A survey on directional information techniques for multi-Objective evolutionary algorithms - Nguyen Long

Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 54 A SURVEY ON DIRECTIONAL INFORMATION TECHNIQUES FOR MULTI-OBJECTIVE EVOLUTIONARY ALGORITHMS Nguyen Long1*, Nguyen Xuan Hung2, Cao Van Huan2, Nguyen Duc Dinh3 Abstract: In many disciplines, optimization problems often have two or more objectives, which are normally in conflict with others, and that we wish to optimize them simultaneously. These problems are called multi-objective optimization problems (MOPs). In fact, MOPs normally give rise not to one, but to a set of solutions (called a Pareto optimal set (POS)) which, in the absence of any further information, are all equally good. An evolutionary algorithms have been very popular for solving MOPs [8, 12] mainly due to their ease of use, work on population and their wide applicability. Evolutionary algorithms allow to find an entire set of Pareto optimal solutions in a single run of the algorithm, instead of having to perform a series of separate runs as in the case of the traditional mathematical programming techniques. Recently, the guided techniques have been discussed, conceptualized and used to guide multi-objective evolutionary algorithms (MOEAs) during the search process towards the POS. Generally, guided information is derived from population, individuals, archives, decision makers. Then those information are used to guide MOEAs during their evolutionary process quickly towards the POS. The good guidance will control MOEAs to obtain the set of solutions towards POSs in a good quality of convergence and diversity. This is a difficult task since the evolutionary process allows randomness so it is hard to maintain the balance between convergence and diversity properties during the search. This paper will discuss the determination and the effective usage of the guided information in MOEAs. The paper also gives a survey on the usage of directional information for best guiding the search to improve the balance between convergence and diversity for multi-objective evolutionary algorithms. Keywords: MOEAs, Gradient descent directions, Improvement directions, PDE, DMEA-II. 1. INTRODUCTION In optimization area, the using evolution algorithms (EAs) brings many effectiveness to solve optimization problems. In fact, evolution algorithms work on population and stochastic mechanism so evolution algorithms can be effectively used to solve difficulty problems which have complex optimal sets in objective space. EAs have widely randomized range so they make the search being not biased towards local optima, that is why EAs are suitable for global optimization problems. When solving multi-objective problems, EAs are adaptively and effectively used to obtain a set of approximated Pareto optimal solutions. However, EAs also have some difficulties such as: the obtained solutions are approximated Pareto optimal solutions so they are not really desired optimal solutions for the problems. It also requires a high number of generations to get a good set of solutions. To avoid these disadvantages, a hybridization model that combines MOEAs with search mechanisms to improve the performance quality of the algorithms. The search techniques are discussed and widely used in multi- objective optimization such as: particle swarm optimization (PSO), ant colony, gradient based directions, differential evolution, direction of improvement, ... These techniques are used to guide the evolutionary process quick towards Pareto Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 55 optimal fronts in objective space (or Pareto optimal sets in decision space), and to avoid being trapped in local optima. This guidance helps MOEAs to be improved in their exploitation and exploration characterises and the quality of the obtained solutions. The using of guided information is a promising technique to get good approximated solutions, it helps MOEAs to be improved in their quality and capacity. In fact, there are many approaches in using guided information in MOEAs, one of these kinds of guided information is directional information in MOEAs, it contains: gradient descent directions, differential directions in differential evolution and directions of improvement. The using of these directions are summarized in following sections. The remainder of this paper is organized as follows. A description of guided directions is given in Section II. The advantages and disadvantages of the usage of directional information are indicated in Section III. Finally, the research directions and trends are shown in conclusions. 