# A primal-dual fixed point algorithm for minimization of the sum of three convex separable functions

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## Abstract

Many problems arising in image processing and signal recovery with multi-regularization and constraints can be formulated as minimization of a sum of three convex separable functions. Typically, the objective function involves a smooth function with Lipschitz continuous gradient, a linear composite nonsmooth function, and a nonsmooth function. In this paper, we propose a primal-dual fixed point (PDFP) scheme to solve the above class of problems. The proposed algorithm for three-block problems is a symmetric and fully splitting scheme, only involving an explicit gradient, a linear transform, and the proximity operators which may have a closed-form solution. We study the convergence of the proposed algorithm and illustrate its efficiency through examples on fused LASSO and image restoration with non-negative constraint and sparse regularization.

## Keywords

primal-dual fixed point algorithm convex separable minimization proximity operator sparsity regularization## 1 Introduction

*B*being the discrete gradient operator together with \(\ell_{1}\) regularization has been adopted in some image restoration applications, for example in [3]. Another useful application corresponds to \(f_{3}=\chi_{C}\), where \(\chi _{C}\) is the indicator function of a nonempty closed convex set

*C*. In this case, the problem reduces to

As far as we know, Combettes and Pesquet first proposed a fully splitting algorithm in [4] to solve monotone operator inclusions problems, which include (1.1) as a special case. Condat [5] tackled the same problem and proposed a primal-dual splitting scheme. Extensions to multi-block composite functions are also discussed in detail. For the special case \(B=I\) (*I* denotes the usual identity operator), Davis and Yin [6] proposed a three-operator splitting scheme based on monotone operators. For the case that the problem (1.1) reduces to two-block separable functions, many splitting and proximal algorithms have been proposed and studied in the literature. Among them, extensive research have been conducted on the alternating direction of multiplier method (ADMM) [7] (also known as split Bregman [3]; see for example [8] and the references therein). The primal-dual hybrid gradient method (PDHG) [9, 10, 11, 12], also known as the Chambolle-Pock algorithm [11], is another class of popular algorithm, largely adopted in imaging applications. In [13, 14, 15, 16], several completely decoupled schemes, such as the inexact Uzawa solver and primal-dual fixed point algorithm, are proposed to avoid subproblem solving for some typical \(\ell_{1}\) minimization problems. Komodakis and Pesquet [17] recently gave a nice overview of recent primal-dual approaches for solving large-scale optimization problems (1.1). A general class of multi-step fixed point proximity algorithms is proposed in [18], which covers several existing algorithms [11, 12] as special cases. In the preparation of this paper, we notice that Li and Zhang [19] also studied the problem (1.1) and introduced a quasi-Newton and the overrelaxation strategies for accelerating the algorithms. Both algorithms can be viewed as a generalization of Condat’s algorithm [5]. The theoretical analysis is established based on the multi-step techniques present in [18].

In the following, we mainly review some most relevant work for a concise presentation. Problem (1.2) has been studied in [20] in the context of maximum a posterior ECT reconstruction, and a preconditioned alternating projection algorithm (PAPA) is proposed for solving the resulting regularization problem. For \(f_{3}=0\) in (1.1), we proposed the primal-dual fixed point algorithm \(\mathrm{PDFP}^{2}\mathrm{O}\) (primal-dual fixed point algorithm based on proximity operator) in [15]. Based on the fixed point theory, we have shown the convergence of the scheme \(\mathrm{PDFP}^{2}\mathrm{O}\) and the convergence rate of the iteration sequence under suitable conditions.

