REVIEW 1 major objections 5 minor 69 references
An augmented Lagrangian method solves constrained nonlinear least-squares by keeping linear constraints explicit and approximating the penalty Hessian with a structured SR1 update.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-14 06:04 UTC pith:LI4HDULN
load-bearing objection Solid specialized AL solver for constrained NLS: careful combination of known pieces, honest numerics, one known gap between theorem and code. the 1 major comments →
An augmented Lagrangian algorithm for constrained nonlinear least-squares
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper establishes that an augmented-Lagrangian outer loop combined with trust-region gradient-projection inner iterations and a hybrid structured-SR1 Hessian approximation converges globally to first-order KKT points of a nonlinear least-squares problem subject to mixed nonlinear and linear constraints, and that the resulting algorithm is practically competitive on a standard test set.
What carries the argument
The structured secant equation for the second-order remainder of the augmented Lagrangian, Bk+1 sk = (Jk+1 − Jk)⊤ rk+1 + (Ck+1 − Ck)⊤ λ̄k+1, which is realized by an SR1 update and optionally switched with pure Gauss–Newton via a residual-ratio test.
Load-bearing premise
The code stops when the reduced gradient is small, yet the global-convergence theorem is proved for the true projected gradient; the two coincide only when active-bound multipliers keep the correct sign at every outer iterate.
What would settle it
Run the published Julia implementation on a problem whose active-bound multipliers change sign near a limit point and check whether the reduced-gradient residual vanishes while the true projected-gradient residual stays bounded away from zero.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes TRAULLS, an augmented Lagrangian algorithm for nonlinear least-squares problems with mixed nonlinear and linear (equality/bound) constraints. Nonlinear constraints are penalized via the AL function (2.1), while linear constraints are retained and handled by a trust-region gradient-projection inner solver (Cauchy point along the projected path plus projected CG minor iterations). The AL Hessian is approximated by a Gauss–Newton term plus a structured SR1 correction of the second-order residual/constraint contribution (secant equation (2.26), update (2.27)), with an optional hybrid switch (2.29). Global convergence of the inner process (Theorem 5) and of the outer AL iterates (Theorem 9 / Lemma 8) is established under standard assumptions (twice continuous differentiability, full-rank active linear constraints, compact level sets, controlled Hessian growth, and rank of C(x*)N* ≥ nc). Numerical experiments on 79 instances compare Hessian variants and show the hybrid-SR1 version competitive with IPOPT in CPU time and superior to Percival.
Significance. The work cleanly combines two mature lines—AL methods with gradient-projection subproblem solves and structured quasi-Newton updates for least-squares—into a solver that treats general nonlinear equalities/inequalities together with linear equalities and bounds. The structured secant equation for the AL second-order term and the polyhedral adaptation of the Cauchy-point path are concrete technical contributions. Global-convergence proofs follow the Conn–Gould–Toint / Conn–Gould–Sartenaer–Toint templates after careful reformulation for polyhedra, and a public Julia implementation (TRAULLS) with reproducible benchmarks is provided. The hybrid-SR1 variant is shown to be competitive with a state-of-the-art interior-point solver on a standard test set while exploiting first-order structure only. These strengths make the paper a useful addition to the constrained NLS literature.
major comments (1)
- End of §3: the proved outer convergence (Theorem 9 / Lemma 8) is stated for the projected-gradient residual (2.6), while the implementation and all numerical claims use the reduced-gradient test (2.10). The authors correctly note that the two coincide only when bound multipliers have the correct sign at every outer iterate; without that condition only the tangential component is controlled and a limit point need not be critical. This is the sole material gap between the theorems and the code. A short clarification—either an explicit additional assumption that the sign condition holds along the generated sequence, a practical safeguard that restores the projected residual when signs are wrong, or a statement that the numerical claims are conditional on the observed sign pattern—would close the gap without altering the rest of the argument.
minor comments (5)
- §2.5 / Figure 1: the hierarchy of outer / inner / minor iterations is clear, but a one-sentence reminder that the outer index K and inner index k are independent (and that B0 is reset to zero at each outer iteration) would help readers who jump between sections.
- §4.1: the performance profiles are informative; adding the absolute number of failures (already mentioned in the text) to the figure captions would make the profiles self-contained.