2. TECHNIQUE OF USING GUIDED DIRECTIONS 2.1. Using gradient descent directions In optimization, gradient based method determines search directions base on gradient vectors of objective functions at a selected point. Because gradient vectors are fast increasing directions of objective functions at a point, so the idea of using negative derivative directions is the fast decreasing directions of objective functions. Other words, solutions are moved by these directions allow to find a optimal point. In multi-objective optimization algorithms, the aim of the evolutionary process is getting an approximated Pareto optimal set. During the search, if the selected point is a Pareto optimal it will be kept for next generation, otherwise, this point need to be moved toward the Pareto optimal front to be an approximated Pareto optimal solution. A method is used as a local search widely used is using gradient based directions at a point, these directions help algorithm to move the selected point toward the Pareto optimal front. For more details of determining and using these directions is described in two popular methods: steepest descent method and Pareto descent method. In gradient based direction methods for multi-objective problems, steepest descent directions at a point are defined as a direction which is combined by descent directions at a that point, they are often determined by solving a quadratic linear programming problem. The illustration for descent directions is shown in Fig. 1. Figure 1. Illustration of descent directions, gi denotes for vector − () . The using of steepest descent directions in multi-objective optimization is proposed in [15], the idea of this method is determining steepest descent direction Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 56 at a selected point, the directions are determined by a linear combination of negative gradient components. These directions improve all objectives simultaneously. The steepest descent directions are determined as follows: ∗ = −∗ (1) The authors use concept of Lagrange to calculate value for m dimensions of a Lagrange multiplier λ: = arg min 1 2 ‖‖ ≥ 0 (2) = 1 This is a quadratic linear programming with m variables and m linear inequalities and an equality. In other propose [16], the authors define Pareto descent directions and use these directions to control evolutionary process towards Pareto optimal front. In this propose, Pareto descent directions at x in decision space are defined as descent directions to which no other descent direction is superior in improving all objective functions. There are often multiple Pareto descent directions. Other words, if direction d is a Pareto descent direction if d can be presented as a convex combination of steepest descent directions [15] of objective functions or there exist ≥ 0 ( = 1,2, , ) such that: = (−∇()) (3) Since both the complete set of descent directions and all convex combinations of the steepest descent directions form convex cones, the union of the two, namely, the complete set of Pareto descent directions, also forms a convex cone. The illustration of descent cone is shown in Fig. 2. Figure 2. Illustration of Pareto descent directions (d2, d3), gi − () . The authors propose a local search model that uses both Pareto descent directions and descent directions when solving multi-objective problems. In this propose, these directions are determined in three cases: all Pareto descent directions are in feasible region, some Pareto descent directions are in infeasible region and there are no Pareto descent directions in feasible region. In case of all Pareto descent directions are in feasible region, a combination of steepest descent directions gives a descent direction, it determined by a linear programming problem: Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 57 = ≥ 0 ( = 1,2, , ) (4) ≪ 1, ≫ 0 ( = 1,2, , ) There, ≠ 0; ∑ ≪ 1 is a weighted vector needs to determine, = ∆()∆(). When the selected solution is located on the boundary of feasible region, some Pareto descent directions in infeasible region, hence, to solve the multi-objective problems in e_ectiveness way, these infeasible directions need to be ignored during the search. For this task, an additional constraint is used: . ≫ 0 (5) There, is a normalized vector of the boundary of feasible region, is Pareto descent directions are determined by the de_nition [16]. If there are no Pareto descent directions in feasible region, descent directions are used for the search, these directions are determined by a linear programming problem: . . (−∇() ( = 1,2, , ) (6) −1 ≪ ≪ 1 ( = 1,2, , ) There, = (, , , ) is a weighted vector, when = 0, it can be presented as a direction, otherwise, when ≠ 0, the directions start from a edge which is calculated by above linear programming problem, these found directions are descent directions. This case is demonstrated in Fig. 3. Figure 3. Demonstration of determination directions in three cases: All Pareto descent directions are in feasible region (left), Some Pareto descent directions are in infeasible region (middle), No Pareto descent directions in feasible region (right). In [27], the authors introduce a directed search bases on gradient information, in this method, a search direction v ∈ Rn in decision space is determined: lim → ( + ) − () = , = 1,2, , (7) There, ∈ is a vector in objective space,: → is an objective function ith of the multi-objective problem. is a step length parameter add into objective functions. A Jacobian matrix is used, then, (7) is represented as matrix vector: () = (8) Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 58 A bellow system of equalities is solved to find direction . When number variables bigger than number of objectives, can be determined: = () (9) With ∈ × is an invertible matrix of ∈ ×, ≪ . When problems contain constraints , , , : → at x then a system of equalities is solved to find : () = () ≪ 0 −1 ≪ ≪ 1 ( = 1,2, , ) ℎ () == ⎝ ⎜ ⎛ ∇() ∇() ∇() ⎠ ⎟ ⎞ ∈ × (10) The search direction is used for directed