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arxiv: 2405.20744 · v5 · pith:RVSZZML6new · submitted 2024-05-31 · 🧮 math.OC

On the sequential convergence of Lloyd's algorithms

Pith reviewed 2026-05-24 00:55 UTC · model grok-4.3

classification 🧮 math.OC
keywords Lloyd's algorithmsequential convergencequantizationoptimal transportanalytic densityKurdyka-Lojasiewicz inequalitysubanalytic integralssemi-discrete optimal transport
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The pith

Lloyd's algorithm iterates converge sequentially to one point under an analytic density assumption.

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

The paper proves that the iterates of two variants of Lloyd's algorithm for quantization converge to a single accumulation point when the target probability measure has an analytic density, possibly restricted to a compact semi-algebraic set. It does so by recasting Lloyd's method as a gradient algorithm on a quantization functional derived from optimal transport, then invoking convergence theory for methods satisfying the Kurdyka-Lojasiewicz inequality, which follows from the log-analytic character of globally subanalytic integrals. A reader would care because Lloyd's algorithm is a standard tool in digital signal processing and data clustering, where guarantees against oscillation among multiple limits improve reliability. The argument also produces definability statements for several other semi-discrete optimal transport losses.

Core claim

Lloyd's algorithm is interpreted as a gradient method on the quantization functional given by optimal transport. Under the assumption that the target probability measure admits an analytic density or an analytic density restricted to a compact semi-algebraic set, the functional is definable in a log-analytic structure. This ensures the Kurdyka-Lojasiewicz property holds, which in turn yields sequential convergence of the iterates to a single accumulation point for both optimal quantization with arbitrary discrete measures and uniform quantization with uniform discrete measures.

What carries the argument

The log-analytic character of globally subanalytic integrals, which transfers definability to the quantization functional and thereby activates Kurdyka-Lojasiewicz convergence for the gradient interpretation of Lloyd's method.

If this is right

  • The iterates reach a single critical point instead of oscillating among several accumulation points.
  • The convergence result covers both the case of arbitrary discrete measures and the uniform discrete-measure case.
  • Definability holds for broader semi-discrete optimal transport losses, including general-cost transport, max-sliced Wasserstein distance, and entropy-regularized transport.
  • The guarantees apply directly to practical settings such as analytic densities truncated to compact semi-algebraic domains.

Where Pith is reading between the lines

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

  • The same definability route might be used to obtain convergence statements for other iterative optimal-transport algorithms beyond Lloyd's method.
  • If analyticity can be weakened while preserving log-analytic integrability, sequential convergence could extend to a larger class of densities.
  • In applications one could verify the analyticity condition on a given density to decide whether the theoretical guarantee applies.

Load-bearing premise

The target probability measure admits an analytic density, possibly restricted to a compact semi-algebraic set.

What would settle it

An explicit analytic density on a compact semi-algebraic set for which the sequence of Lloyd iterates possesses at least two distinct accumulation points would falsify the sequential-convergence claim.

Figures

Figures reproduced from arXiv: 2405.20744 by Edouard Pauwels, Elsa Cazelles, L\'eo Portales.

Figure 1
Figure 1. Figure 1: (Left) Target Gaussian mixture µ with two components truncated on a disk. (Middle) Optimal quantization of µ with 20 points (blue) and their corresponding Voronoi cells (in red) after 250 iterations of Lloyd’s algorithm. (Right) Uniform quantization of µ with 20 points (blue) and the corresponding power cells (in red) after 5 iterations of Lloyd’s algorithm adjusted for uniform quantization. The diameter o… view at source ↗
read the original abstract

Lloyd's algorithm is an iterative method that solves the quantization problem, i.e. the approximation of a target probability measure by a discrete one, and is particularly used in digital applications. This algorithm can be interpreted as a gradient method on a certain quantization functional which is given by optimal transport. We study the sequential convergence (to a single accumulation point) for two variants of Lloyd's method: (i) optimal quantization with an arbitrary discrete measure and (ii) uniform quantization with a uniform discrete measure. For both cases, we prove sequential convergence of the iterates under an analiticity assumption on the density of the target measure. This includes for example analytic densities truncated to a compact semi-algebraic set. The argument leverages the log analytic nature of globally subanalytic integrals, the interpretation of Lloyd's method as a gradient method and the convergence analysis of gradient algorithms under Kurdyka-Lojasiewicz assumptions. As a by-product, we also obtain definability results for more general semi-discrete optimal transport losses such as transport distances with general costs, the max-sliced Wasserstein distance and the entropy regularized optimal transport loss.

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 proves sequential convergence of the iterates for two variants of Lloyd's algorithm—optimal quantization with arbitrary discrete measures and uniform quantization with uniform discrete measures—under the assumption that the target probability measure has an analytic density (or an analytic density restricted to a compact semi-algebraic set). The argument interprets Lloyd iterates as gradient steps on an optimal-transport quantization functional, invokes the Kurdyka-Łojasiewicz inequality via definability of globally subanalytic integrals, and uses the log-analytic character of those integrals; as a byproduct it establishes definability for several semi-discrete OT losses including max-sliced Wasserstein and entropy-regularized transport.

