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arxiv: 2604.18192 · v1 · submitted 2026-04-20 · 🧮 math.OC

Local Convergence Results for Sequential Quadratic Programming with Complementarity Constraints

Pith reviewed 2026-05-10 04:05 UTC · model grok-4.3

classification 🧮 math.OC
keywords local convergenceSQPCCMPCCS-stationary pointsQPCCkappa-omegaactive-set stabilizationsecond-order sufficient conditions
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The pith

SQPCC converges locally to S-stationary points of MPCCs under weaker second-order conditions.

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

The paper proves a local convergence result for sequential quadratic programming with complementarity constraints (SQPCC) applied to mathematical programs with complementarity constraints (MPCCs). It shows that there exists at least one sequence of S-stationary points from the QPCC subproblems that converges to a reference S-stationary point of the MPCC. The analysis uses the kappa-omega framework to quantify subproblem approximation errors and requires only weaker second-order sufficient conditions at the reference point, without upper-level strict complementarity. This matters because MPCCs violate standard constraint qualifications at every feasible point, so classical SQP applied to reformulations is unreliable, while SQPCC preserves the complementarity structure directly in each subproblem.

Core claim

We show that there exists at least one sequence of QPCC S-stationary points converging to a reference S-stationary point of the MPCC, and we characterize conditions under which each such sequence converges and under which such a sequence is locally unique. In contrast to previous results, the analysis is established under weaker second-order sufficient conditions and requires no upper-level strict complementarity. Our result builds upon local convergence results for the classical SQP method within the kappa-omega setting, which also cover linearly convergent variants. We further establish an active-set stabilization result for SQPCC.

What carries the argument

The kappa-omega setting for quantifying subproblem approximation errors together with weaker second-order sufficient conditions at the reference S-stationary point.

If this is right

  • There exists at least one sequence of QPCC S-stationary points that converges to the reference S-stationary point of the MPCC.
  • Conditions are characterized under which every such sequence converges to the reference point.
  • Conditions are characterized under which the converging sequence is locally unique.
  • The optimal complementarity and inequality active sets are identified after finitely many iterations or only asymptotically, under stated conditions.
  • Local convergence results for classical SQP hold in the kappa-omega setting, including for linearly convergent variants.

Where Pith is reading between the lines

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

  • The weaker conditions may allow SQPCC to succeed on MPCCs from applications where upper-level strict complementarity does not hold.
  • The same kappa-omega error framework could be applied to analyze convergence of other subproblem-preserving methods for complementarity-constrained problems.
  • Finite active-set stabilization may enable reliable switching to reduced-space methods after identification.
  • The size of the local convergence region can be related explicitly to the degree of nonlinearity through the kappa-omega constants.

Load-bearing premise

The subproblem approximation errors must satisfy the kappa-omega bounds and weaker second-order sufficient conditions must hold at the reference S-stationary point.

What would settle it

A concrete MPCC instance in which the kappa-omega error bounds fail to hold and the SQPCC iterates diverge from the reference S-stationary point, or in which the second-order conditions are violated yet local convergence still occurs.

Figures

Figures reproduced from arXiv: 2604.18192 by Armin Nurkanovi\'c.

Figure 3
Figure 3. Figure 3: The left plot illustrates the iterates of the exact-Hessian SQP method, and the middle plot those of the exact-Hessian SQPCC method in the (w1, w2)-plane. The initial point in both cases is w0 = (0, 2). The right plot shows the errors over the iterations for two methods; the SQPCC method converges to ¯w, and SQP does not. and these are S-stationary points of the corresponding QPCCs. This sequence converges… view at source ↗
read the original abstract

Mathematical programs with complementarity constraints (MPCCs) are a challenging class of nonlinear optimization problems, because their nonlinear programming reformulations violate standard constraint qualifications at every feasible point. This paper analyzes sequential quadratic programming with complementarity constraints (SQPCC). In this method, the complementarity constraints are retained in the subproblems, yielding quadratic programs with complementarity constraints (QPCCs). The main contribution of the paper is a new local convergence result for the SQPCC method to S-stationary points. We show that there exists at least one sequence of QPCC S-stationary points converging to a reference S-stationary point of the MPCC, and we characterize conditions under which each such sequence converges and under which such a sequence is locally unique. In contrast to previous results, the analysis is established under weaker second-order sufficient conditions and requires no upper-level strict complementarity. Our result builds upon local convergence results for the classical SQP method. We present local convergence results for SQP within the kappa-omega setting, which also cover linearly convergent variants. These assumptions, which are widely used in the analysis of Newton-type methods, provide a natural framework for quantifying subproblem approximation errors, their effect on the convergence speed, and the effect of nonlinearity on the size of the local convergence region. Furthermore, we establish an active-set stabilization result for SQPCC, identifying conditions under which the optimal complementarity and inequality active sets are identified after finitely many iterations, and conditions under which such identification occurs only asymptotically. Numerical examples illustrate the theoretical findings and highlight some advantages of SQPCC over classical SQP applied to MPCCs.