descent direction method, which is incorporated with traditional methods such as weighted sum, goal programming. The authors use this direction based technique to propose a multi-objective algorithm. In this algorithm, a individual at selected point in decision space is moved belong the search direction to be closed Pareto optimal set. Using gradient based algorithm to solve multi-objective problems, it has a good convergence rate but there are some difficulties when MOPs have a complex surface in objective space, a lot of local optimums and the they are non- differentiable, those are difficulty MOPs. There are no way to determine gradient vectors, so gradient based algorithm can not be used. Based on population and stochastic mechanism, EAs can solve above difficulty MOPs, however, EAs also have some difficulties such as: the obtained solutions are approximated Pareto optimal solutions so they are not really desired optimal solutions for the problems. It also requires a high number of generations to get a good set of solutions. Other words, EAs might have problem in convergence rate. The idea of combining gra dient based directions with evolution algorithm in a hybridization algorithm is raised and discussed. These algorithms inherit the advantages of gradient based and evolution algorithms to effective solve multi-objective problems. An evolution algorithm is incorporated with gradient information in local search will be quicker convergence to Pareto optimal front. Here, some gradient based directions are used to guide the evolutionary process quickly convergence to Pareto optimal front and avoid thelocal optimums during the search. According the above idea, in [2] the authors propose an evolution algorithm which combines with discrete gradient information. In this propose, discrete gradient is used as a local search operator for + and (, ) evolution strategies, there is number of randomly selected parents, is number of generated offsprings. The algorithm applies discrete gradient on all individuals in population in the first generation. At later generations, discrete gradient is only applied for the best found individuals. A number of offsprings are generated after recombination and mutation, then µ best individuals are selected in ( + ) (parents and new Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 59 generated offsprings ) or only in (new generated offsprings). The authors use = 2() and = (2) to calculate a step length in ⃗ in the mutation operator individual : , = + (0,1) (11) , = (0,1) + (0,1) In [17], the authors combine concept of evolution computation with approximated gradients to generate offsprings for next generations. In this propose, gradients is not exactly calculated, they are estimated by using minimum solutions in neighbor populations. This propose is implemented in four main steps: Step 1, a center point c and a radius r are initialized, set = ; Step 2, a population with size randomly individuals, each point in population defines a set of vectors such that: = + ; ∈ (1,2, , ) (12) There, a randomized number in (0,1), is a randomized vector and is the current iteration; Step 3: Approximating gradient: Evaluate () ℎ = 1,2, , . When ∃: (min (() ≪ (), set is in problem () = min (()). An approximated gradient vector is defined from the differences between . = − (13) At Step 4, a new center point and new radius are created by: = + ∀ ≫ 1 (14) there, is a control parameter. = ‖ − ‖∀ ≫ 1 (15) In [4, 3], the authors introduce three approaches of using gradient information with MOEAs:  Using conjugate gradients: A local search method which uses gradient with a single objective, an objective is randomly selected in objectives of MOPs. This method depends on relationship between objectives, when an objective is the best improved, it helps to improve the remaining objectives.  Using an alternative repeated line-search: The idea of this approach is to reduce the probability of improving a single objective while making the objective value in the other objective worse, the objective that is searched locally can be altered during local search. The authors propose to perform a linesearch (i.e. find a local minimum with respect to a single direction) in a single, alternatingly chosen objective in the direction of the negative gradient of that objective. This process is repeated until a multiobjective local minimum is found.  Using a combined repeated line-search: A linesearch is performed in a promising direction. When the linesearch terminates, a new linesearch is initiated in a new promising direction found at the new location. A set of non-dominated directions are improvement directions, they are described as bellows: Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 60 = ∑ , ∑ , ∑ ∈[,] (16) When the linesearch terminates, a randomly vector of is given (each ∈ [0,1] and ∑ = 1), the a new direction is determined when putting this randomly vector intoabove inequality. The line-search is not used to minimize a single objective but it used to findnon-dominated solutions belong the line-search. A multi-objective problem need to be solved to find the line-search. In [5], the authors modify the concept of descent cone in the using gradients, the descent cone is defined as interregion of negative space which is generated by gradient vectors over all