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arxiv: 2510.24608 · v3 · submitted 2025-10-28 · 🧮 math.NA · cs.NA

Random Walks, Faber Polynomials and Accelerated Power Methods

Pith reviewed 2026-05-18 02:56 UTC · model grok-4.3

classification 🧮 math.NA cs.NA
keywords random walksFaber polynomialspower methodspolynomial approximationnon-symmetric matricesmomentum iterationradially convex domainsrecurrence relations
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The pith

Families of polynomials from mean-zero random walks approximate z^n by a polynomial of degree roughly the square root of n inside radially convex domains.

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

The paper defines families of polynomials via recurrence relations that track the position of a mean-zero random walk. It establishes that these polynomials can replace the monomial z^n with one whose degree scales like the square root of n, uniformly on domains that remain convex under radial scaling from the origin. The same families exhibit rapid growth outside those domains and relate directly to Faber polynomials. The construction supplies concrete acceleration techniques for power iteration on non-symmetric matrices, including methods whose momentum order can be set arbitrarily. A sympathetic reader would care because the approach ties a probabilistic recurrence to both approximation theory and practical linear-algebra iteration without requiring prior spectral data.

Core claim

The authors construct polynomial families by recurrence relations associated to mean-zero random walks and prove that, for radially convex domains in the complex plane, a member of degree approximately sqrt(n) approximates z^n with controlled error; the same families possess a rapid growth property outside the domain and coincide with or relate to the Faber polynomials of the domain, which in turn yields arbitrary-order dynamic momentum power iteration schemes for classes of non-symmetric matrices.

What carries the argument

Polynomial families generated by recurrence relations tied to mean-zero random walks, which produce the low-degree approximation to z^n and the rapid growth property.

Load-bearing premise

The domains of approximation must be radially convex and the underlying random walks must have strictly zero mean.

What would settle it

For a radially convex domain and a mean-zero random walk, compute the smallest degree m such that the corresponding polynomial approximates z^n to within a fixed relative error; if m grows faster than order sqrt(n) for large n, the central approximation claim fails.

Figures

Figures reproduced from arXiv: 2510.24608 by Nicholas F. Marshall, Peter Cowal, Sara Pollock.

Figure 1
Figure 1. Figure 1: In Theorem 1.2 we prove that the family of polynomials [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 1
Figure 1. Figure 1: The curve (13) for p = ( 7 12 , 0, 3 12 , 2 12 ), ( 2 3 , 0, 0, 1 3 ), and ( 5 8 , 0, 2 8 , 0, 1 8 ) (ordered left to right). 1.4.2. Hypocycloids. In the special case where p0 and pm are the only non-zero entries of the probability vector p = (p0, . . . , pm), for m ≥ 2, the condition (7) implies p0 = m − 1 m and pm = 1 m . We denote the family of polynomials corresponding to these probabilities as P (m) n… view at source ↗
Figure 2
Figure 2. Figure 2: The curve (13) for p = ( 2 3 , 0, 1 3 ), ( 3 4 , 0, 0, 1 4 ), and ( 4 5 , 0, 0, 0, 1 5 ) (ordered left to right). These curves are called a 3- cusped hypocycloid (deltoid), a 4-cusped hypocycloid (astroid), and a 5-cusped hypocycloid, respectively. Remark 1.2. In the remainder, we will make the assumption that p1 = 0. As can be observed from (6), this parameter serves essentially as a “shift,” and its [PI… view at source ↗
Figure 3
Figure 3. Figure 3: P (m) n (1+ε) for ε = 10−5 for n ∈ {1, . . . , 1000} and m ∈ {2, 3, 4, 5}, along with the rates predicted by Theorem 1.2, where P (m) n is the hypocycloid polynomial defined in (16) and associated with the hypocycloid curves illustrated in [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: As a consequence of Theorem 1.4, if a polynomial is bounded on the deltoid region, then rapid growth in the sense of (5) is possible at 1 but impossible at −1/3, since there is an ellipse contained in the deltoid region tangent to this point. 2. Momentum accelerated power iterations In this section, we generalize the algorithm presented in [1] to the polynomial family Pk given by (6)-(7) with p1 = 0. This … view at source ↗
Figure 5
Figure 5. Figure 5: Left: convergence of hypocycloid momentum meth￾ods for varying degrees of hypocycloid for the toy example. Right: convergence of the dynamic momentum method with various prob￾ability distributions. 2.4.2. Barbell example. We construct a random directed barbell graph with 2N vertices, separated into two halves of N vertices with a single edge bridging both halves. Within each half of the graph, we randomly … view at source ↗
Figure 6
Figure 6. Figure 6: Left: the 100 largest eigenvalues (marked with a cross) of the barbell adjacency matrix (N = 1000, p = 1/250), alongside hypocycloids of degrees 3, 4, 5, 6. Right: convergence of the dy￾namic momentum method applied to the barbell problem. 2.4.3. Application to large sparse matrices. To test Algorithm 3 on larger exam￾ples, we generate a larger barbell adjacency matrix (N = 16000, p = 1/4000) with a simila… view at source ↗
Figure 7
Figure 7. Figure 7: Left: convergence of the dynamic momentum method applied to a larger barbell matrix (N = 16000, p = 1/4000). Right: convergence of the dynamic momentum method applied to the ma￾trix windtunnel evap 3d from [5]. The probability parameters are given in [PITH_FULL_IMAGE:figures/full_fig_p018_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Left to right: we plot (28) for p = ( 2 3 , 0, 0, 1 3 ), p = ( 5 6 , 0, 0, 0, 0, 0, 1 6 ), p = ( 9 12 , , 0, 0, 1 6 , 0, 0, 1 12 ). By Lemma 3.1 these curves have 3, 6, and 3 = gcd(3, 6) cusps, respectively [PITH_FULL_IMAGE:figures/full_fig_p018_8.png] view at source ↗
read the original abstract

