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arxiv: 2510.20220 · v3 · submitted 2025-10-22 · 💻 cs.LG · cs.NA· math.NA

Alternatives to the Laplacian for Scalable Spectral Clustering with Group Fairness Constraints

Pith reviewed 2026-05-18 04:24 UTC · model grok-4.3

classification 💻 cs.LG cs.NAmath.NA
keywords spectral clusteringgroup fairnessLagrangian reformulationSherman-Morrison-Woodbury identitybalance constraintsscalable algorithmsnetwork clusteringalgorithmic bias
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The pith

Fair spectral clustering runs twice as fast with new Laplacian choices

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper develops Fair-SMW to make spectral clustering with group fairness constraints more scalable. It reformulates the problem using the Lagrangian method and applies the Sherman-Morrison-Woodbury identity to handle the constraints efficiently. Three alternative Laplacian matrices are introduced, each with distinct spectral gaps, to create variations of the algorithm. Tests on synthetic Stochastic Block Models and real datasets such as LastFM, FacebookNet, Deezer, and German show that the approach delivers clustering balance comparable to prior work but with substantially better runtime, reported as twice as fast and capable of twice the balance. This matters for applying fair clustering to larger networks where previous methods become impractical due to computation cost.

Core claim

Fair-SMW reformulates the constrained optimization problem using a new formulation derived from the Lagrangian method and the Sherman-Morrison-Woodbury identity, resulting in three variants based on alternatives to the Laplacian matrix that achieve clustering solutions with comparable balance to existing algorithms while offering improved runtime performance.

What carries the argument

The Sherman-Morrison-Woodbury identity applied to the Lagrangian reformulation, enabling efficient updates to solve the fairness-constrained eigenvalue problem using smaller matrices.

If this is right

  • Clustering solutions achieve balance comparable to state-of-the-art methods.
  • Computation time improves by a factor of two over existing fair spectral clustering algorithms.
  • The approach is flexible enough to achieve up to twice as much balance in some configurations.
  • Performance is validated on both synthetic Stochastic Block Model data and real-world networks including LastFM, FacebookNet, Deezer, and German.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • This method could be extended to other fairness notions beyond group balance if similar Lagrangian forms can be derived.
  • The choice of which alternative Laplacian to use might provide control over the trade-off between clustering quality and fairness in untested scenarios.
  • Scalability improvements may make fair clustering feasible for very large graphs where current methods time out.
  • Connections to other low-rank update techniques in numerical linear algebra could yield further efficiency gains in related machine learning tasks.

Load-bearing premise

The alternative Laplacian matrices preserve the spectral properties required for accurate clustering and for satisfying the fairness balance constraints when the Sherman-Morrison-Woodbury identity is applied to the Lagrangian reformulation.

What would settle it

Experiments showing that on the tested datasets the runtime of Fair-SMW is not at least twice as fast as the state-of-the-art or that the balance achieved is not comparable or better.

Figures

Figures reproduced from arXiv: 2510.20220 by Iv\'an Ojeda-Ruiz, Leonardo Cambisaca, Malcolm Dickens, Young Ju Lee.

Figure 4.1
Figure 4.1. Figure 4.1: Example of GF = 0 By applying the Sherman-Morrison-Woodbury formula, we obtain (G −1 + µF F T ) −1 = G − µGF(I + µF T GF) −1F T G = G − GF(µ −1 I + F T GF) −1F T G. For µ ≫ 1, we note that the asymptotic behavior is the following: (G −1 + µF F T ) −1 → G − GF(F T GF) −1F (4.10) T G = G(I − F(F T GF) −1F (4.11) T G) We now arrive at the following optimization problem (4.12) max HT H=I HT G(I − F(F T GF) −… view at source ↗
Figure 7.1
Figure 7.1. Figure 7.1: (Left) Balance, (Center) Algorithm Run Time, (Right) Eigensolver Run [PITH_FULL_IMAGE:figures/full_fig_p017_7_1.png] view at source ↗
Figure 7.2
Figure 7.2. Figure 7.2: Comparison of eigenvalue gaps, runtimes, and scalability for SMW and S [PITH_FULL_IMAGE:figures/full_fig_p021_7_2.png] view at source ↗
read the original abstract

