functions; B2S6 removes that requirement at the unit level.
- The reduced learning rate on the input-dependent sampling-interval parameters should be adopted in other long-sequence settings, since the gradient ratios in Theorem 3 grow with sequence length regardless of task.
- On language modeling, B2S6's perplexity tracks S6 closely on SlimPajama-6B, so the long-range fix does not sacrifice the utility that makes Mamba attractive.
- S6's exponential input-magnitude bias predicts that large-magnitude tokens can overwrite memory; B2S6's block split makes this failure unlikely because independent blocks with opposite selection signs cancel the extreme behavior.
- The paper's comparison would be strengthened by a matched S6 baseline: the S6 row in Table 2 is reported without the configuration given for B2S6 in Appendix F, so a re-run with bidirectional S6, complex parameters, and the same schedule would test whether the reported gain comes from the B2S6 changes or from the training setup.
- Theorem 3 fixes $w=0$, meaning the stability analysis holds only for the length-dependent part of the selective mechanism; the input-dependent part remains unanalyzed, so the reduced-learning-rate recommendation is an engineering remedy whose theoretical justification covers a restricted regime.
- The block structure alone restores universal approximation, which suggests that multihead selective models already inherit part of the expressiveness fix; this isolates B2S6's distinctive contribution to the bias term and the learning-rate treatment.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes why Mamba's S6 unit underperforms on Long-Range Arena (LRA) and proposes B2S6, a modification that partitions the input channels into blocks and adds a channel-specific bias term. The theoretical part claims three limitations of S6: single-layer S6 is not a universal approximator (Theorem 1), S6 imposes an exponentially input-magnitude-dependent sensitivity bias (Theorem 2), and S6 is less stable to train than S4D (Theorem 3). The paper then proves that B2S6 regains universality (Theorem 4) and exhibits a milder polynomial sensitivity (Theorem 5). Empirically, Table 2 reports an average LRA accuracy of 87.71 for B2S6 versus 66.59 for S6 (Mamba), and Appendix G reports that B2S6 roughly matches Mamba's perplexity on a SlimPajama subset.
Significance. If the empirical comparison were properly controlled, this paper would be a useful contribution: it identifies a concrete expressiveness bottleneck in S6, proposes a simple architectural repair, and backs the results with detailed proofs. The strengths are the self-contained Lemma 1 construction for the non-universality result, the explicit proof appendices for Theorems 1-5, the reproducible B2S6 configuration table in Appendix F, and the synthetic experiments that directly illustrate the theoretical statements. However, the headline LRA claim is currently confounded by protocol differences, and Theorem 3 explicitly fixes w=0, so the paper's central explanation of Mamba's LRA failure is not fully established. These issues substantially limit the significance of the present version.
major comments (4)
- [Section 7, Table 2; Appendix A; Appendix F] The central empirical claim that B2S6 "resurrects Mamba from failure" on LRA is not established because the S6 baseline is not compared under a matched protocol. Appendix A states that the authors adopt a bidirectional structure in training the LRA tasks, and Appendix F (Table 4) provides full configurations for B2S6 but no configuration for the S6 row. B2S6 also differs from the published S6 model by using complex-valued parameters, normalization choices, and a reduced learning rate for Delta-related parameters. Since bidirectionality and these training choices are known to affect LRA accuracy substantially, the 21-point gap between S6 (66.59) and B2S6 (87.71) cannot be attributed to the proposed block partition and channel bias alone. The authors should either train a same-protocol S6 baseline, or ablate bidirectionality, complex parameterization, and the reduced learning rate to isolate the effect of the proposed mechanism.
- [Section 5, Theorem 3] Theorem 3 fixes w=0, which makes the sampling interval Delta_k = softplus(b) input-independent. The paper itself acknowledges this at the end of Section 5: "We also fixed w = 0, leaving the case when w≠0 for future work." Consequently, Theorem 3 does not analyze the input-dependent selection mechanism that abstract and Section 1 identify as the source of instability. The statement "this instability comes from the input-dependent selection mechanism" is therefore not supported by the theorem. The theorem should be reframed as a statement about S6's shared B/C structure under a fixed sampling interval, or extended to the selective w≠0 case before the paper can claim to explain the role of selectivity in training instability.
- [Section 7, Table 3] The paper claims in Section 1 and Section 7 that B2S6 achieves "state-of-the-art performance" on LRA, but the extended Table 3 directly contradicts this: HOPE-SSM (87.85) and S4D-FT (88.26) have higher average accuracies, and RTF SSM ties B2S6 at 87.71. The claim should be revised to "competitive with published SSMs" or restricted to selective SSMs, for which the claim may still hold. This is a factual inconsistency in a stated contribution and should be corrected.
- [Section 4, Eq. (6); Theorems 2 and 5] The paper interprets the relative gradient sensitivity S_k as a measure of inductive bias and memory retention, but S_k as defined in Eq. (6) measures the sensitivity of the final output to perturbations of each input, not whether information is actually stored or discarded by the recurrence. The statements that S6 "can cause the model to discard useful long-term information too quickly" and that B2S6 "preserves memory" rely on this identification. The authors should either provide direct evidence connecting S_k to long-range memory behavior (e.g., probing or reconstruction tasks), or restrict the claims to gradient sensitivity rather than memory retention.
minor comments (6)
- [Section 2, last paragraph] The sentence "In the next section, show how this distinction limits the 'effective width' and expressiveness of an S6 unit" is missing a grammatical subject; it should be "we show how".