*f*; see (2.2). When \(f_{3}=\chi_{C}\), the proposed algorithm (1.3) is reduced to the PAPA proposed in [20]; see (4.1). For another special case, \(f_{3}=0\) in (1.3), we obtain the \(\mathrm{PDFP}^{2}\mathrm{O}\) proposed in [15]; see (4.2). The convergence analysis of this PDFP algorithm is built upon fixed point theory on the primal and dual pairs. The overall scheme is completely explicit, which allows for an easy implementation and parallel computing for many large-scale applications. This will be further illustrated through application to the problems arising in statistics learning and image restoration. The PDFP has a symmetric form and it is different from Condat’s algorithm [5]. In addition, we point out that the ranges of the parameters in PDFP are larger than those of [5, 19] and the rules for the parameters in PDFP are well separated, which could be advantageous in practice compared to [5, 19].

The rest of the paper is organized as follows. In Section 2, we will present some preliminaries and notations, and deduce PDFP from the first order optimality condition. In Section 3, we will provide the convergence results and the linear convergence rate results for some special cases. In Section 4, we will make a comparison on the form of the PDFP algorithm (1.3) with some existing algorithms. In Section 5, we will show the numerical performance and the efficiency of PDFP through some examples on fused LASSO and pMRI (parallel magnetic resonance image) reconstruction.

## 2 Primal-dual fixed point algorithm

### 2.1 Preliminaries and notations

For the self completeness of this work, we list some relevant notations, definitions, assumption and lemmas in convex analysis. We refer the reader to [15, 22] and the references therein for more details.

*B*satisfy

*f*is defined as \(\operatorname{dom}_{f} = \{x\in\mathbb{R}^{n}\mid f(x) < +\infty\}\).

*f*: \(\operatorname{prox}_{f}\) [21] is defined by

*C*, defined by

*C*,

*i.e.*

*∂f*be the subdifferential of

*f*,

*i.e.*

*f*, defined by

*T*is firmly nonexpansive if

*T*is

*δ*-strongly monotone if there exists a positive real number

*δ*such that

## Lemma 2.1

*For any two functions*\({{f_{2}}} \in\Gamma_{0} (\mathbb{R}^{m})\)

*and*\({{f_{3}}}\in\Gamma_{0} (\mathbb{R}^{n})\),

*and a linear transformation*\(B:{\mathbb{R}^{n}}\rightarrow\mathbb{R}^{m}\),

*satisfying that*\(0\in\operatorname{ri}(\operatorname{dom}_{{f_{2}}}- B(\operatorname{dom}_{{f_{3}}}))\),

*we have*

## Lemma 2.2

*Let*\({f}\in\Gamma_{0}(\mathbb{R}^{n})\).

*Then*\(\operatorname{prox}_{f}\)

*and*\(I-\operatorname{prox}_{f}\)

*are firmly nonexpansive*.

*In addition*,

*If*

*f*

*has*\(1/\beta\)-

*Lipschitz continuous gradient further*,

*we have*

## Lemma 2.3

*Let* *T* *be an operator and* \(u^{*}\) *be a fixed point of* *T*. *Let* \(\{ u^{k+1}\}\) *be the sequence generated by the fixed point iteration* \(u^{k+1}=T(u^{k})\). *Suppose* (i) *T* *is continuous*, (ii) \(\{\|u^{k}-u^{*}\|\} \) *is non*-*increasing*, (iii) \(\lim_{k\to+\infty} \|u^{k+1}-u^{k}\|=0\). *Then the sequence* \(\{u^{k}\}\) *is bounded and converges to a fixed point of* *T*.

The proof of Lemma 2.3 is standard, and we refer the reader to the proof of Theorem 3.5 in [15] for more details.

*γ*and

*λ*be two positive numbers. To simplify the presentation, we use the following notations:

*A*. When \(0<\lambda< 1/\lambda_{\mathrm{max}}(BB^{T})\),

*M*is a symmetric and positive definite matrix, so we can define a norm

### 2.2 Derivation of PDFP

On extending the ideas of the PAPA proposed in [20] and the \(\mathrm{PDFP}^{2}\mathrm{O}\) proposed in [15], we derive the primal-dual fixed point algorithm (1.3) for solving the minimization problem (1.1).

