# RELAXATION HEURISTICS FOR THE SET COVERING PROBLEM

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1 Journal of the Operations Research Society of Japan 2007, Vol. 50, No. 4, RELAXATION HEURISTICS FOR THE SET COVERING PROBLEM Shunji Umetani University of Electro-Communications Mutsunori Yagiura Nagoya University (Received December 19, 2006; Revised June 15, 2007) Abstract The set covering problem (SCP) is one of representative combinatorial optimization problems, which has many practical applications. The continuous development of mathematical programming has derived a number of impressive heuristic algorithms as well as exact branch-and-bound algorithms, which can solve huge SCP instances of bus, railway and airline crew scheduling problems. We survey heuristic algorithms for SCP focusing mainly on contributions of mathematical programming techniques to heuristics, and illustrate their performance through experimental analysis. Keywords: Combinatorial optimization, set covering problem, linear programming, Lagrangian relaxation, subgradient method, heuristic algorithm 1. Introduction The set covering problem (SCP) is one of representative combinatorial optimization problems, which has many practical applications, e.g., bus, railway and airline crew scheduling, vehicle routing, facility location, political districting [5, 23, 32]. More recent applications of SCP are found in probe selection in hybridization experiments in DNA sequencing [16] and feature selection and pattern construction in logical analysis of data [17]. SCP is formally defined as follows. We are given a ground set of m elements i M = {1,..., m}, a collection of n subsets S j M and costs c j for j N = {1,..., n}, where we assume c j > 0 and S j 1 without loss of generality. We say that X N is a cover of M if j X S j = M holds, and X is a prime cover of M if X is a cover of M without any redundant subset. The goal of SCP is to find a minimum cost cover X of M. SCP can be formulated as a 0-1 integer programming (IP) problem as follows: (SCP) minimize z(x) = j N c j x j subject to j N a ij x j 1 (i M) (1.1) x j {0, 1} (j N), where a ij = 1 if i S j and a ij = 0 otherwise; i.e., a column a j = (a 1j,..., a mj ) T of matrix (a ij ) represents the corresponding subset S j by S j = {i M a ij = 1}. It is understood that decision variable x j = 1 if a subset S j is selected in the cover X and x j = 0 otherwise. For notational convenience, for each i M, let N i = {j N a ij = 1} be the set of subsets that contain element i. Since a column j N and a row i M correspond to a subset S j and an element i respectively, we say that a column j covers a row i if a ij = 1 holds. SCP is known to be NP-hard in the strong sense [31], and there is no polynomial time approximation scheme (PTAS) unless P = NP. Furthermore, a number of lower bounds on 350

2 Relaxation Heuristics for SCP 351 approximation ratio for SCP have been shown. Feige [27] proved that, for any ɛ > 0, it is impossible to achieve a polynomial time (1 ɛ) ln n approximation algorithm unless NP has n O(log log n) -time deterministic algorithms, and Trevisan [46] showed that the problem is hard to approximate within a factor ln d O(ln ln d) unless P = NP, where d = max j N S j. However, theoretical results do not necessarily reflect the experimental performance in practice; e.g., we can improve the performance of algorithms for the real-world instances by utilizing their structural property. The continuous development of mathematical programming has much improved the performance of exact branch-and-bound algorithms [3, 4, 7, 12, 30] accompanying with advances in computational machinery. Recent exact branch-and-bound algorithms enable us to solve large SCP instances with up to 400 rows and 4000 columns exactly [3]. Heuristic algorithms have also been studied extensively [2, 8, 11, 18, 28], and several efficient metaheuristic algorithms have been developed to solve huge SCP instances with up to 5000 rows and 1,000,000 columns within about 1% of the optimum in a reasonable computing time [20, 22, 24, 50]. Most of these impressive results were achieved by hybridizing metaheuristics and mathematical programming approaches. For example, Beasley [8] presented a number of greedy algorithms based on a Lagrangian relaxation (called the Lagrangian heuristics), and Caprara et al. [20] introduced variable fixing and pricing techniques into a Lagrangian heuristics. In this paper, we present a review of heuristic algorithms and related mathematical programming techniques for SCP. We mainly focus on contributions of mathematical programming to heuristic algorithms for SCP, and illustrate their performance through experimental evaluation for several classes of benchmark instances. We tested these algorithms on an IBM-compatible personal computer (Intel Xeon 2.8GHz, 2GB memory), and used two well-known sets of benchmark instances. The first set of benchmark instances is Beasley s OR Library [9], which contains 11 classes of SCP instances, namely 4 6 and A H. Each of classes 4 and 5 has 10 instances, and each of classes 6 and A H has 5 instances. In this paper, we denote instances in class 4 as 4.1, 4.2,..., 4.10, and other instances in classes 5, 6 and A H similarly. Another set of benchmark instances is called RAIL arising from a crew pairing problem in an Italian railway company [20, 24], which contains seven instances, namely three small size instances with up to 600 rows and 60,000 columns (RAIL507, 516 and 582), two medium size instances with up to 2600 rows and 1,100,000 columns (RAIL2536 and 2586), and two large size instances with up to 4900 rows and 1,100,000 columns (RAIL4284 and 4872). The data of these instances are given in Table 1, where density is defined by i M j N a ij/mn. This paper is organized as follows. In Section 2, we review representative relaxation techniques for SCP called the linear programming (LP) relaxation, the Lagrangian relaxation and the surrogate relaxation. In Section 3, we explain a well-known approach called the subgradient method, which computes good lower bounds of SCP instances within a short computing time. We also illustrate several techniques for improving the performance of the subgradient method. In Section 4, we explain a pricing method called the sifting method to solve huge relaxation problems of SCP. In Section 5, we illustrate problem reduction rules that test feasibility and reduce the size of SCP instances. In Section 6, we review heuristic algorithms for SCP including construction algorithms, Lagrangian heuristics and the state-of-the-art algorithms based on hybridization of metaheuristics and mathematical programming approaches.