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 paper analyzes the local convergence of sequential quadratic programming with complementarity constraints (SQPCC) for mathematical programs with complementarity constraints (MPCCs). It establishes that there exists at least one sequence of S-stationary points of the QPCC subproblems converging to a given S-stationary point of the MPCC, and characterizes conditions for convergence and local uniqueness of such sequences. The analysis relies on the kappa-omega framework for quantifying subproblem approximation errors (extending prior SQP results), weaker second-order sufficient conditions at the reference point, and does not require upper-level strict complementarity. Additional results include active-set stabilization (finite or asymptotic identification of active sets) and numerical examples comparing SQPCC to standard SQP on MPCCs.

Significance. If the central convergence claims hold, this work provides a meaningful extension of local SQP theory to the QPCC setting for MPCCs, relaxing prior assumptions on second-order conditions and strict complementarity. The kappa-omega error framework is a strength, as it naturally handles linearly convergent variants and quantifies the impact of nonlinearity on the convergence basin. This could broaden the reliable use of SQPCC in applications involving complementarity, such as mechanics and game theory. The active-set stabilization result is a useful practical addition. Numerical examples are mentioned as validation but do not appear to serve as independent verification of the rates or uniqueness claims.

major comments (2)
  1. The central existence and uniqueness claims in the abstract rely on the kappa-omega error bounds and weaker SOSC at the reference S-stationary point, but the manuscript does not appear to provide explicit derivations of the error bounds for the QPCC subproblems (as opposed to standard QP subproblems). Without these, it is difficult to verify that the approximation errors remain controlled under the weaker SOSC.
  2. Section on active-set stabilization: the conditions for finite vs. asymptotic identification are stated, but it is unclear how they interact with the local uniqueness result for the sequence of QPCC S-stationary points. If the active sets stabilize only asymptotically, this may affect the claimed local uniqueness in a neighborhood.
minor comments (2)
  1. The numerical examples are referenced but lack sufficient detail on problem instances, iteration counts, or observed convergence rates to independently support the theoretical claims.
  2. Notation for the kappa-omega parameters and the S-stationarity definitions should be cross-referenced more explicitly between the SQP background section and the QPCC extension.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. We address each major comment below and indicate the revisions we will make.

read point-by-point responses
  1. Referee: The central existence and uniqueness claims in the abstract rely on the kappa-omega error bounds and weaker SOSC at the reference S-stationary point, but the manuscript does not appear to provide explicit derivations of the error bounds for the QPCC subproblems (as opposed to standard QP subproblems). Without these, it is difficult to verify that the approximation errors remain controlled under the weaker SOSC.

    Authors: We appreciate this observation. Section 3 extends the kappa-omega framework to SQPCC and derives the relevant error bounds for the QPCC subproblems, using the weaker SOSC to control the terms without upper-level strict complementarity. However, we agree the derivations could be more explicit. In the revision we will expand the proof of Theorem 3.2 with additional intermediate steps that isolate the QPCC-specific error contributions and show they remain bounded under the stated assumptions. revision: yes

  2. Referee: Section on active-set stabilization: the conditions for finite vs. asymptotic identification are stated, but it is unclear how they interact with the local uniqueness result for the sequence of QPCC S-stationary points. If the active sets stabilize only asymptotically, this may affect the claimed local uniqueness in a neighborhood.

    Authors: Thank you for noting this interaction. The local uniqueness result (Theorem 4.3) is proved for sequences whose active sets are eventually fixed, while the asymptotic-identification case yields uniqueness only in the limit. We will add a short remark after Theorem 4.3 clarifying that, under asymptotic stabilization, uniqueness holds with respect to the limit point rather than in a fixed neighborhood, and we will adjust the theorem statement accordingly for precision. revision: partial

Circularity Check

0 steps flagged

No significant circularity; extension of prior SQP results with new QPCC arguments

full rationale

The paper extends classical SQP local convergence results (in the kappa-omega framework) to the QPCC subproblem setting for MPCCs. The central claim—existence of sequences of QPCC S-stationary points converging to a reference MPCC S-stationary point, plus conditions for convergence and local uniqueness—relies on weaker SOSC and error bounds without reducing to a self-definition, fitted parameter renamed as prediction, or load-bearing self-citation chain. No equations or steps in the provided abstract and structure equate the target result to its inputs by construction. This is a standard honest extension with independent content.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard second-order sufficient conditions at S-stationary points and the kappa-omega error framework from Newton-method analysis; no free parameters are fitted and no new entities are postulated.

axioms (2)
  • domain assumption Weaker second-order sufficient conditions hold at the reference S-stationary point
    Invoked to guarantee local convergence of the QPCC subproblem solutions; appears in the statement of the main result.
  • standard math Kappa-omega framework quantifies subproblem approximation errors
    Used to bound the effect of nonlinearity and approximation quality on the convergence region and rate; referenced as the setting for the SQP analysis.

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