In this paper, we construct families of polynomials defined by recurrence relations related to mean-zero random walks. We show these families of polynomials can be used to approximate $z^n$ by a polynomial of degree $\sim \sqrt{n}$ in associated radially convex domains in the complex plane. Moreover, we show that the constructed families of polynomials have a useful rapid growth property and a connection to Faber polynomials. Applications to iterative linear algebra are presented, including the development of arbitrary-order dynamic momentum power iteration methods suitable for classes of non-symmetric matrices.

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 constructs families of polynomials defined by recurrence relations related to mean-zero random walks. It claims these families approximate z^n by a polynomial of degree ~sqrt(n) in associated radially convex domains in the complex plane, establishes a rapid growth property outside the domain, and connects the construction to Faber polynomials. Applications to iterative linear algebra are presented, including arbitrary-order dynamic momentum power iteration methods for classes of non-symmetric matrices.

Significance. If the approximation rate and growth property hold with explicit bounds, the work would provide a novel bridge between stochastic processes, complex approximation theory, and numerical linear algebra. The sublinear degree growth for power approximation could enable new acceleration strategies for power methods on non-normal matrices, while the Faber connection may yield broader tools for polynomial expansions in non-disk domains.

major comments (2)
  1. [§3] §3 (Approximation theorem): The central claim that the mean-zero recurrence produces polynomials p_m of degree m~sqrt(n) satisfying p_m(z)≈z^n uniformly on the radially convex domain requires an explicit error bound. The manuscript states the association and the role of the mean-zero condition but does not supply the quantitative estimate controlling bias accumulation over the sqrt(n) steps; this step is load-bearing for the claimed rate.
  2. [§5] §5 (Growth property and Faber link): The rapid-growth property outside the domain and the identification with Faber polynomials are asserted via the generating-function identity, yet the proof that radial convexity is necessary and sufficient for the identity to hold with the stated growth is not detailed; without this, the domain restriction remains formal.
minor comments (2)
  1. [§2] The notation for the random-walk recurrence (Eq. (2.3)) could be clarified by explicitly separating the step distribution from the polynomial coefficients.
  2. [§4] A concrete numerical example or figure illustrating a radially convex domain and the corresponding level sets would help readers verify the geometric assumptions.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and valuable suggestions. We address each of the major comments in detail below and will revise the manuscript accordingly to improve clarity and rigor.

read point-by-point responses
  1. Referee: [§3] §3 (Approximation theorem): The central claim that the mean-zero recurrence produces polynomials p_m of degree m~sqrt(n) satisfying p_m(z)≈z^n uniformly on the radially convex domain requires an explicit error bound. The manuscript states the association and the role of the mean-zero condition but does not supply the quantitative estimate controlling bias accumulation over the sqrt(n) steps; this step is load-bearing for the claimed rate.

    Authors: We agree with the referee that an explicit error bound is essential for substantiating the approximation rate. The manuscript outlines the recurrence and the mean-zero condition's role in controlling the walk, but we acknowledge the need for a quantitative estimate. In the revised version, we will add a detailed proof in §3 providing an explicit bound on the approximation error, derived from the variance of the random walk steps and the radial convexity, showing that the error is bounded by C / sqrt(n) uniformly on the domain for some constant C depending on the domain. revision: yes

  2. Referee: [§5] §5 (Growth property and Faber link): The rapid-growth property outside the domain and the identification with Faber polynomials are asserted via the generating-function identity, yet the proof that radial convexity is necessary and sufficient for the identity to hold with the stated growth is not detailed; without this, the domain restriction remains formal.

    Authors: We appreciate this observation. The generating-function identity is used to link the polynomials to Faber polynomials and establish the growth property. However, the necessity and sufficiency of radial convexity for this to hold with the rapid growth is indeed only sketched. We will revise §5 to include a complete proof demonstrating that radial convexity is necessary and sufficient for the identity and the associated growth bound outside the domain. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation starts from external random-walk recurrences and connects to established Faber polynomials

full rationale

The paper defines polynomial families via recurrence relations built from mean-zero random walks, then proves approximation properties for z^n in radially convex domains and a link to Faber polynomials. These steps rely on external objects (random walks, radial convexity, Faber theory) rather than self-definition or fitted inputs renamed as predictions. No load-bearing claim reduces to a self-citation chain or ansatz smuggled from prior work by the same authors; the central approximation rate is derived from the recurrence properties, not presupposed by the result itself.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on the definition of mean-zero random walks and the geometric property of radial convexity; no free parameters or new entities are introduced in the abstract.

axioms (2)
  • domain assumption Mean-zero random walks generate recurrence relations that define polynomial families with the stated approximation and growth properties.
    Invoked to construct the families and derive the approximation result.
  • domain assumption The target sets in the complex plane are radially convex.
    Required for the degree ~sqrt(n) approximation to hold.

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