REVIEW 2 major objections 3 minor 38 references
Contractive difference-of-convex algorithms
T0 review · 2 major / 3 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read The paper proves that each linearized proximal subproblem in a difference-of-convex method is a fixed point problem for a contraction, and uses that reformulation to build cDCA, an algorithm with an adaptive inner stopping rule and global…
desk verdict A clean contraction-based subproblem solver with a genuinely adaptive stopping rule; the whole-sequence convergence claim overreaches for the ℓ1−2 experiments. 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 composite map $\operatorname{Prox}_{\mu g}[T_k^{\lambda,\mu}] = \operatorname{Prox}_{\mu g}[(1-\mu\lambda)(I-\frac{\mu}{1-\mu\lambda}\nabla f)(\cdot)+\mu\lambda x_k+\mu\eta_k]$. It is a contraction because $\nabla f$ is $1/L_f$-cocoercive, so $I-\nu\nabla f$ is nonexpansive for $\nu\le2/L_f$, and $\operatorname{Prox}_{\mu g}$ is firmly nonexpansive; the affine shift preserves the contraction constant $1-\mu\lambda$. This object carries the argument: it turns the convex subproblem into a Banach fixed point problem, justifies Picard iteration, and makes the adaptive stopping rule (16) natural, because inner contraction error can be controlled by outer displacement. The auxiliary function $E$ then supplies the descent framework that drives both subsequential and whole-sequence convergence.
What would settle it
Run cDCA on a one-dimensional problem with $f(x)=x^2/2$, $g=0$, and $h(x)=|x|^{3/2}$, so $\nabla h$ is convex but not Lipschitz on the convex hull of any sequence passing through $0$, while the other hypotheses of Theorem 4.1 hold. If the outer sequence still converges to a critical point, the Lipschitz-gradient assumption in Theorem 4.2 is not needed; if it fails or stalls, that assumption is load-bearing. Also record whether the inner termination rule (16) is ever satisfied when two consecutive outer iterates coincide exactly.
Extended reading notes
Core claim
Lemma 3.1 is the hinge: for $x_k$ and $\eta_k\in\partial h(x_k)$, the unique minimizer of (10) is exactly the unique fixed point of $\operatorname{Prox}_{\mu g}[T_k^{\lambda,\mu}]$, where $T_k^{\lambda,\mu}(x)=(1-\mu\lambda)x-\mu\nabla f(x)+\mu\lambda x_k+\mu\eta_k$. For $\mu\in(0,2/(2\lambda+L_f)]$ the composed operator is a contraction with coefficient $1-\mu\lambda$, and the best coefficient $L_f/(2\lambda+L_f)$ is attained at $\mu=2/(2\lambda+L_f)$. The paper therefore replaces the usual inner proximal-gradient loop by Picard iteration of this contraction, terminated by (16): $\|x_{m_k}^{k+1}-x_{m_k-1}^{k+1}\|\le\delta\|x_{k-1}-x_k\|$ with $\delta\in(0,2\lambda/L_f)$. The convergence analysis shows the auxiliary function $E(x,y)=F(x)+\tau\|x-y\|^2$ decreases along the pairs $(x_k,x_{k-1})$, that outer step lengths are square-summable, and that all cluster points satisfy $0\in\nabla f(x)+\partial g(x)-\partial h(x)$. Theorem 4.2 then upgrades subsequential convergence to whole-sequence convergence when $E$ has the KL property at the limit and $\nabla h$ is Lipschitz on the convex hull of the iterates.
Load-bearing premise
For the whole sequence to converge (not just along subsequences), the proof needs the slope of the convex function $h$ to change at a bounded rate all along the path the iterates travel, and the auxiliary function $E$ to satisfy a standard asymptotic flatness condition; neither follows from the basic assumptions and neither is verified in the experiments.
Editorial extensions
If this is right
- Every outer iteration of cDCA needs only finitely many inner Picard steps when consecutive outer iterates differ, so no user-supplied inner tolerance sequence is required.
- All cluster points of the cDCA sequence are critical points of $F$, and the sequence of objective values converges.
- Adding the KL property and Lipschitz differentiability of $h$ along the iterates upgrades this to convergence of the whole sequence to a critical point, with rates when the KL exponent is known.
- In the reported experiments on $\ell_1\text{-}2$ and logarithmic regularized least squares, cDCA uses fewer total iterations and less CPU time than ADCA and pDCAe at comparable final objective values.
Reading between the lines
- Beyond the paper, the contraction constant $L_f/(2\lambda+L_f)$ gives a direct bound on the inner iteration count: aiming for inner error $\varepsilon$ costs about $\log(\varepsilon)/\log(L_f/(2\lambda+L_f))$ Picard steps, so the adaptive rule can be converted into an explicit worst-case complexity estimate.