objectives. The authors indicate that, offsprings which is generated by a MOEA have to located on this descent cone. A gradient approximation is proposed, the approximated method bases on neighbored information, it really costs less than solving a quadratic linear programming in [15]. In other propose [28], the authors introduce a method that uses generating techniques for optimal Pareto solutions [30] and [26] to improve an mutation on NSGA-II. This propose uses Simultaneous Perturbation (SP)[29] for gradient estimation: () = (∆)() ∆ (17) there, ()is the component i th of a gradient, ∆ is randomized vectors with n dimensions that satisfies the condition in [29]. The randomized component of this randomized vector is generated by the Bernouli distributor. A step-size c at each iteration k is calculated: = /( + 1) . The using of randomized gradient in [30]: A set of non-dominated solutions is determined and used for approximating of Pareto optimal set. From each new non-dominated , a new individual , is generated by = − ∑ ∇ () (18) there, is a is a vector distribution in [0, 1] and t is the step-length at generation j which is calculated: = /, is a positive constant. The generating method in [26] is used: = −() + , = (19) there, > 0, is a Brownian movement, (−q(x)) is a direction in decision space that is a descent direction for all objective functions, this descent direction is determined by a quadratic problem: () = ∑ ∇ () when is minimum These mutation operators is used in NSGA-II as twonew versions of NSGA-II: T- NSGA-FD and T-NSGA-SP. Based on the directed search method in [27], the authors in [19] improve and combine the directed search with MOEA/D [31]. The directed search is improved by other direction determination: () = ∑ , where = () (20) there, ∈ is a guided vector, T (x) ∈ Rk×r k is the number of objectives, r is number of search directions, λ ∈ Rr is step-length vector for search directions. When search directions are found, the selected solution x is moved to new Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 61 location by: = () , t is a step-length is calculated by [22]. The directed search method is applied with MOEA/D as a local search, for the combination, two additional control parameters are given: the first parameter is a ratio for using the local search in each generations, the second parameter is the number of solutions in population which are selected by the local search in each generation. In [24, 25], the authors introduce two version of a MOEA which uses descent directions, they are suitable to solve non-differentiable MOPs, in this propose a specific descent direction determination is introduced to avoid the derivative calculation. A new decision vector is determined: = + ∑ (21) there, is a new generated decision vector, is the current randomized decision vector, is a number is randomly generated from a distributor in [0, 1], is a descent direction for objective, is a step-length at iteration, that is calculated: = max (1, (1 − /), there is a desired number of iterations. To avoid the derivative calculation, the authors propose a descent direction determination for each objective: The first, the population is divided into α equal sub-populations. At current generation, for each sub-population, calculate all points for all objective functions to get a . This is a point that has the lowest values for each objective function. Then a coordinated search is used to find a descent direction for each (randomly generation in variables randomly order, then they are checked for selection), the descent direction for individual i of objective j is determined: , = − + (22) there, is a leader point of a sub-population, is the descent direction of the . This procedure is repeated for the other objectives. At the end, M descent directions are associated with each solution in the population. 2.2. Using differential directions One of widely used methods which uses directional information to guide MOEAs is differential evolution (DE) method. In this method, guided directions are determined on the differences of selected solutions in population. The differences between differential evolution algorithms and evolutionary algorithms in mutation and recombination operators. The perturbation is not like in evolutionary algorithms, in differential evolution, the differences of weighted vectors in decision space are used to perturb the population. In other words, the main technique in differential evolution is the using differential directions which are determined from selected points in decision space for the perturbation. The common steps in differential evolution are described:  Step 1: i = 1.  Step 2: Randomly select 1, 2, 3 ∈ {1,2, , } such that 1 ≠ 2 ≠ 3 ≠ ,i is the indexes of the selected individuals in population.  Step 3: Generate a new child individual , from selected parents ,, , v , with a current individual , at generation . A child individual , is generated by: Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 62 , = , + (, − ,) + (, − ,) (23) there, K and F are control parameters, they are often generated by a uniformly distribution in [0, 1]. , is called main parent, ,, , are called supported parents.  Step 4: i = i + 1.  