** Int'l J. of Communications, Network and System Sciences** Vol.2 No.6(2009), Article ID:699,7 pages DOI:10.4236/ijcns.2009.26061

A Rank-One Fitting Method with Descent Direction for Solving Symmetric Nonlinear Equations^{*}

College of Mathematics and Information Science, Guangxi University, Nanning, Guangxi, China

Email: glyuan@gxu.edu.cn.

Received January 14, 2009; revised March 4, 2009; accepted May 31, 2009

**Keywords:** Rank-One Update, Global Convergence, Nonlinear Equations, Descent Direction

ABSTRACT

In this paper, a rank-one updated method for solving symmetric nonlinear equations is proposed. This method possesses some features: 1) The updated matrix is positive definite whatever line search technique is used; 2) The search direction is descent for the norm function; 3) The global convergence of the given method is established under reasonable conditions. Numerical results show that the presented method is interesting.

1. Introduction

Consider the following system of nonlinear equations:

(1)

where F:R^{n}→R^{n} is continuously differentiable and the Jacobian ▽F(x) of F(x) is symmetric for all x∈R^{n}. Let θ(x) be the norm function defined by

then the nonlinear Equation (1) is equivalent to the following global optimization problem

(2)

The following iterative method is used for solving (1)

(3)

where x_{k} is the current iterative point, d_{k} is a search direction, and a_{k} is a positive step-size.

It is well known that there are many methods [1–9] for the unconstrained optimization problems

where the BFGS method is one of the most effective quasi-Newton methods [10–17]. These years, lots of modified BFGS methods (see [18–23]) have been proposed for UP. Different from their techniques, Xu [24] presented a rank-one fitting algorithm for UP and the numerical examples are very interesting. Motivated by their idea, we give a new rank-one fitting algorithm for (1) which possesses the global convergence, the method can ensure that the updated matrices are positive definite without carrying out any line search, the search direction is descent for the normal function, and the numerical results is more competitive than those of the BFGS method for the test problem.

For nonlinear equations, the global convergence is due to Griewank [25] for Broyden’s rank one method. Fan [1], Yuan [26], Yuan, Lu andWei [27], and Zhang [28] presented the trust region algorithms for nonlinear equations. Zhu [29] gave a family of nonmonotone backtracking inexact quasi-Newton algorithms for solving smooth nonlinear equations. In particular, a GaussNewton-based BFGS method is proposed by Li and Fukushima [30] for solving symmetric nonlinear equations, and the modified methods [31,32] are studied.

The line search rules play an important role for solving the optimization problems. In the following, we briefly review some line search technique to obtain the stepsize a_{k}.

Brown and Saad [33] proposed the following line search method:

(4)

where, i_{k} is the smallest nonnegative integer i such that (4). Zhu [29] gave the nonmonotone line search technique:

and M is a nonnegative integer. Yuan and Lu [32] presented a new backtracking inexact technique to obtain the stepsize a_{k}:

(5)

where δ∈(0,1) is a constant, and d_{k} is a solution of the system of linear Equation (9). Li and Fukashima [11] give a line search technique to determine a positive step-size a_{k} satisfying

(6)

where δ_{1} andδ_{2} are positive constants, and {ε_{k}} is a positive sequence such that

(7)

The Formula (7) means that {F(x_{k})} is approximately norm descent when k is sufficiently large. Gu, Li, Qi, and Zhou [14] presented a descent line search technique as follows

(8)

whereδ_{1} andδ_{2 }are positive constants. In this paper, we also use the Formula (8) as line search to find the step-size a_{k}:

The search direction d_{k}: play a main role in line search methods for solving optimization problems too, and d_{k}: is a solution of the system of linear equation

(9)

where B_{k} is often generated by BFGS update formula

(10)

where

and Is there another way to determine the update formula? Accordingly the search direction d_{k} is determined by the way. In this paper, the updated matrix B_{k} is generated by the following rank-one updated formula

(11)

(12)

where, as k = 0, B_{0} is the given symmetric positive definite matrix,

and

is a positive constant. Then we use the following formula to get the search direction,

(13)

(14)

B_{k} follows (11), a_{k-1} is the steplength used at the previous iteration, and the Equation (14) is inspired by [34]. Throughout the paper, we use these notations: is the Euclidean norm, and F(x_{k}) and F(x_{k+}_{1}) are replaced by F_{k} and F_{k-}_{1}, respectively.

