REVIEW 4 major objections 7 minor 32 references
rDSM -- A robust Downhill Simplex Method software package for optimization problems in high dimensions
T0 review · 4 major / 7 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read The rDSM package claims that reshaping degenerated simplices and re-averaging persistent vertices lets the downhill simplex method reach global minima on noisy and high-dimensional problems where the classic method stalls.
desk verdict Central 2D test is irreproducible as written; the package itself is plausible and deserves a corrected round. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the simplex itself, treated as a geometric figure that can lose dimensionality and as a set of vertices whose recorded costs can be corrupted by noise. Two mechanisms carry the argument. Degeneracy correction compares the shortest edge to the longest (edge test) and the n-th root of the determinant-based volume to the edge lengths (volume test); when either ratio falls below a threshold (default 0.1), the worst vertex is moved by solving a constrained maximization: maximize simplex volume subject to fixed perimeter, using a Newton-type solver, iterating over vertices from worst to best until the simplex is non-degenerate. Reevaluation attaches a counter to each vertex,
What would settle it
Run the 2D obstacle test and the 5D valley test with each enhancement disabled in turn: if turning off degeneracy correction does not degrade rDSM's result, the volume-maximization step is not carrying the claimed gain, and the same test applies to reevaluation on the noisy problems. A sharper check: add a constant positive bias to every evaluation (for instance, U[0.01,0.03] noise); because averaging cannot remove a nonzero mean, rDSM should fail to approach the true optimum. If it converges anyway, the reevaluation mechanism is working differently than the paper's averaging argument suggests
Extended reading notes
Core claim
The central claim is that the two failure modes of the classic downhill simplex method—geometric degeneration and noise-induced spurious minima—can be corrected without leaving the derivative-free framework. A simplex is declared degenerated when its shortest edge is far shorter than its longest, or when its volume is too small relative to its edge lengths; rDSM then moves the worst-scoring vertex to a position that maximizes the simplex volume while keeping the perimeter unchanged. For noise, any vertex that remains in the simplex for more than 1.5n iterations is re-evaluated and its objective value is replaced by the mean of all its past evaluations. On the 2D obstacle problem, DSM stops a
Load-bearing premise
The reevaluation step's claimed benefit depends on repeated evaluations of a fixed vertex averaging out to the true objective value, which requires zero-mean, independent noise; under biased or state-dependent noise, including the positive-mean uniform noise used in some of the paper's own tests, the averaged value is systematically offset.
Editorial extensions
If this is right
- For the 2D obstacle test, rDSM reaches the global-minimum neighborhood (J≈0.0047) while plain DSM stalls at J≈0.2269 on a domain boundary.
- Across 20 noisy runs with uniform and Gaussian perturbations, rDSM converges closer to the known optimum with smaller spread in both endpoint and final cost.
- On the 5D valley function, rDSM reaches objective 2.92e-10 versus DSM's 1.66e-5, but requires about 97 more evaluations and roughly 19x the run time.
- The mechanisms preserve the derivative-free character of DSM, so the package can be applied when gradients are unavailable, including CFD-based or experimental objective functions.
- The paper's own outlook is that rDSM improves applicability to higher dimensions and noise but does not by itself resolve very-high-dimensional scaling; the authors point to dimensionality reduction and hybrid solvers as next steps.
Reading between the lines
- The degeneracy-correction step is formulated as a constrained maximization of the worst vertex, so it could be detached from rDSM and applied to other simplex-based optimizers, including those with adaptive coefficients.
- The reported run-time jump (64.6 s vs 3.4 s in 5D) suggests the volume-maximization subproblem becomes the bottleneck as dimension grows; a testable extension would be to approximate the correction or trigger it less often in higher dimensions.
- The reevaluation trigger (1.5n) is a fixed heuristic; making it depend on the estimated noise variance, or using a weighted average that discounts stale evaluations, could improve the trade-off between evaluation budget and accuracy.