To sum up, we have the following theorem.

## Theorem 2.1

*Suppose that*\(x^{*}\)

*is a solution of*(1.1)

*and*\(v^{*}\in\mathbb{R}^{m}\)

*is defined as*(2.17).

*Then we have*

*i*.

*e*. \({u^{*}}= ({v^{*}},{x^{*}} )\)

*is a fixed point of*

*T*.

*Conversely*,

*if*\({u^{*}}= ({v^{*}},{x^{*}} )\in\mathbb {R}^{m}\times\mathbb{R}^{n}\)

*is a fixed point of*

*T*,

*then*\(x^{*}\)

*is a solution of*(1.1).

It is easy to confirm that the sequence \(\{(v^{k+1}, x^{k+1})\}\) generated by the PDFP algorithm (1.3) is the Picard iteration \((v^{k+1},x^{k+1})={T}(v^{k},x^{k})\). So we will use the operator *T* to analyze the convergence of the PDFP in Section 3.

## 3 Convergence analysis

In the following, let \(\{y^{k+1}\}\) and \(\{u^{k+1}=(v^{k+1},x^{k+1})\}\) be the sequences generated by the PDFP algorithm (1.3), *i.e.* \(y^{k+1}={T}_{0}(v^{k},x^{k})\) and \((v^{k+1},x^{k+1})={T}(v^{k},x^{k})\). Let \({u^{*}}= ({v^{*}},{x^{*}} )\) be a fixed point of the operator *T*.

### 3.1 Convergence

## Lemma 3.1

*We have the following estimates*:

## Proof

_{2}and (2.18) we further have

_{3}(

*cf.*(2.5)), we have

*cf.*(2.3)),

*i.e.*

_{1}, it follows that

## Lemma 3.2

*We have*

## Proof

## Lemma 3.3

*Let* \(0<\lambda<1/{\lambda_{\mathrm{max}}(BB^{T})}\) *and* \(0<\gamma<2\beta\). *Then the sequence* \(\{\|u^{k}-u^{*}\|_{\lambda}\}\) *is non*-*increasing and* \(\lim_{k\to+\infty} \|u^{k+1}-u^{k}\|_{\lambda}=0\).

## Proof

*i.e.*the sequence \(\{\|u^{k}-u^{*}\| _{\lambda}\}\) is non-increasing. Moreover, summing the inequalities (3.4) from \(k=0\) to \(k=+\infty\), we get

*M*is symmetric and positive definite, so (3.8) is equivalent to

As a direct consequence of Lemma 3.3 and Lemma 2.3, we obtain the convergence of the PDFP as follows.

## Theorem 3.1

*Let* \(0<\lambda<1/{\lambda_{\mathrm{max}}(BB^{T})}\) *and* \(0<\gamma<2\beta\). *Then the sequence* \(\{u^{k}\}\) *is bounded and converges to a fixed point of* *T*, *and both* \(\{x^{k}\}\) *and* \(\{y^{k}\}\) *converge to a solution of* (1.1).

## Proof

By Lemma 2.2, both \(\operatorname{prox}_{{\gamma }{f_{3}}}\) and \(I-\operatorname{prox}_{\frac{\gamma}{\lambda}{f_{2}}}\) are firmly nonexpansive, thus the operator *T* defined by (2.9)-(2.12) is continuous. From Lemma 3.3, we know that the sequence \(\{\|u^{k}-u^{*}\| _{\lambda}\}\) is non-increasing and \(\lim_{k\to+\infty} \| u^{k+1}-u^{k}\|_{\lambda}=0\). By using Lemma 2.3, we know that the sequence \(\{ u^{k}\}\) is bounded and converges to a fixed point of *T*. By using Theorem 2.1 and (3.14), we can conclude that both \(\{x^{k}\}\) and \(\{y^{k}\} \) converge to a solution of (1.1). □