3 352 S. Umetani & M. Yagiura Table 1: The size, density and cost range of benchmark instances for SCP Instance Rows Columns Density(%) Cost range [1,100] [1,100] [1,100] A.1 A [1,100] B.1 B [1,100] C.1 C [1,100] D.1 D [1,100] E.1 E [1,100] F.1 F [1,100] G.1 G ,000 2 [1,100] H.1 H ,000 5 [1,100] RAIL , [1,2] RAIL , [1,2] RAIL , [1,2] RAIL ,081, [1,2] RAIL , [1,2] RAIL ,092, [1,2] RAIL , [1,2] 2. Relaxation Problems The relaxation problems give us useful information to solve SCP. We can directly obtain good lower bounds from their solutions, and also good upper bounds by modifying them. In this section, we review three well-known relaxation problems for SCP called the linear programming (LP) relaxation problem, the Lagrangian relaxation problem, and the surrogate relaxation problem Linear programming relaxation The most general technique is the linear programming (LP) relaxation, which is defined by replacing the binary constraints x j {0, 1} with 0 x j 1 for all j N. Since the upper bound on x j is known to be redundant, we can obtain the following LP relaxation problem: (LP) minimize z LP (x) = j N c j x j subject to j N a ij x j 1 (i M) (2.1) x j 0 (j N). Although general-purpose LP solvers give an exact solution of the LP relaxation problem, it has been pointed out in the literature that their computation would be quite expensive because these solvers often suffer various problems caused by the degeneracy of the LP relaxation of SCP instances. However, in recent years, the development of mathematical programming softwares, accompanied with advances in computing machinery, enables us to solve huge LP instances [14]. We accordingly report a computational comparison of the simplex and barrier (or interior-point) methods with two state-of-the-art general purpose LP solvers called GLPK 4.8 (GNU Linear Programming Kit) [52] and CPLEX [51] on the benchmark instances. Table 2 illustrates the optimal value z LP of the LP relaxation problem and computation time in seconds spent by GLPK and CPLEX with the modes of the primal and dual simplex methods and the barrier method, where we set the time limit to be 3600 seconds for each

4 Relaxation Heuristics for SCP 353 Table 2: Comparison of the simplex and barrier methods Instance Rows Columns z LP GLPK 4.8 CPLEX Primal Dual Barrier Primal Dual Barrier A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , RAIL , RAIL , RAIL ,081, >3600 >3600 > RAIL , >3600 >3600 > RAIL ,092, >3600 >3600 >3600 > RAIL , >3600 >3600 >3600 > Note: each result of classes 4 6 and A H reports the average for all instances in the class. run. In Table 2, computation time does not include the time for reading instance data, and each result of classes 4 6 and A H reports the average computation time for all instances in the class, while the results for individual instances are reported for class RAIL. This is the same for all computational results in this paper. We observe that the dual simplex method is faster than the primal simplex method and the barrier method for classes 4 6 and A H. On the other hand, the barrier method of CPLEX is faster than the primal and dual simplex methods for RAIL instances. These general purpose LP solvers still require very large computation time and memory space to solve the LP relaxation of huge SCP instances such as RAIL Lagrangian relaxation Another well-known technique is the Lagrangian relaxation, which is defined by relaxing some constraints while introducing penalty functions. A typical Lagrangian relaxation problem for SCP is defined as follows: (LR(u)) minimize z LR (u) = c j x j + ( u i 1 ) a ij x j j N i M j N = j N c j (u)x j + i M u i (2.2) subject to x j {0, 1} (j N), where u = (u 1,..., u m ) R m + is a vector called the Lagrangian multiplier vector (R + is the set of non-negative real numbers), and c j (u) = c j i M a iju i is called the reduced cost (or relative cost) associated with a column j N. For any u R m +, z LR (u) gives a lower bound on the optimal value of SCP z(x ). The problem to find a Lagrangian multiplier vector u R m + that maximizes z LR (u) is called the Lagrangian dual problem: (LRD) max { z LR (u) u R m +}. (2.3)

5 354 S. Umetani & M. Yagiura For any u R m +, we can easily obtain an optimal solution to LR(u), denoted by x(u) = ( x 1 (u),..., x n (u)), by setting x j (u) 1 if c j (u) < 0 holds and x j (u) 0 if c j (u) > 0 holds, and choosing the value of x j (u) from either zero or one if c j (u) = 0 holds. It is known that any optimal solution to LR(u) is also optimal for its LP relaxation problem (i.e., replacing the binary constraint x j {0, 1} with the linear relaxation 0 x j 1), which is called the integrality property. Hence, the optimal value of the Lagrangian dual problem is equal to that of the LP relaxation problem of SCP [33]. In other words, any optimal solution ū to the dual of the LP relaxation: (LPD) maximize i M u i subject to i M a ij u i c j (j N) u i 0 (i M), (2.4) is also optimal to the Lagrangian dual problem LRD. Note that, even if an optimal solution x(u) of the Lagrangian relaxation problem LR(u) is feasible for the original SCP, it is not necessarily optimal for the original SCP. If all constraints are satisfied with equality (i.e., j N a ij x j (u) = 1 for all i M), then x(u) is optimal for the original SCP. We can define another Lagrangian relaxation problem by changing the set of relaxed constraints. For example, Balas and Carrera [3] defined the following Lagrangian relaxation problem: (LR (u)) minimize c j a ij u i x j + u i j N i M\ M subject to j N a ij x j 1 (i M) x j {0, 1} (j N), i M\ M (2.5) where M is a maximal subset of M satisfying N h N i = for all h, i M, h i, and u is a vector indexed by M \ M. They have compared both Lagrangian relaxations experimentally, and found that their Lagrangian relaxation was more robust across different instance classes and converged faster when applying the subgradient optimization, though the difference were not drastic Surrogate relaxation The surrogate relaxation problem is defined by replacing some constraints into a surrogate constraint. The standard surrogate relaxation of SCP is defined as follows: (S(w)) minimize z S (w) = c j x j j N subject to ( ) w i a ij x j w i i M j N i M (2.6) x j {0, 1} (j N), where w = (w 1,..., w m ) R m + is a given vector called the surrogate multiplier vector. Compared to Lagrangian relaxation problem, there have been less attempts using the surrogate relaxation problem for integer programming (IP) problems including SCP. Lorena and Lopes [45] proposed a heuristic algorithm for SCP based on a continuous surrogate relaxation.