- Appendix A, Algorithm 4: the termination test “|A(ti)| = n−m” is correct under Assumption 4, but a brief remark that the trust-region bounds guarantee eventual full activation would remove any ambiguity for readers unfamiliar with the polyhedral path.
- Several minor typos appear (e.g., “nonlinar”, “Compuations”, “Transcations”, “Reasearch”); a careful proof-reading pass would clean them.
- References [37] and [26] both concern NL2SOL-related work; ensuring the year and author list of [37] match the published version would avoid confusion.
Circularity Check
No circularity: global KKT claims follow from stated assumptions and standard AL/trust-region theory; numerical claims are empirical benchmarks, not forced predictions.
full rationale
The paper's central theoretical claim (Theorem 9 / Lemma 8) is that, under Assumptions 1–9, every limit point of the outer iterates of Algorithm 1 is a first-order KKT point of the constrained NLS problem. The derivation is self-contained: it adapts the projected-gradient path analysis of Conn–Gould–Toint (SBMIN / LANCELOT) to polyhedral sets (Lemmas 1–3, Theorem 4–5) and the AL multiplier/feasibility arguments of Conn–Gould–Sartenaer–Toint (Lemmas 6–8, Theorem 9). No quantity that is later called a “prediction” is fitted from the target KKT residual; the structured SR1 update (2.26)–(2.27) and hybrid switch (2.29) are algorithmic choices whose effect is measured empirically, not asserted as a first-principles derivation. Hyperparameters (μ0, τ, κhyb, trust-region constants) are fixed once for the 79-instance suite and reported; they do not appear as free constants inside the theorems. Self-citations are to standard background (Conn et al., Dennis–Gay–Walsh, Biggs, etc.) that is externally published and independently checkable; none is a uniqueness theorem that forbids alternatives and thereby forces the present result. The only material gap between theory and code—the use of the reduced-gradient test (2.10) instead of the projected-gradient residual (2.6)—is explicitly flagged by the authors at the end of §3 and is a correctness/implementation caveat, not a circular reduction. Consequently the derivation chain does not collapse into its inputs by construction.
Axiom & Free-Parameter Ledger
free parameters (5)
- Initial penalty μ0 and increase factor τ =
μ0=10, τ=100
- Tolerance schedule constants (ω, η, κω, κη, βω, βη) =
as listed in §4
- Hybrid switch threshold κhyb =
0.1
- Trust-region constants (δ0, α1, α2, η1, η2, γ1) =
δ0=0.1, α1=0.25, α2=2.5, η1=0.25, η2=0.75, γ1=0.0625
- Stopping tolerances ω*, η* =
1e-5 / 1e-6
axioms (7)
- domain assumption Residuals r and constraints c are twice continuously differentiable (Assumption 1).
- domain assumption Equality matrix A has full row rank; active-bound matrix à has full row rank at every x in Ω (Assumptions 2, 4).
- domain assumption Linear feasible set Ω is nonempty (Assumption 3).
- domain assumption Level set of φ intersect Ω is nonempty and compact; Hessian approximations satisfy divergent 1/bk series and bk(φk+1−φk)→0 (Assumptions 5–7).
- domain assumption Outer iterates stay in a closed bounded domain; at every limit point, rank(C(x*)N*) ≥ nc (Assumptions 8–9).
- standard math Steps achieve a fixed fraction of Cauchy decrease (κfcd) and standard trust-region acceptance ratios.
- ad hoc to paper When reduced gradient is used as the practical criticality measure, bound multipliers have correct sign so that reduced and projected residuals coincide.
invented entities (2)
-
TRAULLS algorithm (AL + polyhedral gradient projection + structured SR1 of AL Hessian + hybrid switch)
independent evidence
-
Structured secant equation for AL second-order term (Eq. 2.26) with SR1 update (2.27)
independent evidence
read the original abstract
We present an algorithm for solving nonlinear least-squares problems subject to a mix of nonlinear and linear constraints. The nonlinear constraints are handled by reformulating the objective as the augmented Lagrangian function while linear constraints are handled directly. Each iteration consists of approximately solving a linearly constrained problem by means of a gradient projection technique. Our approach also involves a structured approximation of the augmented Lagrangian Hessian. We show global convergence of the method and assess the performance through numerical experiments.