- Beyond the paper, the fixed-point equivalence depends on $g$ only through $\operatorname{Prox}_{\mu g}$, so the same contraction argument should apply to any convex prox-friendly regularizer without changing the inner-loop analysis.
- Beyond the paper, a natural test of Theorem 4.2's necessity is to run cDCA with $h(x)=|x|^{3/2}$ in one dimension; if the whole sequence converges anyway, the $L_h$-Lipschitz assumption could be removed or weakened.
- Beyond the paper, the adaptive stopping criterion (16) is portable: any outer method that produces a stabilizing quadratic term could adopt a relative inner-error rule keyed to the previous outer displacement instead of an absolute tolerance schedule.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the difference-of-convex minimization problem min F(x)=f(x)+g(x)-h(x), where f is smooth convex with Lipschitz gradient, g is prox-friendly convex, and h is convex. Its first contribution is Lemma 3.1, which reformulates the linearized proximal method (LPM) subproblem as a fixed-point problem of a contraction, Prox_{μg}[T_k^{λ,μ}], with contraction coefficient 1-μλ when μ∈(0,2/(2λ+L_f)]. Building on this, the authors propose cDCA, which solves each LPM subproblem approximately by Picard iteration with the adaptive inner stopping rule (16), and an outer stopping rule in Step 2. The main theoretical results are Theorem 4.1, establishing subsequential convergence to a critical point and summability of squared successive displacements, and Theorem 4.2, establishing whole-sequence convergence under a Kurdyka-Łojasiewicz condition on an auxiliary function E and Lipschitz differentiability of ∇h on the convex hull of the generated sequence. Numerical experiments compare cDCA with ADCA and pDCAe on ℓ1-2 and logarithmic regularized least-squares problems.
Significance. The contraction reformulation of the LPM subproblem is clean, and the adaptive inner termination rule is a useful idea: if the theory holds, each outer iteration is computed by finitely many inner steps whose error is controlled by the previous outer displacement, avoiding a fixed inner tolerance schedule. The descent estimate (17) in Theorem 4.1 is carefully derived, and the subsequential convergence proof is largely sound. The main limitation is that the whole-sequence convergence theorem's smoothness assumption is not verified for the two numerical test problems, so the advertised global convergence is broader than what is actually established for those examples.
major comments (2)
- [Section 5, Eqs. (32)–(33) and Theorem 4.2] Theorem 4.2 requires ∇h to be L_h-Lipschitz continuous on co({x_k}). For the ℓ1−2 test problem (32), h(x)=γ‖x‖, which is not differentiable at 0; for the logarithmic problem (33), the chosen h(x)=∑γ[|x_i|/ε − log(|x_i|+ε)+log ε] is also nondifferentiable on coordinate hyperplanes. The sentence after (32) asserting that γ<1/(2‖A^T b‖∞) makes the assumptions of Theorem 4.2 satisfied is not justified: the cited passage in [34] concerns a different algorithm, and no argument is given that the cDCA iterates, or their convex hull, avoid the nondifferentiability set. Consequently the abstract's whole-sequence convergence claim is not established for either numerical test case; only the subsequential Theorem 4.1 is guaranteed there. Please either prove the needed avoidance/differentiability property for cDCA or explicitly state that Theorem 4.2 applies only to problems whose h is differentiable on the generated convex hull.
- [Theorem 4.1(ii), around Eq. (27)] In Step 2 of Algorithm 3.2, when the first inner tolerance test fails, the algorithm sets x_{k+1}=x_{m_k+1}^{k+1}; the stationarity equation (19) and its limit version (27) are written only for the iterate x_{m_k}^{k+1}. The sentence following (24) says the extra case follows 'by a similar derivation', but the analogous equation for x_{m_k+1}^{k+1} is not displayed, and the proof of (ii) invokes (19) directly for the final outer iterate. The gap is easily repaired because the contraction property gives ||x_{m_k+1}^{k+1}−x_{m_k}^{k+1}|| ≤ (1−μλ)||x_{m_k}^{k+1}−x_{m_k−1}^{k+1}|| ≤ δ||x_k−x_{k−1}||, so the same bound (16) holds for the extra iterate; please add this argument.
minor comments (3)
- [Section 3.2, Lemma 3.2] The stopping criterion is only shown to detect a critical point in the exact equality case x_{m_k}^{k+1}=x_k=x_{m_k+1}^{k+1}. In finite precision the algorithm stops when the relative distance is below tol, but no quantitative relation between tol and a subgradient residual of F is given; the text after the lemma calls the output an 'approximate critical point', which is stronger than what is proven. A bound along the lines of Lemma 4.1 would make the termination rule more defensible.
