# Planar embeddability of the vertices of a graph using a fixed point set is NP-hard

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1 Planar embeddability of the vertices of a graph using a fixed point set is NP-hard Sergio Cabello institute of information and computing sciences, utrecht university technical report UU-CS wwwcsuunl

2 Planar embeddability of the vertices of a graph using a fixed point set is NP-hard Sergio Cabello Institute of Information and Computing Sciences Utrecht University The Netherlands Abstract Let G = (V, E) be a graph with n vertices and let P be a set of n points in the plane We show that deciding whether there is a planar straight-line embedding of G such that the vertices V are embedded onto the points P is NP-complete, even when G is 2-connected and 2-outerplanar This settles an open problem posed in [2, 4, 13] 1 Introduction A geometric graph H is a graph G(H) together with an injective mapping of its vertices into the plane An edge of the graph is drawn as a straight-line segment joining its vertices We use V (H) for the set of points where the vertices of G(H) are mapped to, and we do not make a distinction between the edges of G(H) and H A planar geometric graph is a geometric graph such that its edges intersect only at common vertices In this case, we say that H is a geometric planar embedding of G(H) See [14] for a survey on geometric graphs Let P be a set of n points in the plane, and let G be a graph with n vertices What is the complexity of deciding if there is a straight-line planar embedding of G such that the vertices of G are mapped onto P? This question has been posed as open problem in [2, 4, 13], and here we show that this decision problem is NP-complete Let us rephrase the result in terms of geometric graphs Theorem 1 Let P be a set of n points, and let G be a graph on n vertices Deciding if there exists a geometric planar embedding H of G such that V (H) = P is an NP-complete problem The reduction is from 3-partition, a strongly NP-hard problem to be described below, and it constructs a 2-connected graph G The proof is given in Section 2, and we use that the maximal 3-connected blocks of a 2-connected planar graph can be embedded in different faces In a 3-connected planar graph, all planar embeddings are topologically equivalent due to Whitney s theorem [9, Chapter 6] Therefore, it does not seem possible to extend our technique to show the hardness for 3-connected planar graphs Partially supported by the Cornelis Lely Stichting 1

3 Related work A few variations of the problem of embedding a planar graph into a fixed point set have been considered The problem of characterizing what class of graphs can be embedded into any point set in general position (no three points being collinear) was posed in [11] They showed that the answer is the class of outerplanar graphs, that is, graphs that admit a straight-line planar embedding with all vertices in the outerface This result was rediscovered in [5], and efficient algorithms for constructing such an embedding for a given graph and a given point set are described in [2] The currently best algorithm runs in O(n log 3 n) time, although the best known lower bound is Ω(n log n) A tree is a special case of outerplanar graph In this case, we also can choose to which point the root should be mapped See [3, 12, 15] for the evolution on this problem, also from the algorithmical point of view For this setting, there are algorithms running in O(n log n) time, which is worst case optimal Bipartite embeddings of trees were considered in [1] If we allow each edge to be represented by a polygonal path with at most two bends, then it is always possible to get a planar embedding of a planar graph that maps the vertices to a fixed point set [13] If a bijection between the vertices and the point set is fixed, then we need O(n 2 ) bends in total to get a planar embedding of the graph, which is also asymptotically tight in the worst case [16] We finish by mentioning a related problem, which was the initial motivation for this research A universal set for graphs with n vertices is a set of points S n such that any planar graph with n vertices has a straight-line planar embedding whose vertices are a subset of S n Asymptotically, the smallest universal set is known to have size at least 1098n [6], and it is bounded by O(n 2 ) [7, 17] Characterizing the asymptotic size of the smallest universal set is an interesting open problem [8, Problem 45] 2 Planar embeddability is NP-complete It is clear that the problem belongs to NP: a geometric graph H with V (H) = P and G(H) G can be described by the bijection between V (G) and P, and for a given bijection we can test in polynomial time whether it actually is a planar geometric graph; therefore, we can take as certificate the bijection between V (G) and P For showing the NP-hardness, the reduction is from 3-partition Problem: 3-partition Input: A natural number B, and 3n natural numbers a 1,,a 3n with B 4 < a i < B 2 Output: n disjoint sets S 1,,S n {a 1,,a 3n } with S j = 3 and a S j a = B for all S j We will use that 3-partition is a strongly NP-hard problem, that is, it is NP-hard even if B is bounded by a polynomial in n [10] Observe that because B 4 < a i < B 2, it does not make sense to have sets S j with fewer or more than 3 elements That is, it is equivalent to ask for subdividing all the numbers into disjoint sets that sum to B Of course, we can assume that 3n i=1 a i = Bn, as otherwise it is impossible that a solution exists Given a 3-partition instance, we construct the following graph G (see Figure 1): Start with a 4-cycle with vertices v 0,,v 3, and edges (v i 1,v i mod 4 ) The vertices v 0 and v 2 will play a special role For each a i in the input, make a path B i consisting of a i vertices, and put an edge between each of those vertices and the vertices v 0,v 2 2

4 v 1 a 1 (B + 3)n v 0 B i a i v 2 a 3n n 1 times T n 1 v 3 Figure 1: Graph G for the NP-hardness reduction Construct n 1 triangles T 1,,T n 1 For each triangle T i, put edges between each of its vertices and v 2, and edges between two of its vertices and v 0 We call each of these structures a separator (the reason for this will become clear later) Make a path C of (B + 3)n vertices, and put edges between each of the vertices in C and v 0 Furthermore, put an edge between one end of the path and v 1, and another edge between the other end and v 3 It is easy to see that G is planar; in fact, we are giving a planar embedding of it in Figure 1 The idea is to design a point set P such that G \ (B 1 B 3n ) can be embedded onto P in essentially one way Furthermore, the embedding of G \ (B 1 B 3n ) will decompose the rest of the points into n groups, each of B vertices and lying in a different face The embedding of the remaining vertices B 1 B 3n will be possible in a planar way if and only if the paths B i can be decomposed into groups of exactly B vertices, which is equivalent to the original 3-partition instance The following point set P will do the work (see Figure 2): Let K := (B + 2)n Place (B+3)n points at coordinates (0, n),(0, (n 1)),,(0, 1) and at coordinates (0,1),(0,2),,(0,K) 3