Recent research has focused on mitigating algorithmic bias in clustering by incorporating fairness constraints into algorithmic design. Notions such as disparate impact, community cohesion, and cost per population have been implemented to enforce equitable outcomes. Among these, group fairness (balance) ensures that each protected group is proportionally represented within every cluster. However, incorporating balance as a metric of fairness into spectral clustering algorithms has led to computational times that can be improved. This study aims to enhance the efficiency of spectral clustering algorithms by reformulating the constrained optimization problem using a new formulation derived from the Lagrangian method and the Sherman-Morrison-Woodbury (SMW) identity, resulting in the Fair-SMW algorithm. Fair-SMW employs three alternatives to the Laplacian matrix with different spectral gaps to generate multiple variations of Fair-SMW, achieving clustering solutions with comparable balance to existing algorithms while offering improved runtime performance. We present the results of Fair-SMW, evaluated using the Stochastic Block Model (SBM) to measure both runtime efficiency and balance across real-world network datasets, including LastFM, FacebookNet, Deezer, and German. We achieve an improvement in computation time that is twice as fast as the state-of-the-art, and also flexible enough to achieve twice as much balance.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes the Fair-SMW algorithm for spectral clustering under group fairness (balance) constraints. It reformulates the constrained eigenproblem via the Lagrangian and applies the Sherman-Morrison-Woodbury identity to obtain a scalable solver, then introduces three alternative Laplacian matrices with differing spectral gaps to produce algorithm variants. Experiments on the stochastic block model and real networks (LastFM, FacebookNet, Deezer, German) are used to claim that the resulting clusterings achieve balance comparable to prior fair spectral methods while delivering roughly twice the speed and greater flexibility in the balance-runtime trade-off.

Significance. If the central claims hold, the work would offer a practical route to fair clustering on larger graphs by reducing the cost of enforcing linear balance constraints. The idea of swapping in alternative Laplacians to modulate the spectral gap is a potentially useful degree of freedom. However, the significance hinges on whether the reported speed-ups and balance improvements are reproducible and whether the spectral properties required for both clustering quality and constraint satisfaction survive the SMW update; without that, the efficiency gains may not translate to reliable fair partitions.

major comments (2)
  1. [§3.2 and §4.1] §3.2 (alternative Laplacian definitions) and §4.1 (Lagrangian + SMW reformulation): the manuscript asserts that the three proposed Laplacians preserve eigenvectors suitable for partitioning while still allowing the fairness constraints to be enforced after the SMW correction, yet provides neither a perturbation analysis nor a bound on the change in the Rayleigh quotient or null-space. Because the clustering correctness and the reported balance improvements rest on this preservation, the absence of such analysis is load-bearing.
  2. [Experimental section] Experimental section (SBM and real-network results): the abstract and results claim a factor-of-two improvement in runtime and flexibility to achieve twice the balance, but the reported figures lack error bars, explicit baseline implementations, dataset statistics (e.g., number of nodes/edges, group sizes), and ablation isolating the effect of each alternative Laplacian. These omissions prevent verification that the quantitative claims are independent of the chosen parameter regimes or datasets.
minor comments (2)
  1. [§3] Notation for the three alternative Laplacians is introduced without a compact table comparing their definitions, eigenvalues, or spectral gaps; adding such a table would improve readability.
  2. [Related work] The manuscript cites prior fair spectral clustering work but does not discuss how the SMW-based solver differs from existing constrained-eigenvalue techniques that also avoid full matrix inversion.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed comments on our manuscript. We address each of the major concerns below, providing clarifications and indicating the specific revisions we will make to improve the rigor and reproducibility of the work.

read point-by-point responses
  1. Referee: [§3.2 and §4.1] §3.2 (alternative Laplacian definitions) and §4.1 (Lagrangian + SMW reformulation): the manuscript asserts that the three proposed Laplacians preserve eigenvectors suitable for partitioning while still allowing the fairness constraints to be enforced after the SMW correction, yet provides neither a perturbation analysis nor a bound on the change in the Rayleigh quotient or null-space. Because the clustering correctness and the reported balance improvements rest on this preservation, the absence of such analysis is load-bearing.

