Estimating Approximation Errors of Elitist Evolutionary Algorithms
When evolutionary algorithms (EAs) are unlikely to locate precise global optimal solutions with satisfactory performances, it is important to substitute alternative theoretical routine for the analysis of hitting time/running time. In order to narrow the gap between theories and applications, this paper is dedicated to perform an analysis on approximation error of EAs. First, we proposed a general result on upper bound and lower bound of approximation errors. Then, several case studies are performed to present the routine of error analysis, and theoretical results show the close connections between approximation errors and eigenvalues of transition matrices. The analysis validates applicability of error analysis, demonstrates significance of estimation results, and then, exhibits its potential to be applied for theoretical analysis of elitist EAs.
Keywords:Evolutionary Algorithm Approximation Error Markov Chain Budget Analysis.
For theoretical analysis, convergence performance of evolutionary algorithms (EAs) is widely evaluated by the expected first hitting time (FHT) and the expected running time (RT) , which quantify the respective numbers of iteration and function evaluations (FEs) to hit the global optimal solutions. General methods for estimation of FHT/RT have been proposed via theories of Markov chains [2, 3], drift analysis[4, 5], switch analysis  and application of them with partition of fitness levels , etc.
Although popularly employed in theoretical analysis, simple application of FHT/RT is not practical when the optimal solutions are difficult to hit. One of these “difficult” cases is optimization of continuous problems. Optimal sets of continuous optimization problems are usually zero-measure set, which could not be hit by generally designed EAs in finite time, and so, FHT/RT could be infinity for most cases. A remedy to this difficulty is to take a positive-measure set as the destination of population iteration. So, it is natural to take an approximation set for a given precision as the hitting set of FHT/RT estimation [8, 9, 10, 11]. Another “difficult” case is the optimization of NP-complete (NPC) problems that cannot be solved by EAs in polynomial FHT/RT. For this case, it is much more interesting to investigate the quality of approximate solutions obtained in polynomial FHT/RT. In this way, researchers have estimated approximation ratios of approximate solutions that EAs can obtain for various NPC combinatorial optimization problems in polynomial expected FHT/RT [12, 13, 14, 15, 16, 17].
However,the aforementioned methods could be impractical once we have little information about global optima of the investigated problems, and then, it is difficult to “guess” what threshold can result in polynomial FHT/RT. Since the approximation error after a given iteration number is usually employed to numerically compared performance of EAs, some researchers tried to analyze EAs by theoretically estimating the expected approximation error. Rudolph  proved that under the condition , the sequence converges in mean geometrically to , that is, . He and Lin  studied the geometric average convergence rate of the error sequence , defined by Starting from , it is straightforward to claim that .
A close work to analysis of approximation error is the fixed budget analysis proposed by Jansen and Zarges [20, 21], who aimed to bound the fitness value within a fixed time budget . However, Jansen and Zarges did not present general results for any time budget . In fixed budget analysis, a bound of approximation error holds for some small but might be invalid for a large one. He  made a first attempt to obtain an analytic expression of the approximation error for a class of elitist EAs. He proved if the transition matrix associated with an EA is an upper triangular matrix with unique diagonal entries, then for any , the approximation error is expressed by where are eigenvalues of the transition matrix. He et al.  also demonstrated the possibility of approximation estimation by estimating one-step convergence rate , however, it was not sufficient to validate its applicability to other problems because only two studied cases with trivial convergence rates were investigated.
This paper is dedicated to present an analysis on estimation of approximation error depending on any iteration number . We make the first attempt to perform a general error analysis of EAs, and demonstrate its feasibility by case studies. Rest of this paper is presented as follows. Section 2 presents some preliminaries. In Section 3, a general result on the upper and lower bounds of approximation error is proposed, and some case studies are performed in Section 4. Finally, Section 5 concludes this paper.