- A direct prediction is that on smooth, unimodal, noiseless problems where the simplex never degenerates, rDSM behaves like classic DSM; deviations from that baseline would indicate the enhancements are doing more than the paper describes.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces rDSM, a MATLAB software package that extends the classical downhill simplex method (DSM) with two mechanisms: (i) detection and correction of degenerated simplices via constrained volume maximization at fixed perimeter, and (ii) reevaluation of persistent vertices by averaging historical objective evaluations to counter noise. The authors claim improved convergence, better robustness to noisy evaluations, and increased applicability to higher dimensions. They support this with a 2D analytical objective with and without an obstacle, a noisy 2D comparison, and one 5D Rosenbrock run. The code is available in a public repository with metadata and output traceability files.
Significance. If the results are reproducible, the package would be a useful, practical contribution for derivative-free optimization in experimental settings, where gradients are unavailable and evaluations are noisy. The manuscript is clearly written, the software is packaged with useful visualization and logging, and the two enhancements are easy to understand. The degeneracy correction is not entirely novel, since the authors themselves list Luersen and Le Riche [13] as prior work on correcting degenerated simplices, but the explicit implementation and the reevaluation mechanism are of interest. However, the current numerical evidence is substantially weakened by a direct inconsistency in the printed test function, by the absence of a comparison with [13], and by the very limited statistical support for the high-dimensional claim.
major comments (4)
- [§3.1, Eq. (6)] The printed objective function contradicts every reported result. For J(x1,x2)=-(x1-x2)^4+0.5 on [-1,1]^2, evaluation at the claimed optimum (1,-1) gives -15.5, not 0, and the actual minimum over the domain is -15.5 (at (1,-1) and (-1,1)), while 0.5 is the maximum attained along x1=x2. The reported DSM endpoint (0.9999,-0.9998) with J=9.2146×10^-5, the rDSM endpoint with J=0.0047, and all entries in Table 5 are therefore impossible under Eq. (6). Figures 4, 5 and Table 5 appear to have been generated with a different objective function than the one printed, making the central illustration for the claim 'rDSM can identify the global optimum in the scenarios where DSM fails' irreproducible as written. Please correct Eq. (6) or the reported values and regenerate the entire section consistently.
- [§2.4.3 and Table 5] The reevaluation step is justified by the statement that averaging repeated evaluations 'brings the result closer to the true value.' This is only valid for zero-mean noise. The paper's uniform-noise tests U[0,0.01] and U[0,0.02] have positive means 0.005 and 0.01, so the averaged estimate converges to J+0.005 or J+0.01, not to J. Thus the paper's own noise model does not support the unbiased-estimation rationale, and the observed improvement is attributable only to variance reduction. The authors should either use symmetric zero-mean noise (e.g., U[-a,a]) to match the stated rationale, or explicitly acknowledge that averaging reduces variance but does not remove a systematic bias, which can mis-rank vertices when the noise bias is state-dependent.
- [§3.2, Table 6] The high-dimensional claim rests on a single run of the 5D Rosenbrock function from one initial point. The text concludes that 'rDSM consistently identifies the global minimum,' but with n=1 there is no statistical basis for 'consistently.' Moreover, no comparison is made with the prior degeneracy-correcting method of Luersen and Le Riche [13], although Table 2 classifies [13] as already addressing degenerated simplices and the thresholds are said to be 'suggested in [13].' Without a comparison against [13] and without multiple random restarts, the claimed advantage over the closest prior method is not established. Please add repeated trials and a comparative benchmark, or soften the claims accordingly.
- [§3.1 and §2.4.2] The threshold values are adjusted to the specific test cases: θe and θv are said to be 'determined based on prior tests on 2D problems' and set to 0.1, while in §3.2 they are set to 1×10^-5. The reevaluation trigger 1.5n is calibrated on the same U[0,0.02] case that is then reported as the improvement. Since the reported gains are partly a consequence of thresholds tuned on the same examples, the paper should provide evidence that the thresholds transfer across problems (e.g., use fixed thresholds on a suite of benchmark functions) or present a sensitivity analysis showing the results do not depend critically on these choices.
minor comments (7)
- [§3.1] The iteration limit is inconsistent: the text says 'The iterative process is limited to 50 iterations, with a maximum of 100 evaluations,' but later states that rDSM 'ends at point ... after 100 iterations.' Please clarify whether the intended limit is in iterations or evaluations and use consistent wording.