## Remark 3.1

*M*to obtain (3.13) from (3.8). So the condition for the parameter

*λ*is slightly more restricted as \(0<\lambda< 1/\lambda_{\mathrm{max}}(BB^{T})\) in Lemma 3.3 and Theorem 3.1. When \(f_{3}=0\), the conditions in the proof of Lemma 3.3 can also be relaxed to \(0<\lambda\leq1/\lambda_{\mathrm{max}}(BB^{T})\). As a matter of fact, it is easy to check by the definition of \(y^{k+1}\) (see (1.3)

_{1}) and the optimality condition of (2.19) that

*M*is semi-positive definite for \(f_{3}=0\).

## Remark 3.2

For the special case \(f_{1}=0\), the problem (1.1) only corresponds to two proper lower semi-continuous convex functions. The convergence condition \(0<\gamma<2\beta\) in the PDFP becomes \(0<\gamma<+\infty\). Although *γ* is an arbitrary positive number in theory, the range of *γ* will affect the convergence speed and it is also a difficult problem to choose a best value in practice.

### 3.2 Linear convergence rate for special cases

## Theorem 3.2

*Suppose that*(3.17)

*holds and*\(f_{2}^{*}\)

*is strongly convex*.

*Then we have*

*where*\(0<\eta<1\)

*is the convergence rate*(

*indicated in the proof*)

*and*\(\delta>0\)

*is a parameter describing the strongly monotone property of*\(\partial f_{2}^{*}\) (

*cf*. (2.4)).

## Proof

*cf.*(2.7)) to get

_{2}is equivalent to

*δ*-strongly monotone, we have by (3.19) and (3.20)

*i.e.*

Let \(\eta_{3}=1/\sqrt{1+\lambda\delta/\gamma}\) and \(\eta=\max\{ \eta_{2},\eta_{3}\}\). It is clear that \(0<\eta<1\). Hence, according to the notation (2.16), the estimate (3.22) can be rewritten as required. □

We note that a linear convergence rate for strongly convex \(f_{2}^{*}\) and \(f_{3}\) are obtained in [19]. They introduced two preconditioned operators for accelerating the algorithm, while a clear relation between the convergence rate and the preconditioned operators is still missing. Meanwhile, introducing preconditioned operators could be beneficial in practice, and we can also introduce a preconditioned operator to deal with \(\nabla f_{1}\) in our scheme. Since the analysis is rather similar to the current one, we will omit it in this paper.

## 4 Connections to other algorithms

In this section, we present the connections of the PDFP algorithm to some algorithms proposed previously in the literature.

_{1}compared to the extrapolation step in the PDHG method.

Combettes and Pesquet first proposed a fully split algorithm in [4] to solve monotone inclusions with mixtures of composite, Lipschitzian, and parallel-sum type monotone operators, which include (1.1) as a special case. The problem is recast as two-block inclusions and then solved with an error-tolerant primal-dual forward-backward-forward algorithm as studied in [25]. Condat [5] tackled the same problem as given in (1.1) and proposed a primal-dual splitting scheme. For the special case with \(f_{1}=0\), Condat’s algorithm reduces to the PDHG method in [11]. By grouping the multi-block as two blocks, the authors in [24] extended the PDHG algorithm [12] to the minimization of sum of multi-composite functions. The authors in [18] proposed a class of multi-step fixed point proximity algorithms, including several existing algorithms as special examples, for example the algorithms in [11, 12]. In [6], Davis and Yin proposed a three-operator splitting method for solving three-block monotone inclusions in a very tricky way. When solving the problem (1.1) with \(B=I\), the scheme is different from Condat’s algorithm and PDFP algorithm. But it requires subproblem solving if \(B\neq I\). Li and Zhang [19] studied (1.1) based on the techniques present in [18] and including Condat’s algorithm in [5] as a special case, and further introduced quasi-Newton and the overrelaxation strategies to accelerate the algorithms.

