## Abstract

We consider a class of difference-of-convex (DC) optimization problems whose objective is level-bounded and is the sum of a smooth convex function with Lipschitz gradient, a proper closed convex function and a continuous concave function. While this kind of problems can be solved by the classical difference-of-convex algorithm (DCA) (Pham et al. Acta Math Vietnam 22:289–355, 1997), the difficulty of the subproblems of this algorithm depends heavily on the choice of DC decomposition. Simpler subproblems can be obtained by using a specific DC decomposition described in Pham et al. (SIAM J Optim 8:476–505, 1998). This decomposition has been proposed in numerous work such as Gotoh et al. (DC formulations and algorithms for sparse optimization problems, 2017), and we refer to the resulting DCA as the proximal DCA. Although the subproblems are simpler, the proximal DCA is the same as the proximal gradient algorithm when the concave part of the objective is void, and hence is potentially slow in practice. In this paper, motivated by the extrapolation techniques for accelerating the proximal gradient algorithm in the convex settings, we consider a proximal difference-of-convex algorithm with extrapolation to possibly accelerate the proximal DCA. We show that any cluster point of the sequence generated by our algorithm is a stationary point of the DC optimization problem for a fairly general choice of extrapolation parameters: in particular, the parameters can be chosen as in FISTA with fixed restart (O’Donoghue and Candès in Found Comput Math 15, 715–732, 2015). In addition, by assuming the Kurdyka-Łojasiewicz property of the objective and the differentiability of the concave part, we establish global convergence of the sequence generated by our algorithm and analyze its convergence rate. Our numerical experiments on two difference-of-convex regularized least squares models show that our algorithm usually outperforms the proximal DCA and the general iterative shrinkage and thresholding algorithm proposed in Gong et al. (A general iterative shrinkage and thresholding algorithm for non-convex regularized optimization problems, 2013).

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## Notes

We would also like to point to the article “DC programming and DCA” on the person webpage of Le Thi Hoai An: http://www.lita.univ-lorraine.fr/~lethi/index.php/en/research/dc-programming-and-dca.html.

It is also discussed at the end of the numerical section of [17] that suitably incorporating extrapolation techniques into the proximal DCA can accelerate the algorithm empirically.

Indeed, when \(P_2 = 0\), FISTA with fixed restart and FISTA with both fixed and adaptive restart are special cases of \(\mathrm{pDCA}_e\).

\(\lambda _{\max }(A^TA)\) is computed via the MATLAB code lambda = norm(A*A’); when \(m\le 2000\), and by opts.issym = 1; lambda= eigs(A*A’,1,’LM’,opts); otherwise.

These \(\lambda \) satisfy \(\lambda < \frac{1}{2}\Vert A^{T}b\Vert _{\infty }\) for all our random instances.

In the tables, “max” means the number of iterations hits 5000.

The CPU time reported for \(\mathrm{pDCA}_e\) does not include the time for computing \(\lambda _{\max }(A^TA)\).

We set \(\epsilon = 0.5\) in (5.4).

In the tables, “max” means the number of iterations hits 5000.

The CPU time reported for \(\mathrm{pDCA}_e\) does not include the time for computing \(\lambda _{\max }(A^TA)\).

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## Acknowledgements

The authors would like to thank the two anonymous referees for their helpful comments.

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Xiaojun Chen’s work was supported in part by Hong Kong Research Grants Council PolyU153000/15p. Ting Kei Pong ’s work was supported in part by Hong Kong Research Grants Council PolyU253008/15p.

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Wen, B., Chen, X. & Pong, T.K. A proximal difference-of-convex algorithm with extrapolation.
*Comput Optim Appl* **69**, 297–324 (2018). https://doi.org/10.1007/s10589-017-9954-1

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DOI: https://doi.org/10.1007/s10589-017-9954-1