Figures
Reference graph
Works this paper leans on
-
[1]
M. Al-Baali and R. Fletcher. Variational methods for non-linear least-squares.The Journal of the Operational Research Society, 36(5):405–421, 1985. doi: 10.1057/jors.1985.68
-
[2]
R. Andreani, E.G. Birgin, J.M. Martinez, and M.L. Schuverdt. On augmented Lagrangian methods with general lower-level constraints.SIAM Journal on Optimization, 18(4):1286– 1309, 2008. doi: 10.1137/060654797
-
[3]
S. Arreckx, A. Lambe, J.R.R.A. Martins, and D. Orban. A matrix-free augmented La- grangian algorithm with application to large-scale structural design optimization.Opti- mization and Engineering, 17:359–384, 2016. doi: 10.1007/s11081-015-9287-9
-
[4]
Bartholomew-Biggs
M.C. Bartholomew-Biggs. The estimation of the hessian matrix in nonlinear least-squares problems with non-zero residuals.Mathematical Programming, 12:67–80, 1977. doi: 10. 1007/BF01593770
1977
-
[5]
Bellavia, M
S. Bellavia, M. Macconi, and B. Morini. STRSCNE: A scaled trust-region solver for constrained nonlinear equations.Computational Optimization and Applications, 28:31–50, 2004
2004
-
[6]
Bellavia, J
S. Bellavia, J. Gondzio, and B. Morini. Computational experience with numerical methods for nonnegative least-squares problems.Numerical Linear Algebra with Applications, 18 (3):363–385, 2011
2011
-
[7]
S. Bellavia, S. Gratton, and E. Riccietti. A Levenberg-Marquardt method for large nonlin- ear least-squares problems with dynamic accuracy in functions and gradients.Numerische Mathematik, 140:791–825, 2018. doi: 10.1007/s00211-018-0977-z
-
[8]
Stefania Bellavia, Greta Malaspina, and Benedetta Morini. A variable dimension sketching strategy for nonlinear least-squares.SIAM Journal on Scientific Computing, 48(3):A1206– A1234, 2026. doi: 10.1137/25M175980X
-
[9]
E.H. Bergou, Y. Diouane, V. Kungurtsev, and C.W. Royer. A nonmonotone matrix-free algorithm for nonlinear equality-constrained least-squares problems.SIAM Journal on Scientific Computing, 43(5):S743–S766, 2021. doi: 10.1137/20M1349138
-
[10]
J.T. Betts. Solving the nonlinar least square problem: Application of a general method.Journal of Optimization Theory and Applications, 18:469–483, 1976. doi: 10.1007/BF00932656. 31
-
[11]
J. Bezanson, A. Edelman, S. Karpinski, and V.B. Shah. Julia: A fresh approach to numerical computing.SIAM Review, 59(1):65–98, 2017. doi: 10.1137/141000671
-
[12]
L.T. Biegler, J. Nocedal, C. Schmid, and D. Ternet. Numerical experience with a reduced hessian method for large scale constrained optimization.Computational Optimization and Applications, 15:45–67, 2000. doi: 10.1023/A:1008723031056
-
[13]
SIAM, Philadelphia, PA, USA, 2nd edition, 2024
˚Ake Bj¨ orck.Numerical Methods for Least Squares Problems. SIAM, Philadelphia, PA, USA, 2nd edition, 2024. doi: 10.1137/1.9781611977950
-
[14]
R.H. Byrd, J. Nocedal, and R.B. Schnabel. Representations of quasi-Newton matrices and their use in limited memory methods.Mathematical Programming, 63:129–156, 1994. doi: 10.1007/BF01582063
-
[15]
A.R. Conn, N.I.M. Gould, and Ph.L. Toint. Global convergence of a class of trust region method algorithms for optimization with simple bounds.SIAM Journal on Numerical Analysis, 25(2):433–460, 1988. doi: 10.1137/0725029
doi:10.1137/0725029 1988
-
[16]
A.R. Conn, N.I.M Gould, and Ph.L. Toint. Testing a class of methods for solving mini- mization problems with simple bounds on the variables.Mathematics of Computation, 50 (182):399–430, 1988. doi: 10.1090/S0025-5718-1988-0929544-3