- [Eq. (5)] The constraint writes P(x_s1,...,x_sn, y_s^{n+1}) = P(x_s1,...,x_sn, x_s^{n+1}), but y_s^{n+1} is the variable being optimized. Use separate symbols for the candidate point and the fixed old vertex to avoid confusion.
- [Eqs. (1) and (2)] The notation 'i=1,n' for edges is not defined for an n-dimensional simplex, which has n(n+1)/2 edges. Please define the edge indexing and explain why the ratio uses only n edges in the denominator of Eq. (2).
- [§3.2] The sentence 'For the convergence stability, the cost is divided by 10000 during the optimization process' is ambiguous: are the reported Jr values 1.66×10^-5 and 2.92×10^-10 the divided values or the original ones? Please state the convention explicitly.
- [§2.2] There is a typo: 'The “objection function” module' should be 'objective function'.
- [Table 5] The table caption says 'sample means and variances,' but the entries are presented as 'mean ± value.' Specify whether the ± term is a standard deviation, standard error, or confidence interval.
- [§3.2] The running times 3.4 s (DSM) versus 64.6 s (rDSM) imply a 19-fold slowdown while only 97 additional evaluations are performed. The source of this overhead (e.g., Newton–Raphson solves) should be explained, especially because the Impact section stresses efficiency.
Circularity Check
Minor in-sample calibration of the reevaluation threshold; no load-bearing self-citation or definitional circularity.
-
fitted input called prediction
[Sec. 3.1, final paragraph (calibration of csi threshold) and Table 5 (U[0,0.02] row)]
"To calibrate the reevaluation trigger, we compare the thresholds csi ≥1.5n and csi ≥2n by the uniform noise case U[0,0.02]. ... In contrast, when the coefficient is set as 1.5, rDSM can converge to (0.9621±0.0057, -0.9629±8×10−4) ... Table 5: JU[0,0.02] 0.1111±0.0014 0.0239±0.0003"
The reevaluation trigger threshold 1.5n is explicitly selected by comparing rDSM's performance on the U[0,0.02] noise case. The same U[0,0.02] case, with the same endpoint (0.9621, -0.9629) and J=0.0239, is then presented in Table 5 as evidence that rDSM outperforms DSM under noise. That row is therefore the calibration target rather than an independent prediction, so the reported gain partly reflects the chosen constant. Other noise cases and the obstacle/Rosenbrock tests are not calibration targets, so the circularity is partial and does not invalidate the core method.
full rationale
The rDSM derivation does not presuppose its headline result: degeneracy correction and reevaluation are defined from geometric and averaging principles, and the main claims are tested on external benchmark functions (Eq. (6), Eq. (7)) against a re-implemented DSM. No load-bearing uniqueness theorem or ansatz is imported from the authors' prior work; citations [1,12,13,20-28] are external and not used to forbid alternatives. The only circularity-adjacent issue is the in-sample calibration of the reevaluation threshold (1.5n) on the U[0,0.02] case, which is then reused as evidence in Table 5; this row is fitted rather than predicted. The remaining noise configurations and the obstacle/Rosenbrock demonstrations are independent, so the central claim has substantial independent content. Separately, the printed test function Eq. (6) is inconsistent with the stated optimum (J(1,-1)=-15.5, not 0) and with the reported positive J values; this is a correctness/reproducibility defect, not circularity.
Assumptions & free parameters
free parameters (3)
- Edge degeneracy threshold theta_e =
0.1 (2D), 1e-5 (5D Rosenbrock)
- Volume degeneracy threshold theta_v =
0.1 (2D), 1e-5 (5D Rosenbrock)
- Reevaluation trigger coefficient 1.5n =
1.5
assumptions (4)
- standard math The volume of an n-simplex is given by n!^{-1} times the absolute determinant in Eq. (4).
- ad hoc to paper Constrained volume maximization with fixed perimeter, Eq. (5), solved by Newton-Raphson, produces a useful corrected simplex.