6 Relaxation Heuristics for SCP Subgradient Optimization As discussed in Section 2.2, any optimal solution ū to the dual of the LP relaxation problem is also an optimal solution to the Lagrangian dual problem; however, computing such ū by general purpose LP solvers is rather expensive, especially for huge SCP instances as confirmed in Table 2. A common approach to compute a near optimal Lagrangian multiplier vector u is the subgradient method [29, 39]. It uses the subgradient vector s(u) = (s 1 (u),..., s m (u)) R m +, associated with a given u, defined by s i (u) = 1 j N a ij x j (u) (i M). (3.1) This method generates a sequence of nonnegative Lagrangian multiplier vectors u (0), u (1),..., where u (0) is a given initial vector and u (l+1) is updated from u (l) by the following formula: { u (l+1) i max u (l) i + λ z } UB z LR (u (l) ) s s(u (l) ) 2 i (u (l) ), 0 (i M), (3.2) where z UB is an upper bound on z(x), and λ 0 is a parameter called the step size. Various implementations of the subgradient method are possible; we therefore start with the following implementation described in Beasley s tutorial [10], and describe several variants afterwards. Let u = (u 1,..., u m ) and z max LR be the incumbent Lagrangian multiplier vector and lower bound, respectively. The basic subgradient method is described as follows: Basic subgradient method (BSM) Step 1: Set z UB z(x) for a feasible solution x of the original SCP, which is obtained by a greedy algorithm (see Section 6.1). Set u (0) i min{c j / S j j N i } and u i u (0) i for all i M and z max LR z LR (u (0) ). Set λ 2 and l 0. Step 2: Compute the current solution x(u (l) ) and the lower bound z LR (u (l) ). If z LR (u (l) ) > z max LR holds, set z max LR z LR (u (l) ) and u i u (l) i for all i M. If z UB z max LR holds, output u and z max LR and halt (in this case, z UB is optimal for the original SCP). Step 3: Compute the subgradient vector s(u (l) ) for the current solution x(u (l) ). If s i (u (l) ) = 0 holds for all i M, output u and z max LR and halt (in this case, x(u (l) ) is an optimal solution for the original SCP); otherwise compute a new Lagrangian multiplier vector u (l+1) by (3.2). Step 4: If z max LR has not improved in the last 30 iterations with the current value of λ, then set λ 0.5λ. If λ holds, output u and z max LR and halt; otherwise let l l + 1 and return to Step 2. The basic subgradient method requires O(q) time for each iteration, where q = i M j N a ij. Beasley [10] reported a number of detailed observations. For example, the convergence of the subgradient method is relatively insensitive to the initial Lagrangian multiplier vector u (0). Another observation is that it is helpful to set s i (u (l) ) 0 when u (l) i s i (u (l) ) < 0 hold, because s i (u (l) ) 2 factor reduces the change of u (l) h (h i). = 0 and The subgradient method converges very slowly when the current Lagrangian multiplier vector u (l) approaches almost optimal (i.e., the gap z UB z LR (u (l) ) gets close to zero). To overcome this, Beasley proposed to replace z UB in (3.2) with 1.05z UB. Caprara et al. [20]

7 356 S. Umetani & M. Yagiura proposed an adaptive control of the step size λ. Their algorithm starts with λ 0.1. For every 20 iterations, it computes the best and worst lower bounds in the last 20 iterations. If the gap is more than 1%, it decreases the step size by setting λ 0.5λ. On the other hand, if the gap is less than 0.1%, it increases the step size by setting λ 1.5λ. Their algorithm halts when the improvement of the best lower bound z max LR in the last 300 iterations is less than 0.1% and 1.0 in value. We call this rule to update λ adaptively rule ASSC (abbreviation for adaptive step size control), and use it in our computational experiments later. Caprara et al. [20] also dealt with slow convergence due to the degeneracy of the Lagrangian relaxation problem, which often appears for huge SCP instances with relatively uniform costs. In such a situation, after a few subgradient iterations, a large number of columns j N happens to have reduced costs c j (u (l) ) to be very close to zero, and consequently the subgradient vector s(u (l) ) highly oscillates in the subsequent iterations. To overcome this, they proposed a heuristic rule to define a vector s(u (l) ) as follows: Heuristic computation of the subgradient vector (HCSV) Step 1: Set Ñ {j N c j(u (l) ) 0.001} and M j Ñ S j. Step 2: Let R {j Ñ k Ñ\{j} S k = M}, which is the set of redundant columns. Sort all columns j R in the descending order of their reduced cost c j (u (l) ). Step 3: For all j R, according to the above order, set Ñ Ñ \{j} if k Ñ\{j} S k = M holds. Step 4: Set ˆx j 1 for all j Ñ and set ˆx j 0 for all j Ñ. Set s i(u (l) ) = 1 j N a ij ˆx j for all i M. This procedure requires O(n log n) time for sorting the redundant columns, plus O(q) time for the remaining computation. Note that the vector s(u (l) ) obtained by the above heuristic rule is no longer guaranteed to be a subgradient vector. Balas and Carrera [3] proposed a simple procedure that transforms the Lagrangian multiplier vector u into a dual feasible solution of the LP relaxation problem without decreasing (and possibly increasing) the associated lower bound z LR (u). The procedure is shown as follows. Transformation of the Lagrangian multiplier vector (TLMV) Step 1: If there exists a column j N with c j (u) < 0, select a row i S j with u i > 0. Otherwise go to Step 3. Step 2: If u i < c j (u) holds, set u i 0; otherwise set u i u i + c j (u). Return to Step 1. Step 3: Set x j (u) 1 if c j (u) = 0 holds and x j (u) 0 if c j (u) > 0 holds for all j N. If there exists no uncovered row i M by the current solution x(u), output u and halt. Otherwise select an uncovered row i M and set u i u i + min j Ni c j (u), and then return to Step 3. This procedure requires O(q) time. It is not hard to observe that z LR (u) never decreases whenever u is updated. The multiplier vector u is feasible to LPD when the algorithm proceeds from Step 1 to Step 3, and remains feasible during the execution of Step 3. This method can be viewed as a hybrid approach of multiplier adjustment (Steps 1 and 2) and dual ascent (Step 3) methods, whose general ideas are summarized in [10].