- [Sections 5.1 and 5.2] The parameter λ is tuned in Section 5.1 on the same distribution of random instances used for the comparisons in Table 3 and Table 4, while ADCA and pDCAe use literature defaults; the reported advantages in iterations and CPU time should be interpreted with this selection bias in mind.
- [Throughout] Typographical issues include 'ffrstly' and 'ffxed' in the abstract, the corrupted string 'Kurdyka—/suppress Lojasiewicz' in Definition 2.1 and Remark 4.2, and minor grammar issues in the abstract; these should be cleaned up.
Circularity Check
No circularity in the derivation; the fixed-point equivalence is proved from the optimality conditions, and the convergence results rest on explicit assumptions rather than on the authors' own prior results.
full rationale
The paper's central derivation is self-contained. Lemma 3.1 derives the equivalence between the LPM subproblem (10) and the fixed-point equation (12) directly from the first-order optimality condition, displayed in the chain (13); the operator Prox_{μg}[T_k^{λ,μ}] is shown to be a contraction using standard facts (Lemma 2.5 cocoercivity of ∇f, Lemma 2.1 on I−ν∇f, Lemma 2.4 firm nonexpansiveness of Prox), with μ/(1−μλ) ≤ 2/Lf verified from the choice μ ≤ 2/(2λ+Lf). Algorithm 3.2 then approximates that fixed point by Picard iteration with the adaptive stopping rule (16); Theorem 4.1 obtains summability of successive differences from (24), boundedness of the subgradients {η_k} from Lemma 2.2, and subdifferential closure from Lemma 2.3, without invoking any prior result of the authors. Theorem 4.2 uses the external result [22, Thm 3.7] only after Lemma 4.1 establishes the needed subgradient bound under the explicitly stated assumption that ∇h is Lh-Lipschitz on co({x_k}); this is an independent criterion, not an input smuggled in by self-citation. The only self-citation, [9] in Remark 3.1, merely motivates a heuristic choice of the inner-loop initial point, which the theory explicitly permits to be arbitrary, so it is not load-bearing. Two non-circular limitations should be noted separately: tuning λ in Section 5.1 on the same random-instance distribution used in Section 5.2 weakens the experimental comparison but does not enter the derivation chain; and the assertion after Eq. (32) that γ < 1/(2||A^T b||∞) makes the assumptions of Theorem 4.2 satisfied, citing [34], is a scope/correctness question because h = γ||x|| is nondifferentiable at 0, so Theorem 4.2 may not cover the ℓ1−2 test case as stated. Neither issue is a circular reduction of the paper's mathematical claims.
Assumptions & free parameters
free parameters (4)
- lambda =
0.1 L_f (chosen by sweep in Section 5.1)
- delta =
1.99 lambda / L_f
- tol =
varies by regularizer and problem size (e.g., 1e-6 for L1-2, 6.5e-5 or 2e-5 for logarithmic)
- alpha_k and beta_k =
0.6 for all k in experiments
assumptions (4)
- domain assumption f is convex with L_f-Lipschitz gradient; g is proper, lsc, convex, and prox-friendly; h is convex and finite; F = f+g-h is level-bounded.
- standard math The gradient of a convex function with L-Lipschitz gradient is 1/L-cocoercive (Lemma 2.5) and I - nu T is nonexpansive for cocoercive T (Lemma 2.1).
- standard math Prox_{mu g} is firmly nonexpansive and the proximal characterization p = Prox_{alpha phi}(x) iff (x-p)/alpha in partial phi(p).
- domain assumption For Theorem 4.2, E has the Kurdyka-Lojasiewicz property at (x_hat, x_hat) and grad h is L_h-Lipschitz on co({x_k}).
Cite this review
Pith. "Pith review of Contractive difference-of-convex algorithms." pith.science (2026). https://pith.science/paper/RCDNEFE7
@misc{pith2026250510800,
author = {Pith},
title = {Pith review of: Contractive difference-of-convex algorithms},
year = {2026},
howpublished = {\url{https://pith.science/paper/RCDNEFE7}},
note = {Machine review of arXiv:2505.10800}
}
read the original abstract
The difference-of-convex algorithm (DCA) and its variants are the most popular methods to solve the difference-of-convex optimization problem. Each iteration of them is reduced to a convex optimization problem, which generally needs to be solved by iterative methods such as proximal gradient algorithm. However, these algorithms essentially belong to some iterative methods of fixed point problems of averaged mappings, and their convergence speed is generally slow. Furthermore, there is seldom research on the termination rule of these iterative algorithms solving the subproblem of DCA. To overcome these defects, we ffrstly show that the subproblem of the linearized proximal method (LPM) in each iteration is equal to the ffxed point problem of a contraction. Secondly, by using Picard iteration to approximately solve the subproblem of LPM in each iteration, we propose a contractive difference-ofconvex algorithm (cDCA) where an adaptive termination rule is presented. Both global subsequential convergence and global convergence of the whole sequence of cDCA are established. Finally, preliminary results from numerical experiments are promising.
