# The k-center problem February 14, 2007

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1 Sanders/van Stee: Approximations- und Online-Algorithmen 1 The k-center problem February 14, 2007 Input is set of cities with intercity distances (G = (V,V V )) Select k cities to place warehouses Goal: minimize maximum distance of a city to a warehouse Other application: placement of ATMs in a city

2 Sanders/van Stee: Approximations- und Online-Algorithmen 2 Results NP-hardness Greedy algorithm, approximation ratio 2 Technique: parametric pruning Second algorithm with approximation ratio 2 Generalization of Algorithm 2 to weighted problem

3 Sanders/van Stee: Approximations- und Online-Algorithmen 3 Theorem 1. It is NP-hard to approximate the general k-center problem within any factor α. Proof. Reduction from Dominating Set... Dominating set = subset S of vertices such that every vertex which is not in S is adjacent to a vertex in S. Finding a dominant set of minimal size is NP-hard For a graph G, dom(g) is the size of the smallest possible dominating set Dominating set is similar to but not the same as vertex cover!

4 Sanders/van Stee: Approximations- und Online-Algorithmen 4 Dominating set and vertex cover Vertex cover = subset S of vertices such that every edge has at least one endpoint in S The black vertices form a dominating set but not a vertex cover. Also, not every vertex cover is a dominating set.isolated vertex.

5 Sanders/van Stee: Approximations- und Online-Algorithmen 5 Proof We want to find a Dominating Set in G = (V,E). Consider G = (V,V V ) and the weight function 1 if (u,v) E d(u,v) = 2α else

6 Sanders/van Stee: Approximations- und Online-Algorithmen 6 Proof We want to find a Dominating Set in G = (V,E). Consider G = (V,V V ) and the weight function d(u,v) = 1 if (u,v) E 2α else Suppose G has a dominating set of size at most k. Then there is a k-center of cost 1 in G an α-approx. algorithm delivers one with weight α

7 Sanders/van Stee: Approximations- und Online-Algorithmen 7 Proof We want to find a Dominating Set in G = (V,E). Consider G = (V,V V ) and the weight function d(u,v) = 1 if (u,v) E 2α else Suppose G has a dominating set of size at most k. Then there is a k-center of cost 1 in G an α-approx. algorithm delivers one with weight α If there is no such dominating set in G, every k-center has weight 2α > α.

8 Sanders/van Stee: Approximations- und Online-Algorithmen 8 Proof (continued) Assume that there exists an α-approximation algorithm for the k-center problem. Decision algorithm: Run α-approx algorithm on G Solution has weight α dominating set of size at most k exists Else there is no such dominating set.

9 Sanders/van Stee: Approximations- und Online-Algorithmen 9 Metric k-center G is undirected and obeys the triangle inequality u,v,w V : d(u,w) d(u,v) + d(v,w) We show two 2-approximation algorithms for this problem.

10 Sanders/van Stee: Approximations- und Online-Algorithmen 10 The Greedy algorithm Choose the first center arbitrarily At every step, choose the vertex that is furthest from the current centers to become a center Continue until k centers are chosen 2 1 3

11 Sanders/van Stee: Approximations- und Online-Algorithmen 11 Analysis Note that the sequence of distances from a new chosen center, to the closest center to it (among previously chosen centers) is non-increasing Consider the point that is furthest from the k chosen centers We need to show that the distance from this point to the closest center is at most 2 OPT Assume by negation that it is > 2 OPT

12 Sanders/van Stee: Approximations- und Online-Algorithmen 12 Analysis We assumed that the distance from the furthest point to all centers is > 2 OPT This means that distances between all centers are also > 2 OPT We have k + 1 points with distances > 2 OPT between every pair

13 Sanders/van Stee: Approximations- und Online-Algorithmen 13 Analysis Each point has a center of the optimal solution with distance OPT to it There exists a pair of points with the same center X in the optimal solution (pigeonhole principle: k optimal centers, k + 1 points) The distance between them is at most 2 OPT (triangle inequality) Contradiction!

14 Sanders/van Stee: Approximations- und Online-Algorithmen 14 Technique: parametric pruning ausschneiden, abschneiden Idea: remove irrelevant parts of the input Suppose OPT = t We want to show a 2-approximation Any edges of cost more than 2t are useless: if two vertices are connected by such an edge, and one of them gets a warehouse, the other one is still too far away We can remove edges that are too expensive Of course, we don t know OPT. But we can guess.