8 Relaxation Heuristics for SCP 357 Table 3: Lower bounds obtained by the subgradient methods BSM, MSM1 and MSM2 Instance Rows Columns z LP Lower bounds BSM MSM1 MSM A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , RAIL , RAIL , RAIL ,081, RAIL , RAIL ,092, RAIL , Ceria et al. [24] proposed a primal-dual subgradient method, which generates a pair of Lagrangian multiplier vectors (x, u), and approaches the optimal value of the LP relaxation problem from both lower and upper sides. To this end, they first defined a Lagrangian relaxation problem to the LP dual problem (2.4) by introducing a Lagrangian multiplier vector x = (x 1,..., x n ). We can define a Lagrangian dual problem optimizing the Lagrangian multiplier vector x, which is equivalent to the LP relaxation problem for SCP. Accordingly, they proposed a subgradient method, which updates both Lagrangian multiplier vectors x and u simultaneously. We tested the basic subgradient method (BSM) and two modified subgradient methods. The first modified subgradient method (denoted MSM1) uses the rule ASSC to control the step size λ and the heuristic algorithm HCSV to compute the subgradient vector s(u). To be more precise, rules of BSM are modified as follows: (i) The rule to initialize λ in Step 1, the rule to update λ and the stopping criterion in Step 4 are replaced with rule ASSC; (ii) the vector obtained by algorithm HCSV is used instead of the subgradient vector s(u (l) ) in Step 3. The second modified version (MSM2) uses the algorithm TLMV to improve the lower bound at every iteration. More precisely, rules of BSM are modified as follows: Let u be the vector obtained by applying algorithm TLMV to the current multiplier vector u (l), and then use u instead of u (l) in Step 2. The other parts of algorithm MSM1 and MSM2 are exactly the same as BSM. It might seem more natural to use this modified multiplier vector u also in Step 3 to compute the next multiplier vector u (l+1) ; however, we observed through preliminary experiments that the lower bounds obtained with this option is worse than those of BSM. Tables 3 and 4 show the lower bounds, the number of iterations (column iter. ) and the computation time in seconds of BSM, MSM1 and MSM2. From Tables 3 and 4, we can observe that these subgradient methods obtain near optimal lower bounds quickly in spite of their simplicity. We can also observe that the lower bounds of MSM1 are worse than BSM for all instances, and those of MSM2 are slightly better than BSM, though MSM2 consumes

9 358 S. Umetani & M. Yagiura Table 4: Number of iterations and computation time in seconds of the subgradient methods BSM, MSM1 and MSM2 Instance Rows Columns BSM MSM1 MSM2 Iter. Time Iter. Time Iter. Time A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , RAIL , RAIL , RAIL ,081, RAIL , RAIL ,092, RAIL , more computation time than BSM. To see their convergence properties, we illustrate their behavior in Figures 1 and 2, where we imposed no limit on the number of their iterations. From Figures 1 and 2, we can observe that MSM1 and MSM2 obtain comparable lower bounds with smaller numbers of iterations than BSM. 4. Sifting Method In order to solve huge LP relaxation problem, Bixby et al. [15] developed a variant of the column generation method called the sifting method (or pricing method), which generates successive solutions of small subproblems by taking a small subset of columns C N. The sifting method is based on the observation that only a small number of columns j N with negative reduced costs c j (u) < 0 are necessary to compute the current objective value z LP at every iteration of the simplex method. It will therefore be advantageous to solve a number of subproblems called the core problem consisting of a small subset C of columns j with small reduced costs c j (u) and update the current core problem at moderate intervals. Caprara et al. [20] developed a sifting procedure on the subgradient method for SCP. The initial core problem C is defined by taking the columns j N i with the five smallest values of reduced costs c j (u) for each row i M, and the current core problem C is updated every T subgradient iterations. The next core problem C is mainly composed of columns j N with smallest reduced costs c j (u) for the current Lagrangian multiplier vector u. The rule to update the core problem C is formally described as follows. Updating core problem Step 1: Compute the reduced cost c j (u) for all j N. Step 2: Set C 1 {j N c j (u) < 0.1}. For each i M, let C 2 (i) be the columns with the five smallest values of c j (u) among those in N i. Then set C 2 i M C 2(i).