In this paper, we consider a combinatorial optimization problem
where has only finite available values. Denote its optimal solution as , and the corresponding objective value as . Quality of a feasible solution is quantified by its approximation error . Since there are only finite solutions of problem (1), there exist finite feasible values of , denoted as . Obviously, the minimum value is the approximation error of the optimal solution , and so, takes the value 0. We call that is located at the status if . Then, there are totally statuses for all feasible solutions. Status consists of all optimal solutions, called the optimal status; other statuses are the non-optimal statuses.
Suppose that an feasible solution of problem (1) is coded as a bit-string, and an elitist EA described in Algorithm 1 is employed to solve it. When the one-bit mutation is employed, it is called a random local search (RLS); if the bitwise mutation is used, it is named as a (1+1) evolutionary algorithm ((1+1)EA). Then, the error sequence is a Markov Chain. Assisted by the initial probability distribution of individual status , the evolution process of (1+1) elitist EA can be depicted by the transition probability matrix
where is the probability to transfer from status to status .
Since the elitist selection is employed, the probability to transfer from status to status is zero when . Then, the transition probability matrix is upper triangular, and we can partition it as
where , ,
Thus, the expected approximation error at the iteration is
where , , is the sub-matrix representing transition probabilities between non-optimal statuses . Because sum of each column in is equal to 1, the first row can be confirmed by , and in the following, we only consider the transition submatrix for estimation of approximation error. According to the shape of , we can further divide searching process of elitist EAs into two different categories.
Step-by-step Search: If the transition probability satisfies
it is called a step-by-step search. Then, the transition submatrix is
which means the elitist EA cannot transfer between non-optimal statues that are not adjacent to each other;
Multi-step Search: If there exists some such that , we called it a multi-step search. A multi-step search can transfer between inconsecutive statuses, which endows it with better global exploration ability, and probably, better convergence speed.
Note that this classification is problem-dependent because the statuses depend on the problem to be optimized. So, the RLS could be either a step-by-step search or a multi-step search. However, the (1+1)EA is necessarily a multi-step search, because the bitwise mutation can jump between any two statuses. When in (3) is non-zero, column sums of is less than 1, which means it could jump from at least one non-optimal status directly to the optimal status. So, a step-by-step search represented by (7) must satisfies
3 Estimation of General Approximation Bounds
3.1 General Bounds of the Step-by-step Search
Let be the submatrix of a step-by-step search. Its eigenvalues are
which represents the probability of remaining at the present status after one iteration. Then, it is very natural to declare that greater the eigenvalues are, slower the step-by-step search converges. Inspired by this idea, we can estimate general bounds of a step-by-step search by enlarging or reducing the eigenvalues. Achievement of the general bounds is based on the following lemma.
Then, is monotonously increasing with , .
This lemma could be proved by mathematical induction.
When , we have
Note that is not greater than 1 because it is an element of the probability transition matrix . Then, from the truth that , we conclude that is monotonously increasing with , . Meanwhile, (10) also implies that
Suppose that when , is monotonously increasing with for all , and it holds that
First, the monotonicity indicated by (12) implies that
Meanwhile, according to equation (9) we know , that is,
which means is monotonously increasing with for all .
In conclusion, is monotonously increasing with , .∎
If we enlarge or shrink all eigenvalues of to the maximum value and the minimum value, respectively, we can get two transition submatrices and , where , . Then, depicts a searching process converging slower than the one represents, and is the transition submatrix of a process converging faster than what represents.
The expected approximation error of a step-by-step search represented by and is bounded by
where is a non-zero vector composed of non-negative components. Then, by lemma 1 we can conclude that is also monotonously increasing with , . So, we can get the result that
Theorem 3.1 provides a general result about the upper and the lower bounds of approximation error. From the above arguments we can figure out that the lower bounds and the upper bounds can be achieved once the transition submatrix degenerates to and , respectively. That is to say, they are indeed the “best” results about the general bounds. Recall that . Starting from the status, is the probability that the (1+1) elitist EA stays at the status after one iteration. Then, greater is, harder the step-by-step search transfers to the sub-level status . So, performance of a step-by-step search depicted by , for the worst case, would not be worse than that of ; meanwhile, it would not be better than that of , which contributes to a bottleneck for improving performance of the step-by-step search.
