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Generalized Composed Alternating Relaxed Projection Algorithm for Two-Set Feasibility Problem

T0 review · 0 major / 3 minor · reviewed 2026-05-10 · grok-4.3

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Pith's one-line read A generalized algorithm blending relaxed projections and averaging converges to solutions of the two-set convex feasibility problem.

desk verdict gCARPA adds an outer averaging weight and a three-parameter internal relaxation to alternating projections, recovers classical methods as cases, and gives an explicit principal-angle rate formula for subspaces. read the letter →

arxiv 2604.17276 v1 submitted 2026-04-19 math.OC cs.NAmath.NA

Xinxin Li , Yudong Wei , Hao Zhang This is my paper · ORCID
classification math.OCcs.NAmath.NA
keywords two-setfeasibilityrelaxedprojectionDouglas-Rachfordalternatingprojectionsconvergenceanalysissubspaceprincipalanglesparameterselection
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper introduces gCARPA, a generalized composed alternating relaxed projection method for locating a point in the intersection of two closed convex sets in Hilbert space. It combines Douglas-Rachford-type and projection-reflection dynamics through an outer averaging parameter and internal relaxation parameters, recovering several classical projection algorithms as special cases. A non-stationary version with iteration-varying parameters is also defined and shown to converge. For the subspace case an explicit spectral decomposition in terms of principal angles supplies computable factors that guide minimax parameter choice aimed at critical damping.

What carries the argument

The composed gCARPA operator formed by relaxed projections, reflections, and outer averaging, which remains averaged or firmly nonexpansive for suitable parameter ranges and thereby guarantees convergence.

What would settle it

A concrete pair of closed convex sets together with a parameter triple outside the averaged range for which the iterates fail to converge to the intersection would falsify the convergence claim.

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Extended reading notes

Core claim

We propose the gCARPA iteration that blends Douglas-Rachford-type and projection-reflection-type dynamics via an outer averaging step μ and an internal relaxation (γ, θ, η). The algorithm contains several classical projection methods as special cases. We establish convergence of both the stationary and non-stationary variants. For the subspace feasibility model we derive an explicit spectral characterization via principal-angle block decompositions that yields computable subdominant-eigenvalue factors and a minimax parameter-selection recipe in a symmetric regime that targets critical damping on principal-angle planes.

Load-bearing premise

The relaxation parameters must be chosen so that the overall iteration operator stays averaged or firmly nonexpansive.

Editorial extensions

If this is right

  • Classical methods such as alternating projections, relaxed alternating projections, and Douglas-Rachford splitting arise as direct special cases.
  • Convergence holds for both fixed parameters and iteration-dependent parameters provided the averagedness condition is met.
  • In the subspace setting the principal-angle spectral factors allow explicit computation of the linear convergence rate and optimal parameter choice.
  • Numerical behavior can be tuned per problem class by adjusting the outer averaging and internal relaxation values.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The same spectral tuning recipe may apply to other feasibility problems whose linearization admits a principal-angle decomposition.
  • Non-stationary parameter schedules could be adapted on the fly using local estimates of principal angles when the sets change slowly.
  • The unification suggests that performance gains observed in one classical method can be ported to others by appropriate choice of the outer averaging weight.
  • The framework may extend to inconsistent feasibility problems by replacing the intersection condition with a distance-minimization objective.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

0 major / 3 minor

Summary. The manuscript proposes the generalized composed alternating relaxed projection algorithm (gCARPA) for the two-set feasibility problem of finding a point in X ∩ Y where X and Y are closed convex sets in a Hilbert space. The method combines Douglas-Rachford-type and projection-reflection-type dynamics through an outer averaging parameter μ and an internal relaxation triple (γ, θ, η). It recovers several classical projection methods as special cases under suitable parameter choices. Convergence is proved for both the stationary algorithm and its non-stationary variant with iteration-dependent parameters. For the subspace feasibility model, an explicit spectral characterization is derived via principal-angle block decompositions, yielding computable subdominant-eigenvalue factors and a minimax parameter-selection recipe. Numerical experiments illustrate performance relative to baseline methods.