- domain assumption Averaging repeated evaluations of a vertex approximates its true objective value.
- ad hoc to paper The threshold values theta_e=0.1, theta_v=0.1, and the 1.5n trigger transfer across optimization problems.
Cite this review
Pith. "Pith review of rDSM -- A robust Downhill Simplex Method software package for optimization problems in high dimensions." pith.science (2026). https://pith.science/paper/CMPZXRGS
@misc{pith2026250905917,
author = {Pith},
title = {Pith review of: rDSM -- A robust Downhill Simplex Method software package for optimization problems in high dimensions},
year = {2026},
howpublished = {\url{https://pith.science/paper/CMPZXRGS}},
note = {Machine review of arXiv:2509.05917}
}
read the original abstract
The Downhill Simplex Method (DSM) is a fast-converging derivative-free optimization technique for nonlinear systems. However, the optimization process is often subject to premature convergence due to degenerated simplices or noise-induced spurious minima. This study introduces a software package for the robust Downhill Simplex Method (rDSM), which incorporates two key enhancements. First, simplex degeneracy is detected and corrected by volume maximization under constraints. Second, the real objective value of noisy problems is estimated by reevaluating the long-standing points. Thus, rDSM improves the convergence of DSM, and may increase the applicability of DSM to higher dimensions, even in the presence of noise. The rDSM software package thus provides a robust and efficient solution for both analytical and experimental optimization scenarios. This methodological advancement extends the applicability of simplex-based optimization to complex experimental systems where gradient information remains inaccessible and measurement noise proves non-negligible.
Figures
Figures from the paper (2 more)
Reference graph
Works this paper leans on
-
[13]
Globalized nelder–mead method for engineering optimization.Comput Struct, 82(23-26):2251–2260, 2004
Luersen MA and Le Riche R. Globalized nelder–mead method for engineering optimization.Comput Struct, 82(23-26):2251–2260, 2004. https://doi.org/10.1016/j.compstruc.2004.03.072. 18
-
[1]
A Simplex Method for Function Minimization
Nelder JA and Mead R. A Simplex Method for Function Minimization. Comput J, 7(4):308–313, 1965. https://doi.org/10.1093/comjnl/7.4.308
-
[2]
Model-free power optimization of wind farm based on nelder-mead method
Xu Z, Geng H, and Chu B. Model-free power optimization of wind farm based on nelder-mead method. In2020 2nd International Conference on Smart Power & Internet Energy Systems (SPIES), pages 166–171,
-
[3]
Su H, Lan F, He Y, and Chen J. A modified downhill simplex algorithm interpolation response surface method for structural reliability analy- sis.Eng Comput, 37(4):1423–1450, 2020. https://doi.org/10.1108/ec- 03-2019-0085
doi:10.1108/ec- 2020
-
[4]
Wang H, Zhao H, Lin C, and Zhou Y. An optimization method for frame structure of unmanned helicopter based on empirical 17 optimization and downhill simplex method. In2019 Chinese Control And Decision Conference (CCDC), pages 720–725, 2019. https://doi.org/10.1109/CCDC.2019.8832553
-
[5]
Asadi KA, Abbas AA, and Hamdan AN. Optimization of the hy- drological tank model by downhill simplex method.Int J Civ Eng, 18:1433–1450, 2020. https://doi.org/10.1007/s40999-020-00540-5
-
[6]
Hardt M, Schraknepper D, and Bergs T. Investigations on the application of the downhill-simplex-algorithm to the in- verse determination of material model parameters for fe-machining simulations.Simul Modell Pract Theory, 107:102214, 2021. https://doi.org/10.1016/j.simpat.2020.102214
-
[7]