- [References [87] and [88]] References [87] and [88] cite the same paper by Wang and Xue; one of them should be removed or the two citations should be distinguished by their specific claims.
- [Figure 1 caption] The caption uses the block size p and number of blocks h before these are defined in Section 6; a brief parenthetical definition in the caption would improve readability.
- [Section 6, paragraph after Theorem 5] The sentence "the probabilities that a given input satisfies jw^T juk0 < 0 and jw^T juk0 > 0 are equal" refers to random initialization of w; this should be stated explicitly in the main text rather than implied, since the probability statement is over initialization, not over inputs.
- [Appendix F, Table 4] The hyperparameter h is described as "not carefully fine-tuned but rather picked randomly"; reporting a small sensitivity study over h, or at least stating the range tested, would strengthen the empirical claims.
- [Appendix G] The language-modeling experiment reports perplexity only up to 28,830 steps and does not provide final validation perplexity, model size per variant, or token count; a one-epoch preliminary result is acceptable, but the main text should describe it as preliminary with these limitations.
Circularity Check
No circular derivation: theoretical results are self-contained; the LRA baseline asymmetry is a correctness risk, not a definitional reduction.
full rationale
The paper's derivation chain is not circular. Theorem 1 is proved directly from the model equations: the S6 output is reduced to a quadratic encoder and Lemma 1 exhibits a continuous function that no quadratic function can approximate, while the S4D positive result is built on the external Pinkus and Wang-Xue UATs. Theorem 4 reduces B2S6 universality to the S4D construction, and Theorems 2 and 5 compute explicit Jacobian asymptotics from the paper's own recurrence equations rather than assuming their conclusions. The sensitivity statistic S_k in eq. (6) is a defined measure; the interpretive claim that this measure tracks long-range-task suitability is an empirical assumption corroborated by synthetic experiments, not a definitional identity. Theorem 3 is an analytic comparison under the stated assumption w=0, and while this limits the theorem's reach relative to the paper's narrative about the selection mechanism, it is a correctness risk rather than circularity. The empirical LRA claim is also not circular: the S4/S4D rows are external published results, and B2S6 is a genuinely different trained model. However, Appendix F provides configurations for B2S6 but not for the S6 baseline, and Appendix A states that the authors adopt bidirectional structure for their LRA training, so the 21-point gap over the published causal Mamba row may be partly attributable to protocol differences (bidirectionality, complex parameterization, reduced Delta learning rate). That is a confound in the headline comparison, not a reduction of the prediction to its inputs. The only self-citations are minor and non-load-bearing, e.g., [98] is cited for the elementary decomposition of a diagonal SSM into scalar systems, a fact also supported by external results; the central UAT and non-UAT arguments do not depend on the authors' own prior work.
Assumptions & free parameters
free parameters (2)
- Number of blocks h =
8
- Learning-rate multiplier for Delta-related parameters =
not stated
assumptions (6)
- standard math A two-layer wide network with a Lipschitz, non-polynomial activation is a universal approximator of continuous functions (Pinkus).
- standard math LTI state space models with fixed discretization can approximate arbitrary convolutional kernels (Wang and Xue).
- domain assumption A diagonal state matrix A with negative entries suffices to represent the S4D and S6 dynamics analyzed here.
- ad hoc to paper The relative-gradient sensitivity S_k defined in eq. (6) is a meaningful measure of inductive bias for long-range sequence tasks.
- ad hoc to paper Theorem 3 fixes w=0, making S6's sampling interval constant and input-independent.
- domain assumption The 'S6 (Mamba)' baseline in Table 2 was trained under conditions comparable to B2S6.
Cite this review
Pith. "Pith review of Block-Biased Mamba for Long-Range Sequence Processing." pith.science (2026). https://pith.science/paper/N2C53SHL
@misc{pith2026250509022,
author = {Pith},
title = {Pith review of: Block-Biased Mamba for Long-Range Sequence Processing},
year = {2026},
howpublished = {\url{https://pith.science/paper/N2C53SHL}},
note = {Machine review of arXiv:2505.09022}
}
abstract
Mamba extends earlier state space models (SSMs) by introducing input-dependent dynamics, and has demonstrated strong empirical performance across a range of domains, including language modeling, computer vision, and foundation models. However, a surprising weakness remains: despite being built on architectures designed for long-range dependencies, Mamba performs poorly on long-range sequential tasks. Understanding and addressing this gap is important for improving Mamba's universality and versatility. In this work, we analyze Mamba's limitations through three perspectives: expressiveness, inductive bias, and training stability. Our theoretical results show how Mamba falls short in each of these aspects compared to earlier SSMs such as S4D. To address these issues, we propose $\text{B}_2\text{S}_6$, a simple extension of Mamba's S6 unit that combines block-wise selective dynamics with a channel-specific bias. We prove that these changes equip the model with a better-suited inductive bias and improve its expressiveness and stability. Empirically, $\text{B}_2\text{S}_6$ outperforms S4 and S4D on Long-Range Arena (LRA) tasks while maintaining Mamba's performance on language modeling benchmarks.
Figures
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