Significance. If the convergence claims hold, the paper supplies a unifying parametric framework for projection algorithms that extends standard results on averaged and firmly nonexpansive operators while adding a non-stationary extension and an explicit rate formula for the subspace case. The spectral analysis and minimax tuning recipe are particularly useful for applications where subspace models appear, and the recovery of classical methods as special cases provides a clear unification. The numerical illustrations suggest practical benefits in problem-dependent regimes.

minor comments (3)
  1. The statement that the algorithm contains classical methods as special cases would benefit from an explicit table or enumerated list (with the precise parameter settings for each recovered method) to allow immediate verification by readers.
  2. In the subspace spectral analysis, the transition from the principal-angle block decomposition to the explicit subdominant-eigenvalue expression should include a short derivation sketch or reference to the relevant matrix blocks to improve readability.
  3. The numerical experiments section would be strengthened by reporting the specific dimensions, generation procedure for the test sets, and stopping criteria used, so that the observed improvements can be reproduced and compared quantitatively.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive and accurate summary of the manuscript, including the unification of projection methods via gCARPA, the convergence results for both stationary and non-stationary variants, the explicit spectral analysis for subspace feasibility, and the numerical illustrations. We are pleased that the significance of the parametric framework, the minimax tuning recipe, and the recovery of classical methods as special cases was recognized. No major comments were raised in the report.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; derivation is self-contained

full rationale

The manuscript defines gCARPA explicitly via the outer averaging parameter μ and internal relaxations (γ,θ,η), recovers classical methods by direct substitution into those parameters, proves convergence by verifying that the composed operator remains averaged or firmly nonexpansive under the stated ranges (standard facts from convex analysis), and obtains the subspace spectral characterization by block-diagonalizing the iteration operator with respect to principal-angle subspaces. None of these steps reduce to a fitted quantity defined from the target result, a self-citation chain, or an ansatz smuggled in; the central claims follow directly from the operator definitions and the listed hypotheses on X and Y.

Assumptions & free parameters 2 free parameters · 1 assumptions · 0 invented entities

The central claim rests on the standard domain assumption that the sets are closed and convex in a Hilbert space together with the free choice of relaxation parameters that must satisfy averaging conditions for the convergence theorems.

free parameters (2)
  • outer averaging weight μ
    Tunable scalar that blends the two dynamics; its admissible range is part of the algorithm definition.
  • internal relaxation triple (γ, θ, η)
    Three scalars controlling the relaxed projections; chosen once or per iteration in the non-stationary case.
assumptions (1)
  • domain assumption X and Y are closed convex subsets of a Hilbert space
    Invoked to guarantee that the projection operators are well-defined, single-valued, and firmly nonexpansive.

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Pith. "Pith review of Generalized Composed Alternating Relaxed Projection Algorithm for Two-Set Feasibility Problem." pith.science (2026). https://pith.science/paper/2604.17276

@misc{pith2026260417276,
  author       = {Pith},
  title        = {Pith review of: Generalized Composed Alternating Relaxed Projection Algorithm for Two-Set Feasibility Problem},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/2604.17276}},
  note         = {Machine review of arXiv:2604.17276}
}
abstract

We study the two-set feasibility problem of finding a point in the intersection $X\cap Y$ of closed convex sets in a Hilbert space. We propose a generalized composed alternating relaxed projection algorithm (gCARPA) that blends Douglas-Rachford-type and projection-reflection-type dynamics via an outer averaging step $\mu$ and an internal relaxation $(\gamma,\theta,\eta)$. The algorithm contains several classical projection methods as special cases. We also introduce its non-stationary variant, in which $(\gamma_k,\theta_k,\eta_k)$ vary over iterations, and establish its convergence. For the subspace feasibility model, we derive an explicit spectral characterization via principal-angle block decompositions, yielding computable subdominant-eigenvalue factors and a minimax parameter-selection recipe in a symmetric regime that targets critical damping on principal-angle planes. Numerical experiments illustrate that the generalized relaxation and its non-stationary tuning can improve or match baseline methods in problem-dependent regimes.

Figures

Figures reproduced from arXiv: 2604.17276 by the authors.

Figure 1
Figure 1. Predicted linear convergence fac￾tors on subspace feasibility problems with n = 100, p = q = 50, φp = π/2, and angles {φi} defined by (4.4). -1.5 -1 -0.5 0 0.5 1 1.5 -1 -0.5 0 0.5 1 [PITH_FULL_IMAGE:figures/full_fig_p019_1.png] view at source ↗
Figure 3
Figure 3. Subspace feasibility with n = 100, p = q = 50, φp = π/2, and angles given by (4.4). Log-scale plots of FPRk = kz k+1 − z kk/ max{1, kz kk} for DR, CARPA-opt(γ) with θ = η = 1, gCARPA-opt(γ) with θ = η = 0.7, and gCARPA-grid best (γ, θ, η). 4.2.3 Trajectory illustration Finally, we visualize the trajectory behavior on a single principal-angle plane. For subspace feasibility, the DR fixed-point iteration typically exh… view at source ↗
Figure 4
Figure 4. Toy sparse linear inverse instance with ( [PITH_FULL_IMAGE:figures/full_fig_p025_4.png] view at source ↗
Figures from the paper (1 more)
Figure 5
Figure 5. Figure 5: Realistic sparse linear inverse instances: log-s [PITH_FULL_IMAGE:figures/full_fig_p026_5.png]

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