10 Relaxation Heuristics for SCP 359 Lower bound MSM1 LP MSM2 BSM Iteration Figure 1: Comparison of the subgradient methods BSM, MSM1 and MSM2 on instance RAIL516 Step 3: If C 1 > 5m holds, let C 1 be the set of columns j C 1 having the 5m smallest values of c j (u). Set C C 1 C 2. We note that the optimal value for the core problem zlr(u) c = j C c j(u) x j (u)+ i M u i does not necessarily give a lower bound for the original SCP and we need to compute the lower bound z LR (u) by the following formula: z LR (u) = zlr(u) c + min{ c j (u), 0}. (4.1) j N\C c j (u) x j (u) = i M u i + j N One of the most important decisions in implementing the sifting method is how to control the updating interval T. If the sifting method updates the core problem C rarely, the objective value zlr(u) c of the core problem becomes far from the lower bound z LR (u) obtained by the original Lagrangian relaxation problem. On the other hand, if the sifting method updates the core problem C frequently, it consumes much computation time for updating the core problem C and computing the lower bound z LR (u) of the original Lagrangian relaxation problem. They proposed the following sophisticated rule to control the updating interval T. They first set T 10 and compute the relative gap = (zlr(u) z c LR (u))/z UB after each updating, where z UB is the best upper bound obtained by then. If is small, they increase the updating interval T. On the other hand, if is large, they decrease the updating interval T. More precisely, they set 10T T 10 6 < 0.02 T (4.2) 2T 0.02 < > 0.2. Table 5 shows lower bounds and computation time in seconds of two sifting methods, where one sifting method is implemented on the basic subgradient method (BSM) and the other sifting method is a component of CPLEX implemented on the dual simplex

11 360 S. Umetani & M. Yagiura Lower bound MSM2 LP MSM1 BSM Iteration Figure 2: Comparison of the subgradient methods BSM, MSM1 and MSM2 on instance H.1 method. From Table 5, we can see that these sifting methods reduce much computation time without sacrificing the solution quality. Note that, if the sifting method of CPLEX generates a feasible solution u of LPD, it can be used to compute a lower bound of the original SCP. We observed that the lower bounds obtained in this way was very good even with small number of iterations, though the time to compute an exact optimum is more expensive than BSM. 5. Problem Reduction There are many procedures in the literature that test feasibility and reduce the size of an SCP instance by removing redundant rows and columns [7, 21, 30, 32, 43]. Common rules for problem reduction are described as follows: Rule 1: If N i = 0 holds for at least one row i M, the instance is infeasible. Rule 2: If N i = 1 holds for some rows i M, for every such row i, set x j 1 for the unique column j N i, and remove all rows k S j and the column j. Rule 3: If there exists a pair of rows h, i M such that N h N i holds, remove the row i. Rule 4: If there exists a column j N and a set of columns N N \ {j} such that S j ( k N S k ) and c j k N c k hold, set x j 0 and remove the column j. These rules can be applied repeatedly until infeasibility is proven or no rule becomes applicable. If naively implemented, the time complexity of Rules 1 4 are O(m), O(q), O(m 2 n) and O(qn2 n 1 ), respectively. Since a naive implementation of these rules is quite time consuming, they have to be implemented in a careful way, or substituted by other simple rules. For example, Rule 4 is often replaced with the following rule [7, 21, 30]: Rule 4 : If there exists a column j N such that (i) c j > i S j min k Ni c k or (ii) S j > 1 and c j i S j min k Ni c k hold, then set x j 0 and remove the column j.

12 Relaxation Heuristics for SCP 361 Table 5: Computational results of the sifting methods Instance Rows Columns BSM CPLEX9.1.3 z LR Time z LP Time A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , RAIL , RAIL , RAIL ,081, RAIL , RAIL ,092, RAIL , The conditions (i) and (ii) are designed so that they do not remove those columns j that attain the minimum for at least one row (i.e., i, c j = min k Ni c k ); the strict inequality in (i) and the condition S j > 1 in (ii) are necessary for this purpose. The time complexity of this rule is O(q) if appropriately implemented. These rules are usually used for preprocessing, but can be applied at each iteration of heuristic algorithms or each node of branch-and-bound algorithms after some variables are fixed to zero or one. However, since we need much computational effort to reflect the removal of rows and columns on the matrix (a ij ), most algorithms use a part of the problem reduction rules only for preprocessing. We tested the problem reduction rules for the benchmark instances. Since we observed that Rule 3 is not very effective and more time consuming than other rules, we replaced Rule 3 with the following rule: Rule 3 : If there exists a pair of rows h, i M such that N h = N i holds, remove the row i. The time complexity of this rule is O(mn) if appropriately implemented. We also adopt Rule 4 instead of Rule 4 because it is very expensive to check Rule 4 throughly. Since we observed through preliminary experiments that the conditions (i) and (ii) in Rule 4 gave almost the same results, we apply both of them in Rule 4. We apply Rules 1, 2, 3 and 4 repeatedly until no rule is applicable or the infeasibility of the instance is detected. Table 6 reports the size of reduced instances, frequency of each rule (i.e., how many times each rule transforms the instance) and computation time in seconds of the problem reduction procedure. We first observe that only Rule 4 works effectively. However, we also observe that Rule 4 works less effectively for classes E and H, and has very little effect for classes F and RAIL. This is because each column j N covers relatively many rows i M in classes E, F and H and all costs are distributed in a narrow range in RAIL instances.

