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arxiv: 1906.12112 · v1 · pith:6DO3Z2H3new · submitted 2019-06-28 · 🧮 math.OC

Alternating Direction Method of Multipliers with Variable Metric Indefinite Proximal Terms for Convex Optimization

Pith reviewed 2026-05-25 14:02 UTC · model grok-4.3

classification 🧮 math.OC
keywords alternating direction method of multipliersproximal ADMMindefinite proximal termsvariable metricBFGS updateglobal convergenceconvex optimization
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The pith

Variable-metric ADMM with indefinite proximal terms converges globally when the matrices meet stated conditions, and a BFGS construction satisfies them.

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

This paper develops a proximal alternating direction method of multipliers allowing the proximal term to be indefinite and to change at each step through a variable metric. It supplies sufficient conditions on those terms that guarantee global convergence for linearly constrained convex optimization problems. The authors also construct a concrete indefinite term using the BFGS update that meets the required conditions. Earlier fixed or semidefinite versions are extended by this variable indefinite approach. A reader cares because the method keeps subproblems easy while potentially allowing faster practical behavior without losing the convergence guarantee.

Core claim

We consider a variable metric indefinite proximal ADMM, and give sufficient conditions on the proximal terms for the global convergence. Moreover, we propose a new indefinite proximal term based on the BFGS update which can satisfy the conditions for the global convergence.

What carries the argument

The sequence of variable-metric indefinite proximal matrices, generated by a BFGS-style update, that obeys the sufficient conditions required for the convergence analysis.

If this is right

  • Global convergence of the iterates holds for the ADMM even when the proximal terms are allowed to be indefinite.
  • The BFGS update supplies a practical way to generate proximal terms that obey the convergence conditions.
  • The method applies directly to linearly constrained convex optimization problems.
  • Each subproblem remains as easy to solve as in the standard proximal ADMM.

Where Pith is reading between the lines

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

  • The same style of indefinite proximal construction could be tried in other splitting algorithms that rely on proximal terms.
  • Numerical tests on specific convex problems could reveal whether the indefinite terms produce measurable speedups compared with semidefinite alternatives.
  • The sufficient conditions might admit further weakening while still preserving convergence.

Load-bearing premise

The sequence of indefinite proximal matrices must satisfy the specific sufficient conditions on their properties that the convergence proof relies upon.

What would settle it

A convex optimization instance together with a sequence of proximal matrices obeying the stated conditions for which the generated iterates fail to converge to an optimal solution.

read the original abstract

This paper studies a proximal alternating direction method of multipliers (ADMM) with variable metric indefinite proximal terms for linearly constrained convex optimization problems. The proximal ADMM plays an important role in many application areas, since the subproblems of the method are easy to solve. Recently, it is reported that the proximal ADMM with a certain fixed indefinite proximal term is faster than that with a positive semidefinite term, and still has the global convergence property. On the other hand, Gu and Yamashita studied a variable metric semidefinite proximal ADMM whose proximal term is generated by the BFGS update. They reported that a slightly indefinite matrix also makes the algorithm work well in their numerical experiments. Motivated by this fact, we consider a variable metric indefinite proximal ADMM, and give sufficient conditions on the proximal terms for the global convergence. Moreover, we propose a new indefinite proximal term based on the BFGS update which can satisfy the conditions for the global convergence.

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 studies a proximal ADMM with variable metric indefinite proximal terms for linearly constrained convex optimization. It derives sufficient conditions on the sequence of indefinite proximal matrices ensuring global convergence and proposes a BFGS-based construction claimed to satisfy those conditions.

Significance. If the sufficient conditions are correctly stated and the BFGS construction is shown to meet them, the result would extend the theory of proximal ADMM to indefinite metrics while retaining global convergence, providing a theoretical basis for observed practical speedups with slightly indefinite terms.

major comments (2)
  1. [§3] §3 (convergence theorem): the sufficient conditions require a uniform lower bound on the smallest eigenvalue of a linear combination of the metric and constraint matrix together with a summability condition on successive differences; the manuscript must explicitly verify that the proposed BFGS update preserves these bounds for all iterates, as standard BFGS can produce eigenvalue drift when the secant condition holds only approximately.
  2. [§4] §4 (BFGS construction): the claim that the new indefinite proximal term based on the BFGS update satisfies the conditions of §3 is load-bearing for the main contribution, yet the provided derivation does not include an error analysis or inductive argument confirming that the eigenvalue lower bound remains positive uniformly.
minor comments (2)
  1. [Introduction] The reference to the fixed indefinite proximal term result and to Gu-Yamashita should be expanded with precise citations and a short comparison table of convergence rates.
  2. [§2] Notation for the proximal matrix sequence {B_k} should be introduced once and used consistently; several early equations reuse B without the subscript.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The comments correctly identify that the manuscript's main claims rest on the BFGS construction satisfying the convergence conditions, yet the current text does not supply an explicit inductive verification or error analysis. We will revise the paper to include this material.

read point-by-point responses
  1. Referee: [§3] §3 (convergence theorem): the sufficient conditions require a uniform lower bound on the smallest eigenvalue of a linear combination of the metric and constraint matrix together with a summability condition on successive differences; the manuscript must explicitly verify that the proposed BFGS update preserves these bounds for all iterates, as standard BFGS can produce eigenvalue drift when the secant condition holds only approximately.

    Authors: We agree that an explicit verification is required. In the revision we will insert a new lemma (with proof) that proceeds by induction on the iterates. The argument exploits the specific form of the indefinite BFGS update together with the fact that the secant condition is enforced only on the range of the constraint matrix; this prevents the eigenvalue drift that can occur in the classical positive-definite BFGS setting. The summability condition on successive differences will also be verified directly from the update formula. revision: yes

  2. Referee: [§4] §4 (BFGS construction): the claim that the new indefinite proximal term based on the BFGS update satisfies the conditions of §3 is load-bearing for the main contribution, yet the provided derivation does not include an error analysis or inductive argument confirming that the eigenvalue lower bound remains positive uniformly.

    Authors: We acknowledge the gap. The revised manuscript will contain a dedicated subsection (or appendix) that supplies the missing inductive argument and a uniform lower-bound estimate on the relevant eigenvalue. The proof will be self-contained and will not rely on external results beyond standard properties of the BFGS update under the paper's assumptions. revision: yes

Circularity Check

0 steps flagged

Convergence proof and BFGS construction are independent of each other and of prior self-citations.

full rationale

The paper states sufficient conditions on the sequence of indefinite proximal matrices and separately constructs a BFGS-based term asserted to meet those conditions. No equation reduces the claimed convergence to a fitted parameter or to a definition that presupposes the result. The citation to the authors' prior semidefinite BFGS work supplies only motivation and background; the new indefinite conditions and their verification are developed within the present manuscript without load-bearing reliance on unverified self-referential steps. The derivation therefore remains self-contained against external convex-analysis benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Review performed on abstract only; no explicit free parameters, invented entities, or non-standard axioms are stated. The work relies on standard convex optimization assumptions and the existence of the claimed sufficient conditions.

axioms (1)
  • domain assumption The problem is a linearly constrained convex optimization problem.
    Explicitly stated as the setting for which the method is developed.

pith-pipeline@v0.9.0 · 5697 in / 1176 out tokens · 26122 ms · 2026-05-25T14:02:35.915396+00:00 · methodology

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