-
[17]
A.R. Conn, N.I.M. Gould, and Ph.L. Toint. A globally convergent augmented Lagrangian algorithm for optimization with general constraints and simple bounds.SIAM Journal on Numerical Analysis, 28(2):545–572, 1991. doi: 10.1137/0728030
-
[18]
A.R. Conn, N.I.M. Gould, and Ph.L. Toint. Convergence of quasi-Newton matrices gen- erated by the symmetric rank one update.Mathematical Programming, 50:177–195, 1991. doi: 10.1007/BF01594934
-
[19]
A.R. Conn, N.I.M Gould, and Ph.L. Toint.LANCELOT: A Fortran Package for Large- Scale Nonlinear Optimization (Release A). Springer Series in Computational Mathematics. Springer Berlin, Heidelberg, 1992. doi: 10.1007/978-3-662-12211-2
-
[20]
A.R. Conn, N. Gould A. Sartenaer, and Ph.L. Toint. Global convergence of a class of trust region algorithms for optimization using inexact projections on convex constraints.SIAM Journal on Optimization, 3(1):164–221, 1993. doi: 10.1137/0803009
-
[21]
A.R. Conn, N. Gould A. Sartenaer, and Ph.L. Toint. Convergence properties of an augmented Lagrangian algorithm for optimization with a combination of general equal- ity and linear constraints.SIAM Journal on Optimization, 6(3):674–703, 1996. doi: 10.1137/S1052623493251463
-
[22]
A.R. Conn, N.I.M Gould, and Ph.L. Toint.Trust Region Methods. SIAM, Philadelphia, PA, USA, 2000. doi: 10.1137/1.9780898719857
-
[23]
F.E. Curtis, H. Jiang, and D.P. Robinson. An adaptative augmented Lagrangian method for large-scale constrained optimization.Mathematical Programming, Series A, 152:201– 245, 2015. doi: 10.1007/s10107-014-0784-y
-
[24]
Community-Based Service Ecosystem Evolution Analysis
F. Delbos, J.Ch. Gilbert, R.Glowinski, and D. Sinoquet. Constrained optimization in seis- mic reflection tomography: a Gauss-Newton augmented Lagrangian approach.Geophysic Journal International, 164:670–684, 2006. doi: 10.1111/j.1365-246X.2005.02729.x. 32
work page internal anchor Pith review Pith/arXiv arXiv doi:10.1111/j.1365-246x.2005.02729.x 2006
-
[25]
Dennis Jr and R.B
J.E. Dennis Jr and R.B. Schnabel.Numerical Methods for Unconstrained optimization and Nonlinear Equations. Classics in Applied Mathematics. SIAM, Philadelphia, PA, USA,
-
[26]
doi: 10.1137/1.9781611971200
-
[27]
J.E. Dennis Jr, D.M. Gay, and R.E. Walsh. An adaptive nonlinear least-squares algorithm. ACM Transactions on Mathematical Software, 7(3):348–368, 1981. doi: 10.1145/355958. 355965
-
[28]
E.D. Dolan and J.J. Mor´ e. Benchmarking optimization software with performance profiles. Mathematical Programming, Series A 91(2):201–213, 2002. doi: 10.1007/s101070100263
-
[29]
R. Fletcher and C. Xu. Hybrid methods for nonlinear least squares.IMA Journal of Numerical Analysis, 7:373–389, 1987. doi: 10.1093/imanum/7.3.371
-
[30]
P.E. Gill and D.P. Robinson. A primal-dual augmented Lagrangian.Computationnal Optimization and Applications, 51:1–25, 2012. doi: 10.1007/s10589-010-9339-1
-
[31]
G.H. Golub and C.F. Van Loan.Matrix Computations. JHU Press, Maryland, MD, USA, 4th edition, 2013. doi: 10.56021/9781421407944
-
[32]
M.L.N. Gon¸ calves and T.C. Menezes. Gauss–Newton methods with approximate projec- tions for solving constrained nonlinear least squares problems.Journal of Complexity, 58: 101459, 2020. doi: 10.1016/j.jco.2020.101459
-
[33]
N.I.M. Gould, M.E. Hribar, and J. Nocedal. On the solution of equality constrained quadratic programming problems arising in optimization.SIAM Journal on Scientific Computing, 23(4):1376–1395, 2001. doi: 10.1137/S1064827598345667