Figures
Reference graph
Works this paper leans on
-
[34]
Wen, B., Chen, X., Pong, T.K.: A proximal difference-of-conex alg orithm with extrapolation. Comput. Optim. Appl. 69, 297–324 (2018)
work page 2018
-
[1]
Alvarado, A., Scutari, G., Pang, J.S.: A new decomposition method f or mul- tiuser DC-programming and its applications. IEEE Trans. Signal Process. 62, 2984–2998 (2014)
work page 2014
-
[2]
An, L.T.H., Tao, P.D.: The DC (difference of convex functions) prog ramming and DCA revisited with DC models of real world nonconvex optimization prob- lems. Ann. Oper. Res. 133, 23–46 (2005)
work page 2005
-
[3]
Arag´ on Artacho, F.J., Vuong, P.T.: The boosted difference of convex functions algorithm for nonsmooth functions, SIAM J. Optim. 30, 980–1006 (2020)
work page 2020
-
[4]
Banert, S., Bot, R.I.: A general double-proximal gradient algorit hm for d.c. programming. Math. Program. 178, 301–326 (2019)
work page 2019
-
[5]
Bauschke, H.H., Combettes, P.L.: Convex Analysis and Monotone Operator Theory in Hilbert Spaces , 2nd ed. Springer, New York, 2017. 18
work page 2017
-
[6]
Beck, A., Teboulle, M.: A fast iterative shrinkage-thresholding alg orithm for linear inverse problems. SIAM J. Imaging Sci. 2, 183–202 (2009)
work page 2009
-
[7]
Gotoh, J., Takeda, A., Tono, K.: DC formulations and algorithms fo r sparse optimization problems. Math. Program. 169, 141–176 (2018)
work page 2018
Show all 38 references
-
[8]
de Oliveira, W.: The ABC of DC Programming, Set-Valued Var. Anal. 28, 679–706 (2020)
2020
-
[9]
et al.: MiKM: multi-step inertial Kras- nosel’skiˇ ı–Mann algorithm and its applications
Dong, Q.L., Huang, J.Z., Li, X.H. et al.: MiKM: multi-step inertial Kras- nosel’skiˇ ı–Mann algorithm and its applications. J. Glob. Optim. 73, 801–824 (2019)
2019
-
[10]
Le Thi, H.A., Le, H.M., Phan, D.N., Tran, B.: Stochastic DCA for minimiz ing a large sum of DC functions with application to multi-class logistic regre ssion, Neural Networks, 132, 220–231 (2020)
2020
-
[11]
4th International Conference on Advanced Data Mining and Applications
Le Thi, H.A., Nguyen, V.V., Ouchani, S.: Gene selection for cancer c lassifica- tion using DCA. 4th International Conference on Advanced Data Mining and Applications. 5139, 62–72 (2008)
2008
-
[12]
Le Thi, H.A., Pham Dinh, T.: Open issues and recent advances in DC p ro- gramming and DCA. J. Glob. Optim. (2023). https://doi.org/10.1007/s10898- 023-01272-1
2023 doi
-
[13]
Le Thi, H.A., Pham, D.T.: DC programming and DCA: thirty years of d evel- opments. Math. Program. 169, 5–68 (2018)
2018
-
[14]
Lin, D., Liu, C.: The modified second APG method for DC optimization prob- lems. Optim. Lett. 13, 805–824 (2019)
2019
-
[15]
Liu, T., Pong, T.K., Takeda, A.: A refined convergence analysis of pDCAe with applications to simultaneous sparse recovery and outlier detection . Comput. Optim. Appl. 73, 69–100 (2019)
2019
-
[16]
Liu, T., Pong, T.K., Takeda, A.: A successive difference-of-conv ex approxima- tion method for a class of nonconvex nonsmooth optimization proble ms. Math. Program. 176, 339–367 (2019)
2019
-
[17]
Lou, Y., Yan, M.: Fast L1-L2 Minimization via a proximal operator. J. Sci. Comput. 74, 767–785 (2018)
2018
-
[18]
Lu, Z., Zhou, Z.: Nonmonotone enhanced proximal DC algorithms for a class of structured nonsmooth DC programming. SIAM J. Optim. 29(4), 2725–2752 (2019) 19
2019
-
[19]
Lu, Z., Zhou, Z., Sun, Z.: Enhanced proximal DC algorithms with extrapolation for a class of structured nonsmooth DC minimization. Math. Program. 176, 369–401 (2019)