This paper is organized as follows. In the next section, the algorithm is stated. The global convergence convergence is established in Section 3. The numerical results are reported in Section 4.

2. Algorithm

In this section, we state our new algorithm based on Formulas (3), (8), (11), (12) and (13) for solving (1).

Rank-One Updated Algorithm (ROUA).

Step 0: Choose an initial point x_{0}∈R^{n} constants

symmetric positive definite matrices B_{0} and B_{0}^{-}^{1}=H_{0 }. Let: k = 0;

Step 1: If, stop. Otherwise, solving linear Equation (13) to get d_{k};

Step 2: Find a_{k} is the largest number of {1,r,r^{2},…} such that (8);

Step 3: Let the next iterative point be x_{k}_{+1}= x_{k}+a_{k}d_{k};

Step 4: Update B_{k}_{+1} and H_{k}_{+1} by the Formulas (11) and (12) respectively;

Step 5: Set k: = k + 1. Go to Step 1.

In this paper, we also give the normal BFGS method for solving (1), the algorithm which has the same conditions to ROUA is stated as follows.

BFGS Algorithm(BFGSA).

In ROUA, the Step 4 is replaced by: Update Bk+1 by the Formula (10).

Remark 1. a) By the Step 0 of ROUA, there should exist constants λ_{1}≥λ_{0}＞0 such that

(15)

b) By the Step 4 of ROUA, it is easy to deduce that the updated matrices are symmetric

3. Convergence Analysis

This section will establish the global convergence for ROUA. Let Ω be the level set defined by

(16)

In order to establish the global convergence of ROUA, the following assumptions are needed [30,34,35].

Assumption A 1) F is continuously differentiable on an open convex set Ω_{1} containing Ω. 2) The Jacobian of F is symmetric, bounded and uniformly nonsingular on Ω_{1}, i.e., there exist constants M≥m＞0 such that

(17)

and

(18)

Remark Assumption A 2) implies that

(19)

(20)

In particular, for all x∈Ω_{1}, we have

(21)

where x^{*} stand for the unique solution of (1) in Ω_{1}.

Lemma 3.1 Let Assumption A hold. Consider ROUA. Then for any d∈R^{n}, then there exist constants m_{0} such that

(22)

i.e., the matrix B_{k }is positive for all k.

Proof. By ROUA, we know that the initial matrix B0 is symmetric positive, and then we have (15). Using (11), for k≥1, we have

(23)

Let m_{0}=λ_{0}. Then we get (22). The proof is complete.

Since B_{k} is positive definite, then d_{k} which is determined by (13) has the unique solution. The following lemma can found in [34], here we also give the process of this proof.

Lemma 3.2 Let Assumption A hold. If x_{k} is not a stationary point of (2), then there exists a constant a'＞0 depending on k such that when a_{k}_{-}_{1}∈(0, a'), the unique solution d(a_{k}_{-}_{1}) of (13) such that

(24)

Moreover, inequality

(25)

Proof. By (14), we can deduce that

(26)

From (13), we get

(27)

Since x_{k} is not a stationary point of (2), we have ▽F(x_{k})F(x_{k})≠0. By ▽F(x_{k}) is symmetric and B_{k} is positive. We obtain (24).

However, the right hand side of (25) is O(a_{k}_{-}_{1}). Thus, inequality (25) holds for all a_{k}_{-1}＞0 sufficiently small. The proof is complete.