13 362 S. Umetani & M. Yagiura Table 6: Computational results of the problem reduction procedure Instance Original size Reduced size Frequency Time Rows Columns Rows Columns R2 R3 R A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , , RAIL , , RAIL , , RAIL ,081, ,081, RAIL , , RAIL ,092, ,092, RAIL , , From these results, it is effective to apply the problem reduction procedure with Rules 1, 2, 3 and 4 since its computation time is short. However, taking account of the tradeoff between implementation effort and efficiency, it is worthwhile to apply the reduction procedure only with Rule 4 only if an SCP instance has low density and widely distributed costs. Another type of problem reduction rules are derived from the reduced cost c j (u) of the Lagrangian relaxation problem. We are given a solution x(u) = ( x 1 (u),..., x n (u)) of the Lagrangian relaxation problem LR(u). If we impose an additional constraint x j = 1 for a particular column j N with x j (u) = 0, then we obtain a better lower bound z LR (u)+ c j (u). Similarly, if we impose an additional constraint x j = 0 for a particular column j N with x j (u) = 1 then we obtain a better lower bound z LR (u) c j (u) (recall that x j (u) takes one when c j (u) is negative). Accordingly, we can describe the following problem reduction rules: Rule 5: If x j (u) = 0 and z LR (u) + c j (u) > z UB hold, set x j 0 and remove the column j. Rule 6: If x j (u) = 1 and z LR (u) c j (u) > z UB hold, set x j 1, and remove all rows i S j and the column j. These techniques have often been used in branch-and-bound algorithms and are called the variable fixing (or pegging test). Balas and Carrera [3] proposed an improved variable fixing technique. They first define a new Lagrangian relaxation problem by the removal of the covered rows i S j according to fixing a variable x j to one for a particular column j N. This is done by setting u i 0 for all i S j on the original Lagrangian relaxation problem. Then, they recompute the reduced costs c j (u) for all columns j N i and apply Rule 5. We note that these variable fixing techniques for a variable x j never work when the gap between the upper bound z UB and the lower bound z LR (u) is larger than c j. We also note that it is often complicated or quite time consuming to change the data structure of the

14 Relaxation Heuristics for SCP 363 Table 7: Computational results of the variable fixing Instance Original size Reduced size Frequency Time Rows Columns Rows Columns R5 R A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , , RAIL , , RAIL , , RAIL ,081, ,081, RAIL , , RAIL ,092, ,092, RAIL , , SCP instance when some reduction rules are successfully applied. Beasley [10] reported that it is computationally worthwhile to reflect the removal of rows and columns on the matrix (a ij ) when a significant amount of reduction have been achieved, e.g., 10% of variables are fixed to zero or one. Table 7 shows the size of reduced instances, frequency of each rule and computation time in seconds of the basic subgradient method (BSM) including the time for checking Rules 5 and 6, which are applied at every iteration. From Table 7, we observe that Rule 5 fixes many variables with little computational effort for classes 4 6 and A H while fixing no variable for RAIL instances. From this, it is worth applying Rule 5 if an SCP instance has widely distributed costs. We also illustrate the number of fixed variables at every iteration in Figure 3. We can observe that BSM with Rules 5 and 6 fixes many variables with a small number of iterations; the number of fixed variables reaches 5000 very quickly, and becomes more than 7000 after a few hundred iterations. 6. Heuristic Algorithms 6.1. Construction algorithms Several construction algorithms with performance guarantee have been developed for SCP. These construction algorithms are not only interesting in theoretical aspect but contribute to develop efficient heuristic algorithms in practical applications. In this section, we explain five representative construction algorithms and compare their experimental performance. One of the most representative construction algorithms for SCP is the greedy algorithm, which iteratively selects the most cost effective column j N until all rows i M are covered. Greedy algorithm Step 1: Set M and x j 0 for all j N. Step 2: If M = M holds, go to Step 3. Otherwise, find a column j N with the

15 364 S. Umetani & M. Yagiura Number of fixed variables Iteration Figure 3: Number of fixed variables at every iteration on instance H.1 minimum cost effectiveness c j / S j \ M among those with x j = 0, set x j 1 and M M S j, and return to Step 2. Step 3: If there exists a redundant column j N (i.e., x j = 1 and j N\{j} a ij x j = 1 for all i M), set x j 0 and return to Step 3; otherwise output x and halt. In the above algorithm, redundant columns are usually removed in the reverse order of selecting columns, which is called the reverse delete step. The greedy algorithm is known to be an H n -approximation algorithm [25], where H n is called the Harmonic series defined as follows H n = n, and is roughly estimated by O(log n). Ho [40] showed that all variants of the greedy algorithm on the cost effectiveness functions c j c j c j, S j \ M, c j log 2 S j \ M, S j \ M log 2 S j \ M, c j S j \ M ln S j \ M, c j S j \ M, (c j ) 1/2 2 S j \ M 2 have the same worst case behavior. Balas and Ho [4] and Vasko and Wilson [47] showed experimental performance of the greedy algorithm with various cost effectiveness functions. Although a naive implementation of the greedy algorithm requires O(mn) time, Caprara et al. [20] developed O(rn + q) time implementation of the greedy algorithm by devising an efficient procedure to update cost effectiveness, where r = j N x j. Since the term rn, which is the total time for finding the most cost effective column, is typically much larger than q for SCP instances of low density, they also developed an efficient implementation to reduce the practical computation time of this part substantially (though its worst case time complexity is the same). Another natural algorithm is obtained by rounding an optimal solution x = ( x 1,..., x n ) of the LP relaxation problem into an integer solution x = (x 1,..., x n ). We first define f = max i M N i, which is the frequency of the most frequent column. The rounding algorithm for SCP gives an integer solution x = (x 1,..., x n ) as follows: { 1 x j 1/f x j (6.1) 0 otherwise.