Step 5: Repeat Step 2 if it is not enough desired child individuals and ≠ . In differential evolution, the population is randomly generated in ranges for each decision variable in decision space. At each generation, population is perturbed, the index i of current individual , is initialized by 1 at the first step. At the second step, three independent individuals are randomly selected in population (size N), their indexes are: 1, 2, 3. The parameter K is a rate for the combination between xr3,tt and current individual xi,tt. The parameter F is a step- length for vector (, − ,). Step 3, these selected individuals are used to generate new individual , for next generation + 1: In single optimization, a new individual , is compared to the current individual ,, if the new individual is better than the current one, it replaces the current individual in next generation. At Step 5, the terminated condition is checked, it repeats until getting enough individuals or N individuals are checked. In multi-objective optimization, the dominance relationship between new individual and the current individual is examined for the replacement. Some proposed multi-objective evolutionary algorithms which use differential evolution is short described as bellows: According the concept of differential evolution, the authors in [1] introduce a MOEA uses DE. In this propose, the authors modify standard DE as bellows:  Using Gaussian distribution N (0, 5, 0.15) for population initialization.  Parental individuals ,, , v , are only selected in non-dominated solutions in each generation.  Offspring is generated by: , = , + (, − ,) + (, − ,) (24) As a standard DE, but step-length F for vector (, − ,) is randomly picked from Gaussian distribution N (0, 1).  The boundary constraints are preserved either by reversing the sign if the variable is less than 0 or keeping subtracting 1 if it is greater than 1 until the variable is within its boundaries.  Offspring u, are placed into the population if they dominate the main parent , − ,. A combined version of NSGA-II with DE is introduced in [11], in this propose, the individual , in standard DE is replaced by , which has the best rank in dominated relationship in population at generation tt, the individual , is skipped. Here, offspring for generation + 1 are determined: , = , + , − , 2 ≠ 3, 2, 3 ∈ [1, ] (25) Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 63 there, 2, 3 are randomly selected in [1,N], , is the individual which has the best rank in dominated relationship in population. In addition, the crossover operator generates a trial vector by: , = ,, () ≪ = () , , () > ≠ () (26) there, is a crossover rate, j is the decision variable jth of solution ith, r(j) is a number which is randomly picked in [0,1], , will use mutation vector ,, () ≪ . Otherwise, , is used. In [21], the authors introduce a new version of MOEA/D which uses DE (MOEA/D-DE). In this propose, a method for individual replacement and randomized search rate in mutation operators is suggested. In MOEA/D, the maximum number of solutions that can be replaced by a child solution is the size of the neighborhood, T, whose disadvantage is shown in [20]. MOEA/D-DE improves the replacement mechanism by adding a bound nr, which is much smaller than T. A high quality child solution can replace nrcurrent solutions at most, which helps the diversity maintenance. The Euclidian distances from current selected solution to other solutions are calculated to select nr solutions which have the shortest distances, they are entered next generation with mutation operator: , = , + , − , (27) there, 1, 2 ∈ [0, ] 1 ≠ 2, is picked from Gaussian distribution in [0, 1] is step-length parameter for (, − ,) each generation. 2.3. Using direction of improvement The effectiveness of using directional information has been discussed in multi- objective evolutionary algorithmic design. Beside of gradient based technique and differential evolution to guide MOEAs, the using of direction of improvement is raised and indicated as a promising technique to obtain set of good approximated Pareto optimal solutions. This techique helps MOEAs to maintain exploitation and exploration characteristics. Follow the aspect of improving MOEAs, the direction of improvement is defined as directions which are determined from population in decision space or objective space. These directions are archived and used for moving solutions belong the directions to make MOEAs to be improved in convergence rate and diversity. There are some proposes which use direction of improvement: In [18], based on the concept of differential evolution, the authors propose an using direction of improvement for two important characteristics of MOEAs: convergence and diversity. In this propose, two types of direction are defined for NSGA-II [13] as bellows: Directional convergence: In NSGA-II, at each generation, the individuals are ranked in anumber of classes by dominance relationship in population. Individuals are of the same rank which are put into the same class. A directional convergence is defined as a vector from (, − ,), there xi,tt is a current selected individual, , is a main parent in DE, which is randomly selected from higher rank individuals in the population (see Fig. 4). This direction points toward the Pareto optimal set. The meaningful of using this direction is guiding the Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 64 evolutionary process quickly