Kočí V, Kočí J, Čáchová M, Vejmelková E, and Černý R. Multi- parameter optimization of lime composite design using a modified downhill simplex method.Compos Part B-Eng, 93:184–189, 2016. https://doi.org/10.1016/j.compositesb.2016.03.025
Show all 32 references
-
[8]
Multi-startdownhillsimplexmethodforspatio-temporalsourcelocaliza- tion in magnetoencephalography.Electroencephalogr Clin Neurophysiol, 108(1):32–44, 1998
Huang M, Aine CJ, Supek S, Best E, Ranken D, and Flynn ER. Multi-startdownhillsimplexmethodforspatio-temporalsourcelocaliza- tion in magnetoencephalography.Electroencephalogr Clin Neurophysiol, 108(1):32–44, 1998. https://doi.org/10.1016/S0168-5597(97)00091-9
1998 doi
-
[9]
Detection and remediation of stagnation in the nelder– mead algorithm using a sufficient decrease condition.SIAM J
Kelley CT. Detection and remediation of stagnation in the nelder– mead algorithm using a sufficient decrease condition.SIAM J. Optim, 10(1):43–55, 1999. https://doi.org/10.1137/S1052623497315203
1999 doi
-
[10]
Enhancement of the downhill simplex method of optimiza- tion
Koshel JR. Enhancement of the downhill simplex method of optimiza- tion. InInternational Optical Design Conference, page ITuC2. Optica Publishing Group, 2002. https://doi.org/10.1364/IODC.2002.ITuC2
2002 doi
-
[11]
Simplex optimization method for illumination design.Optics letters, 30(6):649–651, 2005
Koshel JR. Simplex optimization method for illumination design.Optics letters, 30(6):649–651, 2005. https://doi.org/10.1364/OL.30.000649
2005 doi
-
[12]
Implementing the nelder-mead simplex algorithm with adaptive parameters.Comput Optim Appl, 51:259–277, 2012
Gao F and Han L. Implementing the nelder-mead simplex algorithm with adaptive parameters.Comput Optim Appl, 51:259–277, 2012. https://doi.org/10.1007/s10589-010-9329-3
2012 doi
-
[14]
A hybrid global optimization method for inverse esti- mation of hydraulic parameters: Annealing-simplex method.Water Re- sour Res, 34(9):2261–2269, 1998
Pan L and Wu L. A hybrid global optimization method for inverse esti- mation of hydraulic parameters: Annealing-simplex method.Water Re- sour Res, 34(9):2261–2269, 1998. https://doi.org/10.1029/98WR01672
1998 doi
-
[15]
Downhill simplex methods for optimizing simulated anneal- ing are effective
Bangert P. Downhill simplex methods for optimizing simulated anneal- ing are effective. InProceedings of ALGORITMY, pages 341–347, 2005
2005
-
[16]
Application of the genetic algorithm and downhill simplex methods (nelder–mead methods) in the search for the optimum chiller configuration.Appl Therm Eng, 61(2):433–442, 2013
Maehara N and Shimoda Y. Application of the genetic algorithm and downhill simplex methods (nelder–mead methods) in the search for the optimum chiller configuration.Appl Therm Eng, 61(2):433–442, 2013. https://doi.org/10.1016/j.applthermaleng.2013.08.021
2013 doi
-
[17]
Stabilization of the fluidic pinball with gradient- enriched machine learning control.J Fluid Mech, 917:A42, 2021
Cornejo Maceda GY, Li Y, Lusseyran F, Morzyński M, and Noack BR. Stabilization of the fluidic pinball with gradient- enriched machine learning control.J Fluid Mech, 917:A42, 2021. https://doi.org/10.1017/jfm.2021.301
2021 doi
-
[18]
Sta- bilization of a multi-frequency open cavity flow with gradient- enriched machine learning control.J Fluid Mech, 955:A20, 2023
Cornejo Maceda GY, Varon E, Lusseyran F, and Noack BR. Sta- bilization of a multi-frequency open cavity flow with gradient- enriched machine learning control.J Fluid Mech, 955:A20, 2023. https://doi.org/10.1017/jfm.2022.1050
2023
-
[19]
Cluster- based control for net drag reduction of the fluidic pinball.Phys Fluids, 35(2):023601, 2023