16 Relaxation Heuristics for SCP 365 The solution x is feasible for the original SCP (i.e., j N a ijx j 1 for all i M) since j N a ij x j 1 for each i M and hence at least one j N i must satisfy x j 1/ N i 1/f. The rounding algorithm requires O(n) time in addition to the computation time for solving the LP relaxation problem, and is known to be an f-approximation algorithm [48]. Note that, since we have N i = 2 for all rows i M in the vertex cover problem (VCP), any f-approximation algorithm for SCP is a 2-approximation algorithm for VCP. The primal-dual method is one of the most representative approaches for designing approximation algorithms. It is based on the following characterization of optimal solutions ( x, ū) to a pair of primal and dual LP problems, called the primal and dual complementary slackness conditions, respectively: x j > 0 i M a ij ū i = c j (j N), (6.2) ū i > 0 j N a ij x j = 1 (i M), (6.3) where (6.2) is also written as x j > 0 c j (u) = 0. Hochbaum [41] proposed a simple primal-dual algorithm. It first finds an optimal solution ū of the dual LP relaxation problem, and set x j 1 for all j N satisfying i M a ijū i = c j and x j 0 for the others. To see x is feasible to the original SCP, observe that, if there exists a row i that satisfies i M a ijū i < c j for all j N i, then we can increase ū i slightly without violating the dual feasibility, which contradicts with the optimality of ū. The running time of this algorithm is O(n) in addition to the computation time for solving the LP relaxation problem. Since the solution x obtained by this algorithm satisfies the primal complementary slackness condition, we observe that c j x j = ( ) a ij ū i x j = ( ) a ij x j ū i max N i ū i f z LP. (6.4) i M j N j N i M i M j N i M Hence, this algorithm is an f-approximation algorithm. A typical primal-dual algorithm starts from a pair of primal infeasible solution x and dual feasible solution u. It iteratively modifies the current solution (x, u) to improve primal feasibility and dual optimality while satisfying the primal complementary slackness condition (6.2) until a primal feasible solution is obtained. During the iterations, the primal solution x is always modified integrally, so that eventually we obtain an integer solution. We describe a basic primal-dual algorithm for SCP by Bar-Yehuda and Even [6]. Basic primal-dual algorithm Step 1: Set x j 0 for all j N and u i 0 for all i M. Step 2: If there exists no uncovered row i M, go to Step 3. Otherwise select an uncovered row i M and set { u i min j N i c j h M,h i a hj u h } Then, set x j 1 for a column j N i satisfying h M a hju h = c j and return to Step 2. Step 3: If there exists a redundant column j N, set x j 0 and return to Step 3; otherwise output x and halt..

17 366 S. Umetani & M. Yagiura Note that we do not need to solve LP relaxation problems though the algorithm is based on the properties of LP relaxation. The basic primal-dual algorithm can be easily implemented with O(q) time. Since the above analysis on the performance guarantee (6.4) is also applicable to the basic primal-dual algorithm by replacing ū with the u obtained by the algorithm, it is also an f-approximation algorithm. Agrawal et al. [1] developed an improved primal-dual algorithm for network design problems, which increases simultaneously and at the same speed all the dual variables that can be increased without violating the dual feasibility. They also showed the primal-dual algorithms with such uniform increase rule have better performance guarantees for several network design problems than the basic primal-dual algorithms [1, 34]. Goemans and Williamson [35] applied this method to SCP and showed detailed analysis of its performance guarantee. Let M be the set of covered rows i M and be the increase of the dual variables u i for uncovered rows i M \ M at each iteration. When dual variables u i for all i M \ M are increased by, the reduced cost c j (u) for each j N decreases S j \ M. From this, at each iteration is computed by the following formula: min j N i,i M c j (u) S j \ M. (6.5) Primal-dual algorithm with uniform increase rule Step 1: Set M, x j 0 for all j N and u i 0 for all i M. Step 2: If M = M holds, go to Step 3. Otherwise compute by (6.5) and set u i u i + for all i M \ M. Set x j 1 and M M S j for all columns j N satisfying i M a iju i = c j, and then return to Step 2. Step 3: If there exists a redundant column j N, set x j 0 and return to Step 3; otherwise output x and halt. Since the improved primal-dual algorithm selects columns j N with the minimum score c j (u)/ S j \M at each iteration, we can regard the primal-dual algorithm as a variant of the greedy algorithm. Although the running time of a naive implementation of the primal-dual algorithm is O(mn), it can be improved in the same way as the greedy algorithm. There have been a few computational studies of construction algorithms. Grossman and Wool [37] conducted a comparative study of nine construction algorithms for unweighted SCP (i.e., all columns have a uniform cost). Gomes et al. [36] investigated empirical performance of seven construction algorithms for VCP and SCP. We now compare five construction algorithms: (i) the greedy algorithm (denoted GR), (ii) the basic primal-dual algorithm (denoted BPD), (iii) the primal-dual algorithm with uniform increase rule (denoted PD-UI), (iv) the primal-dual algorithm using a dual optimal solution ū of the LP relaxation problem (denoted PD-LP) and (v) the rounding algorithm (denoted RND). We note that PD-LP and RND use primal and dual optimal solutions ( x, ū) of the LP relaxation problem obtained by CPLEX We also note that GR, BPD and PD-UI remove redundant columns by the reverse delete step and PD-LP and RND remove redundant columns in the ascending order of values of the optimal solution x of the LP relaxation problem. Tables 8 and 9 show upper bounds and computation time in seconds respectively, where computation time of PD-LP and RND does not include time for solving the LP relaxation problem. From Tables 8 and 9, we can observe that BPD is quite fast but much worse than the other algorithms. This is because the dual feasible solution u obtained by BPD is much

18 Relaxation Heuristics for SCP 367 Table 8: Computational results of the construction algorithms Instance Rows Columns GR BPD PD-UI PD-LP RND A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , RAIL , RAIL , RAIL ,081, RAIL , RAIL ,092, RAIL , worse than that obtained by solving the LP relaxation problem such as PD-LP and RND. We also observe that GR is most effective for its computational effort because PD-LP and RND need much computational effort for solving the LP relaxation problem. On the other hand, PD-LP and RND give better upper bounds than those of GR for RAIL instances. This is because many candidates have the same cost effectiveness for RAIL instances Lagrangian heuristics Solutions of the Lagrangian relaxation problem can be used to construct feasible solutions to the original SCP, which is called the Lagrangian heuristics. The Lagrangian heuristics starts with an optimal solution of the Lagrangian relaxation problem x(u) and tries to convert it into a feasible solution x to the original SCP, where the greedy algorithm and the primal-dual algorithm are often used for this purpose. Beasley [8] and Haddadi [38] proposed a Lagrangian heuristic algorithm, which generates a feasible solution for the original SCP at every iteration of the subgradient method. We describe the basic procedure to generate a feasible solution x = (x 1,..., x n ) for the original SCP from an optimal solution x(u) = ( x 1 (u),..., x n (u)) of the Lagrangian relaxation problem. Basic Lagrangian heuristic algorithm Step 1: Set x j x j (u) for all j N and M j N, x j (u)=1 S j. Step 2: If M = M holds, go to Step 3. Otherwise, select an uncovered row i M \ M, set x j 1 and M M S j for the column j N i with the minimum cost c j and return to Step 2. Step 3: If there exists a redundant column j N, set x j 0 and return to Step 3; otherwise output x and halt. A number of computational studies have shown that almost equivalent near optimal Lagrangian multiplier vectors can produce upper bounds of substantially different quality. It is therefore worthwhile applying the Lagrangian heuristics for several near optimal Lagrangian