convergence to the Pareto optimal set, it helps MOEAs to be improved in convergence rate. Directional spread: This direction is defined from (, − ,), there, two individuals xr1,tt, xr2,tt are randomly selected from current population such that ,, , are of the same rank (see Fig. 4). This direction is used to guide solution to be distributed belong the Pareto optimalset. It helps MOEAs to be improved in diversity. Figure 4. Illustration of directional convergence and directional spread, f is the currentindividual, individuals a and b are of the same rank, c, d, e are of the same rank. Directionalconvergence (left) and directional spread (right) in decision space. Using the directions of improvement: The authors give three ways to use the directions of improvement:  NSDE-DC: A version of NSGA-II which only uses directional convergence, the indexes 1, 2 such that 1 ≠ 2 ≠ 3 ≠ are randomized selected in the indexes of the current population.  NSDE-DS: A version of NSGA-II which only uses directional spread, the index 3 such that r1 ≠ r2 ≠ r3 ≠ i is randomized selected in the indexes of the current population.  NSDE-DCS: A version of NSGA-II which uses both of directional convergence and directional spread. In all ways of using the directions of improvement, offspring is generated by: , = , + (, − ,) + (, − ,) (28) In the experimental validation, the authors set F = 0.8 and K = 0.4. In [9, 6, 7] the authors introduce a guidance technique for multi-objective algorithm in both decision space and objective space in a local search model. In this propose, decision space space is divided into several non-overlap spheres, each sphere si is defined that includes a pair of a centroid and a radius: S = [s0, s1, . . . , sn]v si = (ci, ri). At initialized step, all radiuses ri are set the same value r. Each sphere is defined as a local area in decision space. To ensure a sphere is not overlapped to others, a constraint is used: ≫ 2 (29) there, is the euclidian distance between sphere A and sphere B. The points in a sphere is uniformly distributed, the minimized distance from a point to others in the same sphere is , the distance between two points i and j is , is the centroid of the sphere X, the constraint conditionis described as bellows: Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 65 ≪ , ≪ , (30) ≫ The authors use Cartesian coordinate to generate values for points x = (x1, x2, . . . , xn) in a sphere bases on the radius of the sphere and a set of − 1 randomized angles. The local model is definedas a system of spheres in decision and they are kept by a movement operator. The main steps are described as bellows:  Step 1: Define spheres: The parameters are declared: number of spheres SP, initialized radius such that satisfies (29) and the minimized distance between two spheres β.  Step 2: Initilize spheres: Spheres are initialized by a method of randomly angle generating which satisfies the condition in ( 30).  Step 3: Implement evolutionary iterations for each sphere.  Step 4: Move spheres belong the predefined strategy, a non-communicated strategy is suggested, where a centroid is calculated which bases on entire non-dominated in the sphere: = ∑ ( ) (31) where, is a set of N obtained non-dominated individuals, a radius ri of the sphere i is determined: = , > , ℎ (32)  Step 5: The termination condition is examined, if it is not satisfied then go to Step 3. Figure 5. The movement of a centroid to determine a direction of improvement: I) currentsphere where the big black dot is its centroid; II) The transition state in which the big white dot is the new centroid; III) The new sphere with new generated o_springs (small black dots). The big white arrow is determined direction of improvement. In this propose, the movement direction of a sphere is determined by a vector which is created from a pair of an old centroid and a new one (see Fig. 5), this direction is defined as a direction of improvement. The combined direction of the system is defined as a combination of all directions of improvement of the spheres: ⃗ = ⃗ (33) Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 66 there, is the number of spheres in decision space. Based on the concept of particle swarm optimization (PSO) rules [14], a new individual is generated by an adaption: ⃗ = ⃗ + ⃗ (34) there, ⃗ is the vector of a direction, ⃗ is a individual which is generated by a crossover operator, is a new generated individual from above adaption. The directions of improvement which are created by movement direction of spheres, they are used to guide the evolutionary process towards difference areas in Pareto optimal front in objective space, it helps to maintain the diversity of the population during generations. With the aim of improve the exploitation and exploration of MOEAs, the authors in [10] introduce a direction based multi-objective evolutionary algorithm (DMEA), there are two types of improvement directions are used. These directions of improvement are defined as bellows: Convergence direction: This direction is defined as a direction from a solution to a better solution in objective space, in multi-objective optimization problems, the convergence direction is a direction from a dominated solution to a non- dominated solution. If non-dominated solutions are maintained globally, this direc- tion is