Wang X, Deng N, Cornejo Maceda GY, and Noack BR. Cluster- based control for net drag reduction of the fluidic pinball.Phys Fluids, 35(2):023601, 2023. https://doi.org/10.1063/5.0136499
2023 doi
-
[20]
The generalized cross product and the volume of a sim- plex.Appl Math Lett, 4(6):51–53, 1991
Cho EC. The generalized cross product and the volume of a sim- plex.Appl Math Lett, 4(6):51–53, 1991. https://doi.org/10.1016/0893- 9659(91)90074-6
1991 doi
-
[21]
Springer Berlin, Heidelberg, 2009
Abraham A, Hassanien AE, Siarry P, and Engelbrecht A.Founda- tions of Computational Intelligence. Springer Berlin, Heidelberg, 2009. https://doi.org/10.1007/978-3-642-01085-9
2009 doi
-
[22]
An additive approximation to multiplicative noise.Journal of Mathematical Imaging and Vision, 62:1227–1237, 2020
Nicholson R and Kaipio JP. An additive approximation to multiplicative noise.Journal of Mathematical Imaging and Vision, 62:1227–1237, 2020. https://doi.org/10.1007/s10851-020-00984-3
2020 doi
-
[23]
Noisy optimization prob- lems - a particular challenge for differential evolution? In Proceedings of the 2004 Congress on Evolutionary Computation (IEEE Cat
Krink T, Filipic B, and Fogel GB. Noisy optimization prob- lems - a particular challenge for differential evolution? In Proceedings of the 2004 Congress on Evolutionary Computation (IEEE Cat. No.04TH8753), volume 1, pages 332–339, 2004. https://doi.org/10.1109/CEC.2004.1330876. 19
2004 arXiv
-
[24]
A new particle swarm optimization algorithm for noisy optimization problems.Swarm Intelligence, 10:161–192, 2016
Taghiyeh S and Xu J. A new particle swarm optimization algorithm for noisy optimization problems.Swarm Intelligence, 10:161–192, 2016. https://doi.org/10.1007/s11721-016-0125-2
2016 doi
-
[25]
Distributed evolutionary optimization, in manifold: Rosenbrock’s function case study.In- form Sciences, 122(2):141–159, 2000
Bouvry P, Arbab F, and Seredynski F. Distributed evolutionary optimization, in manifold: Rosenbrock’s function case study.In- form Sciences, 122(2):141–159, 2000. https://doi.org/10.1016/S0020- 0255(99)00116-4
-
[26]
A note on the extended rosen- brock function.Evol Comput, 14(1):119–126, 2006
Shang Y and Qiu Y. A note on the extended rosen- brock function.Evol Comput, 14(1):119–126, 2006. https://doi.org/10.1162/evco.2006.14.1.119
2006 doi
-
[27]
Multiplicative update rules for accel- erating deep learning training and increasing robustness.Neurocomput- ing, 576:127352, 2024
Kirtas M, Passalis N, and Tefas A. Multiplicative update rules for accel- erating deep learning training and increasing robustness.Neurocomput- ing, 576:127352, 2024. https://doi.org/10.1016/j.neucom.2024.127352
2024
-
[28]
A benchmark of kriging- based infill criteria for noisy optimization.Struct Multidiscip O, 48:607–626, 2013
Picheny V, Wagner T, and Ginsbourger D. A benchmark of kriging- based infill criteria for noisy optimization.Struct Multidiscip O, 48:607–626, 2013. https://doi.org/10.1007/s00158-013-0919-4
2013 doi
-
[29]
Aerodynamic characterization of a fan-array wind generator.AIAA J, 62(1):291–301,
Li S, Liu Y, Jiang Z, Hu G, Noack BR, and Raps F. Aerodynamic characterization of a fan-array wind generator.AIAA J, 62(1):291–301,
-
[30]
Aero- dynamic characterization of a wind generator with 40×40 in- dividually controllable fans.Phys Fluids, 37(2):025117, 2025
Liu Y, Noack BR, Hu G, Chen J, Gao N, and Raps F. Aero- dynamic characterization of a wind generator with 40×40 in- dividually controllable fans.Phys Fluids, 37(2):025117, 2025. https://doi.org/10.1063/5.0243991. 20
2025 doi
-
[2020]
http://10.1109/SPIES48661.2020.9243030
2020
-
[2024]
https://doi.org/10.2514/1.J063114
Reviewed August 5, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.