15 Sanders/van Stee: Approximations- und Online-Algorithmen 15 Technique: parametric pruning We can order the edges by cost: cost(e 1 )... cost(e m ) Let G i = (V,E i ) where E i = {e 1,...,e i ) The k-center problem is equivalent to finding the minimal i such that G i has a dominating set of size k Let i be this minimal i Then, OPT = cost(e i )

16 Sanders/van Stee: Approximations- und Online-Algorithmen 16 Graph squaring For a graph G, the square G 2 = (V,E ) where (u,v) E if there is a path of length at most 2 between u and v in G (and u v) G G 2

17 Sanders/van Stee: Approximations- und Online-Algorithmen 17 Lemma 2. For any independent set I in G 2, we have I dom(g). Proof. Let D be a minimum dominating set in G. (The size of D is dom(g).)

18 Sanders/van Stee: Approximations- und Online-Algorithmen 18 Lemma 2. For any independent set I in G 2, we have I dom(g). Proof. Let D be a minimum dominating set in G. Then G contains D stars spanning all vertices (the nodes of D are the centers of the stars).

19 Sanders/van Stee: Approximations- und Online-Algorithmen 19 Lemma 2. For any independent set I in G 2, we have I dom(g). Proof. Let D be a minimum dominating set in G. Then G contains D stars spanning all vertices (the nodes of D are the centers of the stars). A star in G becomes a clique in G 2.

20 Sanders/van Stee: Approximations- und Online-Algorithmen 20 Lemma 2. For any independent set I in G 2, we have I dom(g). Proof. Let D be a minimum dominating set in G. Then G contains D stars spanning all vertices (the nodes of D are the centers of the stars). A star in G becomes a clique in G 2. So G 2 contains D = dom(g) cliques spanning all vertices.

21 Sanders/van Stee: Approximations- und Online-Algorithmen 21 Lemma 2. For any independent set I in G 2, we have I dom(g). Proof. Let D be a minimum dominating set in G. Then G contains D stars spanning all vertices (the nodes of D are the centers of the stars). A star in G becomes a clique in G 2. So G 2 contains D = dom(g) cliques spanning all vertices. There can only be one vertex of each clique in I.

22 Sanders/van Stee: Approximations- und Online-Algorithmen 22 Algorithm We use that maximal independent sets can be found in polynomial time. Construct G 2 1,G2 2,...,G2 m Find a maximal independent set M i in each graph G 2 i Determine the smallest i such that M i k, call it j Return M j. Lemma 3. For this j, cost(e j ) OPT. Lemma 4. This algorithm gives a 2-approximation.

23 Sanders/van Stee: Approximations- und Online-Algorithmen 23 Lemma 3. For this j, cost(e j ) OPT. Proof. For every i < j... M i > k by the definition of our algorithm dom(g i ) > k by Lemma 2 Then i > i Therefore, i j.

24 Sanders/van Stee: Approximations- und Online-Algorithmen 24 Lemma 4. This algorithm gives a 2-approximation. Proof. My my my... Any maximal independent set I in G 2 j is also a dominating set (if some vertex v were not dominated, I v were also independent)

25 Sanders/van Stee: Approximations- und Online-Algorithmen 25 Lemma 4. This algorithm gives a 2-approximation. Proof. My my my... Any maximal independent set I in G 2 j is also a dominating set (if some vertex v were not dominated, I v were also independent) In G 2 j, we have M j stars centered on the vertices in M j

26 Sanders/van Stee: Approximations- und Online-Algorithmen 26 Lemma 4. This algorithm gives a 2-approximation. Proof. My my my... Any maximal independent set I in G 2 j is also a dominating set (if some vertex v were not dominated, I v were also independent) In G 2 j, we have M j stars centered on the vertices in M j These stars cover all the vertices

27 Sanders/van Stee: Approximations- und Online-Algorithmen 27 Lemma 4. This algorithm gives a 2-approximation. Proof. My my my... Any maximal independent set I in G 2 j is also a dominating set (if some vertex v were not dominated, I v were also independent) In G 2 j, we have M j stars centered on the vertices in M j These stars cover all the vertices Each edge used in constructing these stars has cost at most 2 cost(e j ) 2 OPT The last inequality follows from Lemma 3.

28 Sanders/van Stee: Approximations- und Online-Algorithmen 28 Lemma 5. If P NP, no approximation algorithm gives a (2 ε)-approximation for any ε > 0. We again use a reduction from Dominating Set This time, the graph must satisfy the triangle inequality We define G as follows: d(u,v) = 1 if (u,v) E 2 else This graph satisfies the triangle inequality (proof?)

29 Sanders/van Stee: Approximations- und Online-Algorithmen 29 Suppose G has a dominating set of size at most k. Then there is a k-center of cost 1 in G a (2 ε)-approx. algorithm delivers one with weight < 2 If there is no such dominating set in G, every k-center has weight 2 > 2 ε. Thus, a (2 ε)-approximation algorithm for the k-center problem can be used to determine whether or not there is a dominating set of size k.