    Authors: We thank the referee for identifying this gap. The three alternative Laplacians are constructed by adding or scaling specific rank-one or low-rank updates to the standard normalized Laplacian, which by design preserves the kernel (the all-ones vector for connected components) while increasing the algebraic connectivity. The SMW identity is then applied to the Lagrangian-augmented system, so the fairness constraints remain linear and are satisfied exactly at the solution of the corrected eigenproblem. We agree that an explicit perturbation argument would make this preservation clearer. In the revised manuscript we will add a short subsection in §3.2 that derives a first-order bound on the change to the Rayleigh quotient induced by the Laplacian modification and shows that the null-space perturbation remains O(ε) where ε is the size of the added update; we will also note how this bound interacts with the SMW correction to keep the fairness constraints feasible. revision: yes

  2. Referee: [Experimental section] Experimental section (SBM and real-network results): the abstract and results claim a factor-of-two improvement in runtime and flexibility to achieve twice the balance, but the reported figures lack error bars, explicit baseline implementations, dataset statistics (e.g., number of nodes/edges, group sizes), and ablation isolating the effect of each alternative Laplacian. These omissions prevent verification that the quantitative claims are independent of the chosen parameter regimes or datasets.

    Authors: We acknowledge that the current experimental presentation is insufficient for full verification. In the revision we will (i) report all runtime and balance metrics with error bars computed over 10 independent random seeds, (ii) add a table listing the number of nodes, edges, and protected-group sizes for every dataset (SBM instances and the four real networks), (iii) describe the baseline fair spectral clustering implementations in greater detail, including the exact solver used for the original constrained eigenproblem, and (iv) include an ablation table that isolates the contribution of each of the three alternative Laplacians to both runtime and achieved balance. These additions will demonstrate that the reported speed-up and balance flexibility are robust across the tested regimes. revision: yes

Circularity Check

0 steps flagged

No circularity: derivation uses standard Lagrangian + SMW on proposed alternative Laplacians, with empirical validation

full rationale

The paper reformulates the fairness-constrained spectral clustering problem via Lagrangian and the Sherman-Morrison-Woodbury identity to obtain Fair-SMW, then substitutes three alternative Laplacian matrices chosen for their spectral gaps. Clustering quality and balance are evaluated empirically on SBM synthetic graphs and real networks (LastFM, FacebookNet, Deezer, German), reporting runtime and balance metrics. No equation reduces a claimed prediction to a fitted parameter by construction, no load-bearing uniqueness theorem is imported via self-citation, and the central spectral-preservation assumption is presented as a hypothesis tested on held-out data rather than asserted by definition. The reported speed and balance gains are therefore independent empirical outcomes, not tautological restatements of the inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The approach rests on standard linear-algebra identities and domain assumptions about graph Laplacians and fairness constraints; no new entities are introduced.

axioms (2)
  • standard math The Sherman-Morrison-Woodbury identity can be applied to the matrix updates arising from the Lagrangian formulation of the fairness-constrained problem.
    Invoked to obtain the efficient Fair-SMW algorithm.
  • domain assumption Alternative Laplacian matrices with different spectral gaps still support valid spectral clustering while allowing the fairness balance constraints to be satisfied.
    Central modeling choice that enables the three Fair-SMW variants.

pith-pipeline@v0.9.0 · 5765 in / 1261 out tokens · 38880 ms · 2026-05-18T04:24:45.511234+00:00 · methodology

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Reference graph

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