*i.e.*

*λ*to be close to \(1/\lambda_{\mathrm{max} }(BB^{T})\) and

*γ*to be close to 2

*β*for most of tests. Moreover, the results of \(x^{k}-\gamma\nabla{f_{1}}(x^{k})\) and \({\lambda} B^{T} v^{k+1}\) can be stored as two intermediate variables that can be reused in (1.3)

_{1}and (1.3)

_{3}during the iterations. Nevertheless, PDFP has an extra step (1.3)

_{1}compared to Condat’s algorithm and the computation cost may increase due to the computation of \(\operatorname{prox} _{{\gamma}{f_{3}}}\). In practice, this step is often related to the \(\ell_{1}\) shrinkage or projection operation being easy to implement, so the cost could be still ignorable in practice.

**The comparison between Condat (** \(\pmb{\rho_{k}=1}\) **) and PDFP**

| | |
---|---|---|

Form | \(\overline{v}^{k+1}=\operatorname{prox}_{\sigma {f_{2}^{*}}}(\sigma Bx^{k}+\overline{v}^{k})\), \(x^{k+1}=\operatorname{prox}_{{\tau}{f_{3}}}(x^{k}-\tau \nabla {f_{1}}(x^{k})-{\tau} B^{T} (2\overline{v}^{k+1}-\overline{v}^{k}))\) | \(y^{k+1}=\operatorname{prox}_{{\gamma}{f_{3}}}(x^{k}-\gamma \nabla{f_{1}}(x^{k})-{\gamma} B^{T} \overline{v}^{k})\), \(\overline{v}^{k+1}=\operatorname{prox}_{\frac{\lambda }{\gamma }{f_{2}^{*}}}(\frac{\lambda}{\gamma}By^{k+1}+\overline{v}^{k})\), \(x^{k+1}=\operatorname{prox}_{{\gamma}{f_{3}}}(x^{k}-\gamma \nabla {f_{1}}(x^{k})-{\gamma} B^{T} \overline{v}^{k+1})\) |

\(f_{1}\neq0\) | \(\sigma\tau\lambda_{\mathrm{max} }(BB^{T})+\tau/(2\beta)\leq1\) | \(0<\lambda< 1/\lambda _{\mathrm{max}}(BB^{T})\), 0< |

\(f_{1}=0\) | \(0<\sigma\tau\leq1/\lambda_{\mathrm{max} }(BB^{T})\) | \(0<\lambda< 1/\lambda_{\mathrm{max}}(BB^{T})\), 0< |

Relation | |

## 5 Numerical experiments

In this section, we will apply the PDFP algorithm to solve two problems: the fused LASSO penalized problem and parallel magnetic resonance imaging (pMRI) reconstruction. All the experiments are implemented under MATLAB 7.00 (R14) and conducted on a computer with Intel (R) core (TM) i5-4300U CPU@1.90G.

### 5.1 The fused LASSO penalized problem

*A*: \(A_{i}\) for \(i=1,2,\ldots, r\) represent the

*i*th observation of the independent variables and \(a_{i}\) denotes the response variable, and the vector \(x\in\mathbb {R}^{n}\) is the regression coefficient to recover. The first term is corresponding to the data-fidelity term, and the last two terms aim to ensure the sparsity in both

*x*and their successive differences in

*x*. Let

*a*is generated as \(Ax+\alpha e\), where

*A*and

*e*are random matrices whose elements are normally distributed with zero mean and variance 1, and \(\alpha=0.01\), and

*x*is a generated sparse vector, whose nonzero elements are showed in Figure 1 by green ‘+’. We set \(\mu_{1}=200\), \(\mu _{2}=20\), and the maximum iteration number as \(\mathit{Itn}=1\text{,}500\).