19 368 S. Umetani & M. Yagiura Table 9: Computation time in seconds of the construction algorithms Instance Rows Columns GR BPD PD-UI PD-LP RND <0.01 <0.01 <0.01 <0.01 < <0.01 <0.01 <0.01 <0.01 < <0.01 <0.01 <0.01 <0.01 <0.01 A.1 A <0.01 <0.01 <0.01 <0.01 <0.01 B.1 B <0.01 <0.01 <0.01 <0.01 C.1 C <0.01 <0.01 <0.01 <0.01 D.1 D <0.01 <0.01 <0.01 <0.01 <0.01 E.1 E <0.01 <0.01 <0.01 <0.01 F.1 F < <0.01 <0.01 G.1 G ,000 <0.01 < <0.01 <0.01 H.1 H , < <0.01 <0.01 RAIL , < <0.01 RAIL , < <0.01 RAIL , < <0.01 RAIL ,081, RAIL , RAIL ,092, RAIL , Note: The computation time of PD-LP and RND does not include the time for solving the LP relaxation problem. multiplier vectors. As in the case of the greedy algorithm, the Lagrangian heuristics also has many criteria to select the next column; e.g., the reduced cost c j (u) is often used instead of the original cost c j. This is based on the observation that the reduced cost c j (u) gives a reliable information on the attractiveness of letting x j 1, because each column j N with x j = 1 in an optimal solution x of the original SCP tends to have a small reduced cost c j (u). Balas and Carrera [3] proposed another Lagrangian heuristic algorithm, which first transforms the Lagrangian multiplier vector u into a dual feasible solution of the LP relaxation problem and then applies the basic primal-dual algorithm. Tables 10 and 11 show upper bounds and computation time in seconds of four variants of Lagrangian heuristic algorithms: (i) the greedy algorithm using original cost c j (denoted LH- OC), (ii) the greedy algorithm using reduced cost c j (u) (denoted LH-RC), (iii) the greedy algorithm using cost effectiveness c j / S j \ M (denoted LH-CE) and (iv) the basic primaldual algorithm (denoted LH-PD), where each Lagrangian heuristic algorithm is applied at every iteration and removes redundant columns by the reverse delete step. From Tables 10 and 11, we observe that LH-RC, LH-CE and LH-PD are more promising than LH-OC for RAIL instances, while only LH-CE is more promising than LH-OC for classes 4 6 and A H, although computing the cost effectiveness is expensive. We note that these Lagrangian heuristic algorithms are not necessarily applied at every iteration in the subgradient method since it is quite time consuming for huge SCP instances. We also note that the Lagrangian heuristics is helpful to fix variables x j to zero or one because it often improves the best upper bound z UB, and our computational results show that we can increase the number of fixed variables by 9.5% on the average using the Lagrangian heuristics.

20 Relaxation Heuristics for SCP 369 Table 10: Computational results of the Lagrangian heuristic algorithms Instance Rows Columns LH-OC LH-RC LH-CE LH-PD A.1 A B.1 B C.1 C D.1 D E.1 E F.1 F G.1 G , H.1 H , RAIL , RAIL , RAIL , RAIL ,081, RAIL , RAIL ,092, RAIL , The state-of-the-art heuristic algorithms In this section, we review the most effective heuristic algorithms for SCP. Many of them are based on the Lagrangian relaxation. Since a better Lagrangian multiplier vector u does not necessarily derive a better upper bound z(x), many heuristic algorithms explore both good Lagrangian multiplier vectors u and feasible solutions x of the original SCP simultaneously. Ceria et al. [24] proposed a Lagrangian heuristic algorithm based on the primal-dual subgradient method as described in Section 3. To deal with huge SCP instances, their algorithm first defines a good core problem C N based on a near optimal Lagrangian multiplier vector u obtained by the primal-dual subgradient method. Differently from the sifting method in Section 4, their algorithm determines the core problem C in a careful way and never changes it afterwards. Their Lagrangian heuristic algorithm applies the primal-dual subgradient method, fixes a variable x j to one which is selected by its reduced cost c j (u) and primal Lagrangian multiplier, and applies a greedy algorithm for remaining columns to generate a feasible solution x of the original SCP. This procedure is repeated with different parameters for the variable selection and the greedy algorithm. Caprara et al. [20] proposed a three phase heuristic algorithm. The first one is called the subgradient phase shown in Section 3. The second one is called the heuristic phase, which generates a sequence of near optimal Lagrangian multiplier vectors u and applies a greedy algorithm using reduced costs c j (u). The third one is called the column fixing phase, which fixes the first k columns selected by the greedy algorithm in order to reduce the size of rows and columns to be explored by the three phase heuristic algorithm. The above three phases are repeated until the incumbent solution x cannot be improved further. The three phase heuristic algorithm works on a core problem C N defined by a small subset of columns, which is periodically redefined as described in Section 4. Wedelin [49] proposed another type of Lagrangian heuristics for a special type of 0-1 integer programming problem, which is a generalization of the set covering and partitioning problems. Differently from the subgradient method, he used the coordinate ascent method (or nonlinear Gauss-Seidel method) [13] to solve the Lagrangian dual problem, which iteratively

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