-
[34]
Michel Grenier, J. Gagnon, C. Mercier, and J. Richard. Short-term load forecasting at Hydro-Qu´ ebec Trans´Energie. In2006 IEEE Power Engineering Society General Meeting, Montreal, QC, Canada, 2006. doi: 10.1109/PES.2006.1709029
-
[35]
M.R. Hestenes. Multipler and gradient methods.Journal of Optimization Theory and Applications, 4:303–320, 1969. doi: 10.1007/BF00927673
-
[36]
W. Hock and K. Schittkowski.Test Examples for Nonlinear Programming Codes, volume 187 ofLecture Notes in Economics and Mathematical Systems. Springer Berlin, Heidelberg, 2nd edition, 1980. doi: 10.1007/978-3-642-48320-2
-
[37]
J. Huschens. On the use of product structure in secant methods for nonlinear least squares problems.SIAM Journal on Optimization, 4(1):108–129, 1994. doi: 10.1137/0804005
-
[38]
J.E.Dennis Jr, H.J. Martinez, and R.A. Tapia. Convergence theory for the structured BFGS method with an application to nonlinear least squares.Journal of Optimization Theory and Applications, 61(2):161–178, 1999. doi: 10.1007/BF00962795
-
[39]
K. Levenberg. A method for the solution of certain non-linear problems in least squares. Quarterly of Applied Mathematics, 2:164–168, 1944. doi: 10.1090/qam/10666
-
[40]
Z. Li, M. Osborne, and T. Prvan. Adaptative algorithm for constrained least-squares problems.Journal of Optimization Theory and Applications, 114:423–441, 2002. doi: 10.1023/A:1016043919978. 33
-
[41]
Lin and J.J Mor´ e
C.-J. Lin and J.J Mor´ e. Newton’s method for large bound-constrained optimiza- tion problems.SIAM Journal on Optimization, 9(4):1100–1127, 1999. doi: 10.1137/ S1052623498345075
1999
-
[42]
Lukˇ san and J
L. Lukˇ san and J. Vlˇ cek. Sparse and partially separable test problems for unconstrained and equality constrained optimization. Technical Report 767, Institute of Computer Science, Academy of Sciences of the Czech Republic, 1999
1999
-
[43]
Lukˇ san, C
L. Lukˇ san, C. Matonoha, and J. Vlˇ cek. Hybrid methods for nonlinear least squares prob- lems. Technical Report 1246, Institute of Computer Science, Academy of Sciences of the Czech Republic, 2019
2019
-
[44]
N. Mahdavi-Amiri and R.H. Bartels. Constrained nonlinear least squares: An exact penalty approach with projected structured quasi-Newton update.ACM Transcations on Mathematical Software, 15(3):220–242, 1989. doi: 10.1145/66888.66891
-
[45]
D.W. Marquardt. An algorithm for least squares estimation of non-linear parameters. SIAM Journal, 11:431–441, 1963. doi: 10.1137/0111030
doi:10.1137/0111030 1963
-
[46]
Migot, D
T. Migot, D. Monnet, D. Orban, and S. Abel Soares. JSOSolvers.jl: Unconstrained and bound-constrained optimization solvers.Journal of Open Source Software, 11(117):9467,
-
[47]
URLhttps://doi.org/10.21105/joss.09467
doi: 10.21105/joss.09467. URLhttps://doi.org/10.21105/joss.09467
-
[48]
Mor´ e and G
J.J. Mor´ e and G. Toraldo. On the solution of large quadratic programming problems with bound constraints.SIAM Journal on Optimization, 1(1):93–113, 1991. doi: 10.1137/ 0801008
1991
-
[49]
B.A. Murtagh and M.A. Saunders. Large-scale linearly constrained optimization.Mathe- matical Programming, 14:41–72, 1978. doi: 10.1007/BF01588950
-
[50]
J. Nocedal and S.J. Wright.Numerical Optimization. Springer series in Operation Reasearch and Financial Engineering. Springer, New York, NY, USA, 2nd edition, 2006. doi: 10.1007/978-0-387-40065-5
-
[51]