The above lemma shows that line search technique (8) is reasonable, and the given algorithm is well defined. Lemma 3.2 also shows that the sequence {θ(x_{k})} is strictly decreasing. By Lemma 3.2, it is not difficult to get the following lemma.

Lemma 3.3 Let {x_{k}} be generated by ROUA. Consider the line search (8). Then {x_{k}}∈Ω moreover, converges.

Lemma 3.4 Let Assumption A hold and

be generated by ROUA. Then we have

(28)

and

(29)

Proof. By the line search (8), we get

(30)

Summing these inequalities (30) for k from 0 to ∞ we obtain (28) and (29). Then we complete the proof of this Lemma.

Lemma 3.5 Let Assumption A hold. Consider ROUA. Then converges, for all k and any d∈R^{n} then there exist constants m_{0} and M_{0} such that

(31)

and

(32)

which mean that the updated matrices are all positive by ROUA.

Proof. By the updated Formula (11), we have

(33)

By (28), we know that

is convergent. Then we can deduce that is convergent. So there exists a constant M_{0} such that

for all k (34)

Accordingly, we get (28). By (32), (31), and the Remark 1(b), we can deduce that the updated matrices are all symmetric and positive. Consider we obtain (32) immediately. So, we complete the lemma.

By (32), (31), and (34), we have

(35)

Now we establish the global convergence theorem of ROUA.

Theorem 3.1 Let Assumption A hold and be generated by ROUA. Then the sequence {x_{k}} converges to the unique solution x^{*} of (1) in sense of

(36)

Proof. By Lemma 3.3, we know that converges. By Lemma 3.4, we get

(37)

then, we have

(38)

or

(39)

Therefore, we only discuss the case of (38). In this case, for all k sufficiently large and

by (8), we obtain

(40)

By Lemma 3.3, we know that {x_{k}}∈Ω is bounded, considering (35), it is easy to deduce that {q_{k}(a_{k-1}) and {d_{k}} are bounded. Let {x_{k}} and {d_{k}(a)} converge to x^{*} and dx^{*}, respectively. Then we have

(41)

Let both sides of (40) be divided by a_{k}' and take limits as k→∞ we obtain

(42)

By (31) and (13), we have

(43)

As k→∞ taking limits in both of (43) yields

This together with (42) implies d^{*}=0. From (35), we have

which together with (41), we obtain

(44)

By and using is nonsingular, we have. This implies (36). The proof is complete.

Table 1. Test results for ROUA.

Table 2. Test results for BFGSA.

4. Numerical Results

In this section, we report results of some preliminary numerical experiments with ROUA. Problem. The discretized two-point boundary value problem is similar to the problem in [36]

where A is the n×n tridiagonal matrix given by

and

with In the experimentsthe parameters in ROUA were chosen as，. The program was coded in MATLAB Subsection 6.5.1. We stopped the iteration when the condition was satisfied.

The columns of the tables have the following meaning:

Dim: the dimension of the problem.

NI: the total number of iterations.

NG: the number of the function evaluations.

GF: the function norm evaluations.

In the next table, the numerical results are to test ROUA.

In the Table 2, the numerical results are to test BFGSA.

From these two tables, we can see that the numerical results of the two methods are all interesting. The numerical results of the proposed method perform better, and more stationary than the method BFGSA. Moreover, for the method ROUA, the initial points and the dimension do not influence the number of iterations very much. However, for the BFGSA, the number of the iteration will increase quickly with the dimension becoming larger. One thing we like to point out is that δ_{0} should be chosen in such a way that it is not too large. Overall, from the numerical results, we can see that the ROUA is one of the robust methods for symmetric nonlinear equations.

5. Acknowledgements

We are very grateful to anonymous referees and the editors for their valuable suggestions and comments, which improve our paper greatly.

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NOTES

^{*}National Natural Science Foundation of China (10761001) and the Scientific Research Foundation of Guangxi University (Grant No. X081082).