We compare the PDFP algorithm with Condat’s algorithm [5]. For the PDFP algorithm, the parameters *λ* and *γ* are chosen according to Theorem 3.1. In practice, we set *λ* to be close to \(1/\lambda_{\mathrm{max}}(BB^{T})\) and *γ* to be close to 2*β*. Here we set \(\lambda=1/4\) as the \(n-1\) eigenvalues of \(BB^{T}\) can be analytically computed as \(2-2 \cos(i\pi /n)\), \(i = 1, 2,\ldots, n-1\) and \(\gamma=1.99/\lambda_{\mathrm{max}}(A^{T}A)\). For Condat’s algorithm, we set \(\lambda= 0.19/4\), \(\gamma=1.9/\lambda _{\mathrm{max}}(A^{T}A)\), which is chosen for a relative better numerical performance. The computation time, the attained objective function values, and the relative errors to the true solution are close for Condat’s algorithm and PDFP. From Figure 1, we see that both Condat’s algorithm and PDFP can quite correctly recover the positions of the non-zeros and the values.

### 5.2 Image restoration with non-negative constraint and sparse regularization

*A*is some linear operator describing the image formation process, \(\|Bx\|_{1}\) is the usual \(\ell_{1}\) based regularization in order to promote sparsity under the transform

*B*, \(\mu>0\) is the regularization parameter. Here we use the isotropic total variation as the regularization functional, thus the matrix

*B*represents the discrete gradient operator. For this example, we can set \(f_{1}(x)=\frac{1}{2}\|Ax-a\|^{2}\), \(f_{2}=\mu\|\cdot\|_{1}\), and \({f_{3}}=\chi_{C}\).

We consider pMRI reconstruction, where \(A=(A_{1}^{T},A_{2}^{T},\ldots, A_{N}^{T})^{T}\) for each \(A_{j}\) is composed of a diagonal downsampling operator *D*, the Fourier transform *F*, and a diagonal coil sensitivity mapping \(S_{j}\) for receiver *j*, *i.e.* \(A_{j}=DFS_{j}\) and \(S_{j}\) are often estimated in advance. It is well known in the total variation application that \(\lambda_{\mathrm{max}}(BB^{T})=8\). The related Lipschitz constant of \(\nabla f_{1}\) can be estimated as \(\beta =1\). Therefore the two parameters in PDFP are set as \(\lambda=1/8\) and \(\gamma=2\). The same simulation setting as in [15] is used in this experiment and we still use the artifact power (AP) and the two-region signal to noise ratio (SNR) to measure the image quality. We refer the reader to [15, 27] for more details.

## 6 Conclusion

We have extended the algorithm PAPA [20] and \(\mathrm{PDFP}^{2}\mathrm{O}\) [15] to derive a primal-dual fixed point algorithm PDFP (see (1.3)) for solving the minimization problem of three-block convex separable functions (1.1). The proposed PDFP algorithm is a symmetric and fully splitting scheme, only involving explicit gradient and linear operators without any inversion and subproblem solving, when the proximity operator of nonsmooth functions can easily be handled. The scheme can easily be adapted to a variety of inverse problems involving many terms minimization and it is suitable for large-scale parallel implementation. In addition, the parameter range determined by the convergence analysis is rather simple and clear, and it could be useful for practical applications. Finally, as discussed in Section 5 in [5], we can also extend the current PDFP algorithm to solve multi-block composite (more than three) minimization problems. Preconditioning operators, as proposed in [16, 19, 24, 28], can also be introduced to accelerate the PDFP, which could be a future work for some specific applications.

## Notes

### Acknowledgements

P Chen was partially supported by the PhD research startup foundation of Taiyuan University of Science and Technology (No. 20132024). J Huang was partially supported by NSFC (No. 11571237). X Zhang was partially supported by NSFC (Nos. 91330102 and GZ1025) and 973 program (No. 2015CB856004). We thank the reviewer for pointing out the references [4, 14, 16] and for the pertinent comments and suggestions, which greatly improved the early version of this paper.

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