2019
-
[20]
Nesterov, Y.E.: A method for solving the convex programming pr oblem with convergence rate O(1/k2), Dokl. Akad. Nauk SSSR , 269, 543–547 (1983) (in Russian)
1983
-
[21]
Oblomskaja, L.: Methods of successive approximation for linear equations in Banach spaces, USSR Compt. Math. and Math. Phys. 8, 239–253 (1968)
1968
-
[22]
Imaging Sci
Ochs, P., Chen, Y., Brox, T., Pock, T.: iPiano: Inertial proximal a lgorithm for nonconvex optimization, SIAM J. Imaging Sci. 7(2), 1388–1419 (2014)
2014
-
[23]
Pang, J.S., Razaviyayn, M., Alvarado, A.: Computing B-stationar y points of nonsmooth DC programs. Math. Oper. Res. 42(1), 95–118 (2017)
2017
-
[24]
Pham, T.N., Dao, M.N., Amjady, N., Shah, R.: A proximal splitting algo - rithm for generalized DC programming with applications in signal recov ery, 2024 https://arxiv.org/abs/2409.01535
2024 arXiv
-
[25]
Acta Math
Pham, D.T., Le Thi, H.A.: Convex analysis approach to DC programm ing: theory, algorithms and applications. Acta Math. Vietnam. 22, 289–355 (1997)
1997
-
[26]
optimization algorithm for solving the trust-region subproblem
Pham, D.T., Le Thi, H.A.: A D.C. optimization algorithm for solving the trust-region subproblem. SIAM J. Optim. 8, 476–505 (1998)
1998
-
[27]
Fermat Day 85: Mathematics for Optimization, North Holland 1986
Pham Dinh, T., Souad, E.B.: Algorithms for solving a class of nonconvex opti- mizations problems: methods of subgradient . Fermat Day 85: Mathematics for Optimization, North Holland 1986
1986
-
[28]
In: Pro- ceedings of the 27th International Joint Conference on Artificial Intelligence, IJCAI-18, pp
Phan, D.N., Le, H.M., Le Thi, H.A.: Accelerated difference of convex func- tions algorithm and its application to sparse binary logistic regression . In: Pro- ceedings of the 27th International Joint Conference on Artificial Intelligence, IJCAI-18, pp. 1369–1375. International J...
2018
-
[29]
Liang, J.: Geometry of first-order methods and adap tive acceleration, arXiv:2003 .03910
Poon, C.. Liang, J.: Geometry of first-order methods and adap tive acceleration, arXiv:2003 .03910
2003
-
[30]
Rockafellar, R.T.: Convex Analysis , Princeton University Press, Princeton, 1970
1970
-
[31]
Sun, W.Y., Sampaio, R.J.B., Candido, M.A.B.: Proximal point algorithm f or minimization of DC Functions. J. Comput. Math. 21(4), 451–462 (2003) 20
2003
-
[32]
Syrtseva, K., de Oliveira, W., Demassey, S., van Ackooij, W.: Minimiz ing the difference of convex and weakly convex functions via bundle method . Pac. J. Optim. 20(4), 499-741, (2024)
2024
-
[33]
Springer, Berlin 2016
Tuy, H.: Convex Analysis and Global Optimization, 2nd edn. Springer, Berlin 2016
2016
-
[35]
Xu, H.K.: Averaged mappings and the gradient-projection algor ithm. J. Optim. Theory Appl. 150, 360–378 (2011)
2011
-
[36]
Ye, J.J., Yuan, X., Zeng, S., Zhang, J.: Difference of convex algor ithms for bilevel programs with applications in hyperparameter selection.Math. Program. 198, 1583–1616 (2023)
2023
-
[37]
Yin, P., Lou, Y., He, Q., Xin, J.: Minimization of ℓ1−2 for compressed sensing. SIAM J. Sci. Comput. 37(1), A536–A563 (2015)
2015
-
[38]
Yu, P., Pong, T.K., Lu, Z.: Convergence rate analysis of a sequen tial convex programming method with line search for a class of constrained differ ence-of- convex optimization problems, SIAM J. Optim. 31, 2024–2054 (2021) 21
2021
Reviewed August 15, 2026 · model on record in the stance chip above.
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