Significance. If the proofs are correct, the result supplies rigorous sequential-convergence guarantees for a classical algorithm under a broad and practically relevant analyticity hypothesis, while the definability corollaries enlarge the set of OT functionals known to lie in o-minimal structures. These contributions are of clear interest to the quantization, clustering, and optimal-transport communities.

major comments (2)
  1. [§3] §3 (gradient interpretation) and the subsequent KL analysis: the manuscript invokes external theorems on gradient methods under KL assumptions, but does not explicitly verify that the quantization functional satisfies the required desingularizing function with exponent θ<1 when the density is merely analytic on a semi-algebraic set; a short self-contained check or reference to the precise exponent obtained from the log-analytic integral would remove any ambiguity.
  2. [Theorem 4.1] Theorem 4.1 (sequential convergence for optimal quantization): the passage from definability of the subanalytic integral to the KL inequality is stated as a direct consequence of the log-analytic character, yet the precise o-minimal structure and the resulting exponent are not recorded; without this datum it is impossible to confirm that the convergence rate implied by the KL property is compatible with the claimed sequential (rather than merely subsequential) convergence.
minor comments (2)
  1. [Abstract] Abstract: 'analiticity' is a typographical error for 'analyticity'.
  2. [Theorem 4.2] The statement of the uniform-quantization result (Theorem 4.2) repeats the same definability argument as the optimal case; a one-sentence remark on any structural differences would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading, positive evaluation, and constructive suggestions. We address the two major comments below and will incorporate clarifications in a revised version.

read point-by-point responses
  1. Referee: [§3] §3 (gradient interpretation) and the subsequent KL analysis: the manuscript invokes external theorems on gradient methods under KL assumptions, but does not explicitly verify that the quantization functional satisfies the required desingularizing function with exponent θ<1 when the density is merely analytic on a semi-algebraic set; a short self-contained check or reference to the precise exponent obtained from the log-analytic integral would remove any ambiguity.

    Authors: We agree that an explicit reference to the exponent would remove ambiguity. The log-analytic character of the globally subanalytic integrals (as established in the paper via the analytic density on a compact semi-algebraic set) yields a desingularizing function with exponent θ = 1/2, which is strictly less than 1 and compatible with the cited gradient-method theorems. In the revision we will add a short paragraph in §3 with this self-contained check and a pointer to the relevant property of log-analytic functions in the o-minimal structure of globally subanalytic sets. revision: yes

  2. Referee: [Theorem 4.1] Theorem 4.1 (sequential convergence for optimal quantization): the passage from definability of the subanalytic integral to the KL inequality is stated as a direct consequence of the log-analytic character, yet the precise o-minimal structure and the resulting exponent are not recorded; without this datum it is impossible to confirm that the convergence rate implied by the KL property is compatible with the claimed sequential (rather than merely subsequential) convergence.

    Authors: The o-minimal structure employed is that of globally subanalytic sets, and the log-analytic character directly supplies the KL inequality with exponent θ = 1/2. This exponent guarantees the sequential convergence claimed in Theorem 4.1 via the standard results on gradient flows under KL assumptions. We will record these two pieces of information explicitly in the statement and proof of Theorem 4.1 in the revised manuscript. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper establishes a conditional theorem: sequential convergence of two Lloyd variants holds when the target measure has an analytic density (or analytic density on a compact semi-algebraic set). The proof chain interprets Lloyd iterates as a gradient method, invokes the Kurdyka-Łojasiewicz inequality via definability of globally subanalytic integrals, and applies known convergence results for gradient algorithms under KL assumptions. These are external, independently established facts from real algebraic geometry and optimization theory; the derivation does not reduce any claimed prediction or uniqueness result to a quantity defined from the paper's own fitted parameters, self-citations, or ansatzes. The by-product definability statements for other OT losses are likewise derived from the same external machinery rather than circularly from the convergence claim itself.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on two external mathematical properties: the Kurdyka-Lojasiewicz inequality for the quantization functional and the log-analytic character of globally subanalytic integrals. No free parameters or invented entities are introduced.

axioms (2)
  • domain assumption The quantization functional satisfies the Kurdyka-Lojasiewicz inequality
    Invoked to obtain sequential convergence from the gradient-method interpretation of Lloyd's algorithm.
  • standard math Globally subanalytic integrals are log-analytic
    Used to establish the definability results for the semi-discrete optimal transport losses.

pith-pipeline@v0.9.0 · 5726 in / 1326 out tokens · 21854 ms · 2026-05-24T00:55:11.380426+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Weighted quantization using MMD: From mean field to mean shift via gradient flows

    stat.ML 2025-02 unverdicted novelty 6.0

    Derives MSIP algorithm from MMD gradient flows for weighted quantization, extending mean shift and relating to preconditioned gradient descent and Lloyd's clustering.

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