D. Orban. BenchmarkProfiles.jl, 2019. URLhttps://doi.org/10.5281/zenodo. 4630955
doi:10.5281/zenodo 2019
-
[52]
Orban and A.S
D. Orban and A.S. Siqueira. A regularization method for constrained nonlinear least squares.Computational Optimization and Applications, 76(3):961–989, 2020. doi: 10. 1007/s10589-020-00201-2
2020
-
[53]
Porcelli
M. Porcelli. On the convergence of an inexact Gauss-Newton trust-region method for nonlinear least-squares problems with simple bounds.Optimization Letters, 7:447–465,
-
[54]
doi: 10.1007/s11590-011-0430-z
-
[55]
M.J.D. Powell. A method for nonlinear constraints in minimization problems. In R. Fletcher, editor,Optimization, pages 283–298. Academic Press, London, 1969
1969
-
[56]
H. Pu, Z. Chen, B. Wang, and W. Xia. Constrained least squares algorithms for nonlin- ear unmixing of hyperspectral imagery.IEEE Transactions on Geoscience and Remote Sensing, 53(3):1287–1303, 2015. doi: 10.1109/TGRS.2014.2336858. 34
-
[57]
R.T. Rockafellar. The multiplier method of Hestenes and Powell applied to convex pro- gramming.Journal of Optimization Theory and Applications, 12(6):555–562, 1973. doi: 10.1007/BF00934777
-
[58]
Schittkowski.More Test Examples for Nonlinear Programming Codes
K. Schittkowski.More Test Examples for Nonlinear Programming Codes. Lecture Notes in Economics and Mathematical Systems. Springer Berlin, Heidelberg, 1987. doi: 10.1007/ 978-3-642-61582-5
1987
-
[59]
Soares Siqueira and D
A. Soares Siqueira and D. Orban. NLSProblems.jl, 2019. URLhttps://doi.org/10. 5281/zenodo.4605405
2019
-
[60]
Soares Siqueira and D
A. Soares Siqueira and D. Orban. SolverBenchmark.jl, 2020. URLhttps://doi.org/10. 5281/zenodo.3948381
2020
-
[61]
Soares Siqueira and D
A. Soares Siqueira and D. Orban. NLPModelsIpopt.jl, 2026. URLhttps://doi.org/10. 5281/zenodo.19420624
2026
-
[62]
R. Tapia. On secant updates for use in general constrained optimization.Mathematics of Compuations, 51(183):181–202, 1988. doi: 10.2307/2008585
-
[63]
I.B. Tjoa and L.T. Biegler. Simultaneous solution and optimization strategies for pa- rameter estimation of differential-algebraic equation systems.Industrial & Engineering Chemistry Research, 30(2):376–385, 1991. doi: 10.1021/ie00050a015
-
[64]
Ph.L. Toint and D. Tuyttens. LSNNO, a FORTRAN subroutine for solving large-scale nonlinear network optimization problems.ACM Transactions on Mathematical Software, 18(3):308–328, 1992. doi: 10.1145/131766.131771
-
[65]
A. W¨ achter and L.T. Biegler. On the implementation of an interior-point filter line-search algorithm for large-scale nonlinear programming.Mathematical Programming, Series A, 106:25–57, 2006. doi: 10.1007/s10107-004-0559-y
-
[66]
H. Yabe and T. Takahashi. Factorized quasi-Newton methods for nonlinear least squares problems.Mathematical Programming, 51:75–100, 1991. doi: 10.1007/BF01586927
-
[67]
Zhang, L.H
J.Z. Zhang, L.H. Chen, and N.Y. Deng. A family of scaled factorized broydon-like methods for nonlinear least squares problems.SIAM Journal on Optimization, 10(4):1163–1179,
-
[68]
doi: 10.1137/S105262349834530
-
[69]
W. Zhou and X. Chen. Global convergence of a new hybrid Gauss-Newton structured BFGS method for nonlinear least squares problems.SIAM Journal on Optimization, 20 (5):2422–2441, 2010. doi: 10.1137/090748470. 35 Appendix A Cauchy point computation For the sake of clarity, we omit the iteration index during the rest of this appendix. We describe the procedur...
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