global direction. In unconstraint MOPs, a guidance to move a dominated solution belong this direction helps evolutionary process to find better area in decision space. Illustration of convergence direction is shown in Fig.6, there A is a dominated solution, B, C, D, E, F, tt are non-dominated solutions, all directions from A to B, C, D, E, F, tt are determined as convergence directions. Spread direction: This direction is defined as a direction from two equivalence solutions, in multi-objective optimization problems, the spread direction is a direction from a non-dominated solution to other non-dominated solution. If a solution is perturbed by this direction, it helps the population to be more spreading, other words, the obtained solutions will be distributed belong Pareto optimal front in objective space. Illustration of convergence direction is shown in Fig.6, all directions between B, C, D, E, F, tt are determined as spread directions. Figure 6. An illustration of convergence (black arrows) and spread (hollow arrows) directions in objective space (left) and decision variable space (right). The propose directions of improvement are used as bellows: Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 67 A dominated solution Par is selected with variables () + in decision space ∈ {1,2, , }, aperturbation rate 0 < p < 1 and a convergence d1, a individual S1(i) is perturbed by convergence direction by: () ≡ () + ()() (35) there, is a scalar parameter is randomly picked in a distribution U(0,2) this is a step-length to avoid local optimums, RND(p) is a randomized number in distribution U(0,1) it equals 1 if U(0,1) < and 0 otherwise. A non-dominated solution Par is selected with variables () + is perturbed by a spread direction d2 by: () ≡ () + ()() (36) In this propose, the directions of improvement are calculated by: ≡ − | − | (37) ≡ − | − | (38) there, , and are three randomly selected solutions in the archive ( an external populationis used to keep elitist solutions during generations). In an additional technique, a system of rays inobjective space is used for niching operator in DMEA. Based on the distance between from solutions to the nearest ray, a ray based niching is introduced to use during the search. This is an interesting technique which is incorporated with directions of improvement to effective guide the evolutionary process to maintain the exploitation and exploration of the algorithm. In the new version of DMEA, DMEA-II[23], an explicit niching operator is used with a set of rays which divides the space evenlyfor the selection of non-dominated solutions to ll the solution archive and the population of the next generation. We found that, with DMEA-II solutions will effectively converge to the Pareto optimalset under the guidance of the ray system. 3. ADVANTAGES AND DISADVANTAGES Through above short survey on techniques of using directional information as guided direction for multi-objective algorithms, the advantages and disadvantages of each technique are indicated as bellows: 3.1. Gradient based direction technique Solving multi-objective optimization problems by gradient based directions is early discussed and used in difference approaches. In fact, gradient based multi- objective algorithms have some advantages: This algorithms can be used to solve complex differentiable MOPs, gradient based directions are used so it makes multi- objective algorithms to be good convergence rate, when incorporating with evolution strategy in a hybridization MOEA, the algorithms can have a good convergence rate and avoid the local optimums during the search. However, there some difficulties in using gradient based directions: The algorithms can not be used with non-differentiable MOPs, it requires a hight performance cost to determine gradient based directions. In hybridization MOEAs which use gradient directions in local search model but it still has difficulties in determining descent Công nghệ thông tin N. Long, , N. D. Dinh, “A survey on directional information evolutionary algorithms.” 68 directions, Pareto descent directions and directed directions, non-differentiable MOPs and maintaining the balance between exploitation and exploration of the algorithms globally. 3.2. Differential direction technique To date, evolutionary algorithms which use concept of differential evolution is known as a powerful an effective algorithm to solve single optimization problems. However, in multi-objective optimization, although it still has the advantages like in single optimization, but a multi-objective evolutionary which uses differential directions by the concept of differential evolution has some difficulties: MOEAs with DE have hight convergence rate but it is difficulty to keep diversity for the population. This disadvantage can be solved if some mechanisms for maintaining diversity of the population are incorporated with the algorithms. One other difficulty is that MOEAs with DE only work on real decision space, so it can not be used when decision space is binary space. This difficulty can be solved when use an additional codding technique for a space transformation. 