3.2 General Bounds of the Multi-step Search
Denoting the transition submatrix of a multi-step search as
we can bound its approximation error by defining two transition matrices
It is trivial to prove that . Because has part of non-zero elements of , is a partial sum of . Since all elements included in are nonnegative, it holds that .
Moreover, the second inequality can be proved by mathematical induction. Denote
where . Combining with the fact that , we know that
In conclusion, it holds that .∎
The approximation error of the multi-step search defined by (17) is bounded by
3.3 Analytic Expressions of General Bounds
Analytic Expression of : The submatrix can be split as , where
Because multiplication of and is commutative, the binomial theorem  holds and we have
Note that is a nilpotent matrix of index 111In linear algebra, a nilpotent matrix is a square matrix such that for some positive integer . The smallest such is called the index of ., and
Analytic Expression of : For the diagonal matrix , it holds that
4 Case-by-case Estimation of Approximation Error
In section 3 general bounds of approximation error are obtained by ignoring most of elements in the sub-matrix . Thus, these bounds could be very general but not tight. In this section, we would like to perform several case-by-case studies to demonstrate a feasible routine of error analysis, where the RLS and the (1+1)EA are employed solving the popular OneMax problem and the Needle-in-Haystack problem.
4.1 Error Estimation for the OneMax Problem
Application of RLS on the unimodal OneMax problem generates a step-by-step search, the transition submatrix of which is
Eigenvalues and corresponding eigenvectors of are
The expected approximation error of RLS for the OneMax problem is
The expected approximation error of (1+1)EA for the OneMax problem is bounded from above by
According to the definition of population status, we know that the status index is the number of 0-bits in . Once one of 0-bits is flip to 1-bit and all 1-bits keep unchanged, the generated solution will be accepted, and the status transfers from to . Recalling that the probability this case happen is , we know that
and we know that
With distinct eigenvalues, can be diagonalized:
where , . and are the eigenvalues and the corresponding eigenvectors:
It is obvious that is invertible, and its inverse is
Similar to the result illustrated in (39), we know that
4.2 Error Estimation for the Needle-in-Haystack Problem
Landscape of the Needle-in-Haystack problem has a platform where all solutions have the same function value , and only the global optimum has a non-zero function value . For this problem, the status is defined as total number of 1-bits in a solutions .
The expected approximation error of RLS for the Needle-in-Haystack problem is bounded by
When the RLS is employed to solve the Needle-in-Haystack problem, the transition submatrix is
we can conclude that
Theorem 4.3 indicates that both the upper bound and the lower bound converge to the positive when , which implies the fact that RLS cannot converge in mean to global optimal solution of the Needle-in-Haystack problem. Because the RLS searches adjacent statuses and only better solutions can be accepted, it cannot converge to the optimal status once the initial solution is not located at the status .
The expected approximation error of (1+1)EA for the Needle-in-Haystack problem is bounded by
When the (1+1)EA is employed to solve the Needle-in-Haystack problem, the transition probability submatrix is
we can conclude that
To make theoretical results more instructional to algorithm developments and applications, this paper proposes to investigate performance of EAs by estimating approximation error for any iteration budget . General bounds included in Theorems 3.1 and 3.2 demonstrate that bottlenecks of EAs’ performance are decided by the maximum and the minimum eigenvalues of transition submatrix . Meanwhile, theorems 4.1,4.2,4.3, and 4.4 present estimations of approximation error for RLS and (1+1)EA for two benchmark problems, which shows that our analysis scheme is applicable for elitist EAs, regardless the shapes of transition matrices. Moreover, the estimation results demonstrate that approximation errors are closely related to eigenvalues of the transition matrices, which provide useful information for performance improvements of EAs. Our future work is to further perform error analysis on real combinatorial problems to show its applicability in theoretical analysis of EAs.
This work was supported in part by the National Nature Science Foundation of China under Grants 61303028 and 61763010, in part by the Guangxi “BAGUI Scholar” Program, and in part by the Science and Technology Major Project of Guangxi under Grant AA18118047.
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