30 Sanders/van Stee: Approximations- und Online-Algorithmen 30 Weighted k-center problem Input is set of cities with intercity distances (G = (V,V V )) Each city has a cost Select cities of cost at most W to place warehouses Goal: minimize maximum distance of a city to a warehouse

31 Sanders/van Stee: Approximations- und Online-Algorithmen 31 Ideas We use the same graphs G 1,...,G m as before Let wdom(g) be the weight of a minimum weight dominating set in G We look for the smallest index i such that wdom(g i ) W We also use graph squaring again

32 Sanders/van Stee: Approximations- und Online-Algorithmen 32 The set of light neighbors Let I be an independent set in G 2 For any node u, let s(u) be the lightest neighbor of u Here, we also consider u to be a neighbor of itself Let S = {s(u) u I} We claim w(s) wdom(g) (Compare the unweighted problem, where we had I dom(g))

33 Sanders/van Stee: Approximations- und Online-Algorithmen 33 Lemma 6. w(s) wdom(g) Proof. Let D be a minimum weight dominating set in G. Then G contains D stars spanning all vertices (the nodes of D are the centers of the stars). A star in G becomes a clique in G 2. So G 2 contains D cliques spanning all vertices. There can only be one vertex of each clique in I. For each vertex in I, the center of the corresponding star is available as a neighbor in G (this might not be the lightest neighbor). Therefore w(s) wdom(g).

34 Sanders/van Stee: Approximations- und Online-Algorithmen 34 Algorithm Let s i (u) denote a lightest neighbor of u in G i. Construct G 2 1,...,G2 m Compute a maximal independent set M i in each graph G 2 i Compute S i = {s i (u) u M i } Find the minimum index i such that w(s i ) W, say j Return S j

35 Sanders/van Stee: Approximations- und Online-Algorithmen 35 Lemma 7. This algorithm achieves a 3-approximation. As before we have OPT cost(e j ) For every i < j... w(s i ) > W by the definition of our algorithm wdom(g i ) > W by Lemma 6 Then i > i Therefore, i j.

36 Sanders/van Stee: Approximations- und Online-Algorithmen 36 Lemma 7. This algorithm achieves a 3-approximation. As before we have OPT cost(e j ) M j is a dominating set in G 2 j It is a maximal independent set

37 Sanders/van Stee: Approximations- und Online-Algorithmen 37 Lemma 7. This algorithm achieves a 3-approximation. As before we have OPT cost(e j ) M j is a dominating set in G 2 j We can cover V with stars of G 2 j centered in vertices of M j

38 Sanders/van Stee: Approximations- und Online-Algorithmen 38 Lemma 7. This algorithm achieves a 3-approximation. As before we have OPT cost(e j ) M j is a dominating set in G 2 j We can cover V with stars of G 2 j centered in vertices of M j These stars as before use edges of cost at most 2 cost(e j ) (triangle inequality)

39 Sanders/van Stee: Approximations- und Online-Algorithmen 39 Lemma 7. This algorithm achieves a 3-approximation. As before we have OPT cost(e j ) M j is a dominating set in G 2 j We can cover V with stars of G 2 j centered in vertices of M j These stars as before use edges of cost at most 2 cost(e j ) (triangle inequality) Each star center is adjacent to a vertex in S j, using an edge of cost at most cost(e j )

40 Sanders/van Stee: Approximations- und Online-Algorithmen 40 hi 2cost(e j) u cost(e ) j s (u) j A star in G 2 jand some additional nonsense to keep Latex happy

41 Sanders/van Stee: Approximations- und Online-Algorithmen 41 3cost(e ) j u s (u) j A star in G 2 j with redefined centers and some additional nonsense to keep Latex happy Thus every node in G j can be reached at cost at most 3 cost(e j ) from some vertex in S. This completes the proof.

42 Sanders/van Stee: Approximations- und Online-Algorithmen 42 Lower bound for this algorithm M M M a b c d There are n nodes of weight M. The bound W = 3. All edges not shown have weight equal to the length of the shortest path in the graph that is shown For i < n + 3, G i is missing at least one edge of weight One vertex will be isolated (also in G 2 i ) so it will be in S i

43 Sanders/van Stee: Approximations- und Online-Algorithmen 43 Lower bound for this algorithm M M M a b c d There are n nodes of weight M. The bound W = 3. All edges not shown have weight equal to the length of the shortest path in the graph that is shown For i = n + 3, {b} is a maximal independent subset If our algorithm chooses {b}, it outputs S n+3 = {a}. Cost is 3.

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