3.3. Direction of improvement technique The using of direction of improvements in MOEAs is known as an effective technique because of the aim of the directions of improvement. The directions of improvement are used to guide the evolutionary process to make the population to be quickly converged and uniformly distributed belong to Pareto optimal front. It helps to improve the convergence rate and diversity for obtained population. Using direction of improvement is overcame almost difficulties in using of gradient based direction and differential direction: It is quite simple to determine directions of improvement because these directions are determined by dominance relationship of solutions (or individuals) in population (or an external population); The directions of improvement are used for a movement of solution follows two aspects: being closed and uniformly distributed the Pareto optimal front. It helps to ensure convergence rate and diversity of the population so it promises to obtain a good approximated Pareto optimal set, the primary aim of improving MOEAs in multi-objective optimization area. However, there are some difficulties on using of direction of improvement: keeping the balance between exploitation and exploration is difficult because evolutionary process follows the stochastic mechanism. This difficult mights be a reason of reducing convergence rate and diversity of the population. Almost directions of improvement are used in a local search model for MOEAs, so it mights not a effective algorithm for global multi- objective optimization. In recently proposes which use direction of improvement, the authors in [18] defined directions of improvement bases on dominance relationship in ranked classes of individuals so the search is locally. Other local search uses directions of improvement in [9, 6, 7] when the direction of improvement is determined be the moving of local spheres in decision space. The global search uses directions of improvement is introduced in [10] when the directions of improvement are determined by dominance relationship on individuals entire the main population and the archive during generations. Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san CNTT, 12 - 2017 69 4. CONCLUSIONS This paper analysts the using of directional information to guide evolutionary process in MOEAs, the paper also indicates that: The using of direction of improvement is an effective technique, it promises for MOEAs to obtain a good approximated Pareto optimal front through the search guidance which follows directions of improvement for the exploitation and exploration of the algorithms. It helps the population is improved in convergence rate and diversity, however the balance between exploitation and exploration need to be kept during evolutionary processes. In fact, when using directions of improvement, offspring is generated by each direction often in a balanced rate [18,10], It clears that this is a ’hard’ constraint of the algorithms when quality of population in convergence and diversity are differences at difference states of evolutionary process. At the early states, the population is quite diversity, it needs to increase the convergence rate at that time. Otherwise, at the late states, the population is strongly converged to Pareto optimal front, it needs to make population to be more diversity to keep the balance between convergence and diversity. Hence, the search guidance needs to adaptive use to keep the balance between exploitation and exploration throughout evolutionary process. Beside the search guidance belongs the directions of improvement automatically, the search guidance needs to reach decision maker’s preferred regions in objective space for acompleted effective guidance. 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Giải thuật tiến hóa đã trở thành phổ biến trong giải các bài toán tối ưu đa mục tiêu [8, 12] với lý do dễ sử dụng, làm việc theo quần thể và khả năng ứng dụng rộng rãi của nó. Giải thuật tiến hóa cho phép tìm một tập giải pháp tối ưu Pareto trong một lần chạy của giải thuật thay vì phải thực hiện một vài lần chạy nhưng kỹ thuật lập trình toán học truyền thống. Gần đây, kỹ thuật chỉ dẫn được thảo luận, nghiên cứu và sử dụng để chỉ dẫn cho giải thuật tiến hóa tối ưu đa mục tiêu trong quá trình tìm kiếm theo tập tối ưu Pareto. Nói chung, thông tin chỉ dẫn được lấy từ quần thể, các thể, tập lưu trữ ngoài và người ra quyết định, sau đó thông tin này được sử dụng để chỉ dẫn tiến trình tiến hóa nhanh chóng hội tụ đến tập POS. Kỹ thuật chỉ dẫn tốt sẽ điều khiển cho giải thuật tiến hóa tối ưu đa mục tiêu để đạt được tập giải pháp tối ưu Pareto có chất lượng về tính hội tụ và đa dạng. Đây là một việc khó khi mà tiến trình tiến hóa có tính chất ngẫu nhiên và nó rất khó để duy trì tính cần bằng của hội tụ và đa dạng trong quá trình tìm kiếm. Bài báo này sẽ khảo sát và bàn luận cách thức xác định và sử dụng hiệu quả thông tin chỉ dẫn trong giải thuật tiến hóa tối ưu đa mục tiêu. Bài báo cũng tập trung khảo sát về việc sử dụng hướng cải thiện để chỉ dẫn tốt nhất cho quá trình tìm kiếm nhằm duy trì cân bằng giữa tính hội tụ và đa dạng của giải thuật tiến hóa tối ưu đa mục tiêu. Keywords: MOEAs, Hướng trượt Gradient, Hướng cải thiện,Thông tin định hướng, PDE, DMEA-II. Nhận bài ngày 16 tháng 8 năm 2017 Hoàn thiện ngày 26 tháng 11 năm 2017 Chấp nhận đăng ngày 28 tháng 11 năm 2017 Address: 1National Defense Academy; 2Le Quy Don Technical University; 3MITI, Military Academy of Science and Technology. * Email: longn@mta.edu.vn.