5 K p 1 B B n times 0 1 n B p 0 p 2 n 0 1 K 2K p 3 Figure 2: Point set P for the NP-hardness reduction K = (B + 2)n Place points p 0 := (1,0),p 1 := (K,K),p 2 := (2K,0),p 3 := (K, n) In the figure, these points are shown as boxes and are labeled For each i {0,,n 1}, place the group of B points (K, 2+(B +2)i+1),(K, 2+ (B + 2)i + 2),,(K, 2 + (B + 2)i + B) For each i {1,,n 1}, place the group of three points (K,(B + 2)i 3),(K,(B + 2)i 2),,(K +1,(B +2)i 3) In the figure, these points are shown as empty circles Let CH (P) be the points in the convex hull of P Notice that CH(P) consists of the points with x-coordinate equal to 0, and the points p 1,p 2,p 3 The rest of the proof goes in two steps Firstly, we will show that, in any geometric planar graph H such that G(H) is isomorphic to G and V (H) = P, the vertices of C {v 1,v 2,v 3 } are mapped to CH (P), and the vertices v 0,v 1,v 2,v 3 are mapped either to p 0,p 1,p 2,p 3, respectively, or to p 0,p 3,p 2,p 1, respectively In particular, v 0,v 2 are always mapped to p 0,p 2, respectively Secondly, we will discuss why a mapping of the rest of the vertices, namely vertices in G \ (C {v 0,v 1,v 2,v 3 }), onto the rest of the points, namely P \ (CH (P) {p 0 }), provides a geometric planar graph if and only if the 3-partition instance has a solution For the first step, consider the subgraph G of G induced by the vertices of C and v 0,,v 3 ; see Figure 3 The graph G is a subdivision of a 3-connected graph; consider replacing the path v 1,v 2,v 3 by the edge v 1,v 3 Therefore, because of Whitney s theorem [9, Chapter 6], in any topological planar embedding of G, the faces are always induced by the same cycles In particular, in any planar embedding of G we will get the same faces as shown in Figure 3 Furthermore, in any planar embedding of G, all the vertices of G \ G have to be drawn inside the cycle v 0,v 1,v 2,v 3 (notice that this cycle may be the outerface) This means that in any planar embedding of G we have the faces induced by G plus the faces induced by G \ G inside the face v 0,v 1,v 2,v 3 Any planar embedding of G has Bn+3(n 1) vertices placed inside the cycle v 0,v 1,v 2,v 3, 4

6 v 1 (B + 3)n v 0 v 2 v 3 Figure 3: Subgraph G of G induced by C and v 0,,v 3 It is a subdivision of a 3-connected graph; we obtain it by inserting the vertex v 2 in the middle of the edge v 1,v 3 and so any face inside the cycle v 0,v 1,v 2,v 3 has fewer than Bn + 3(n 1) + 4 < (B + 3)n + 3 vertices However, we always have a face consisting of (B + 3)n + 3 vertices, namely the outerface of G in Figure 3 We conclude that, in any planar embedding of G, there is always a unique face with (B + 3)n + 3 vertices, and it is induced by the vertices in C {v 1,v 2,v 3 } Recall that CH (P) denotes the points in the convex hull of P, and observe that it consists of exactly (B + 3)n + 3 points In any geometric planar graph, all the points in the convex hull have to be part of the outerface, and therefore, the vertices in C {v 1,v 2,v 3 } have to be mapped onto the points CH (P) Next, observe that the vertex v 0 has an edge with each point in C {v 1,v 2 }, and the rest of the vertices have to lie inside one single face, namely v 0,v 1,v 2,v 3 Among the points P \CH (P), only p 0 has this property, and we conclude that the vertex v 0 has to be mapped to the point p 0 Furthermore, the vertices v 0,v 1,v 2,v 3 have to be mapped to either p 0,p 1,p 2,p 3, respectively, or to p 0,p 3,p 2,p 1, respectively In any case, v 0 is always mapped to the point p 0, and v 2 is mapped to the point p 2 This concludes the first step of the proof Figure 4 shows with solid straight segments the part of G which has been already embedded onto P For the second step, the only points of P that do not have vertices assigned to them are P := P \(CH (P) {p 0 }) Consider one of the triangles T i G To realize it, we need to find three points p,q,r P such that r is the only point contained in the triangle p 2 pq, and the triangle p 0 pq contains no points Only the groups of three adjacent points that are marked with empty circles have this property, that is, the separators that were added in the last item when generating the point set P There are n 1 triangles T i, and n 1 separators, so each triangle gets assigned to one separator The induced edges are drawn with dotted segments in Figure 4 We are left with n faces F 1, F n, each containing exactly B points; see Figure 4 For a geometric planar embedding H of G having V (H) = P, each of the paths B i has to lie completely inside some face F ji Therefore, a geometric planar embedding of G is possible if and only if B 1,,B 3n can be arranged in groups such that each of the groups has exactly B vertices However, because V (B i ) = a i, such a geometric planar embedding of G is possible if and only if the 3-partition instance has a solution Because 3-partition is NP-hard even when B is bounded by a polynomial in n, the graph G has a polynomial number of vertices, and the point set P also has a polynomial number of points Furthermore, the coordinates of the points in P are bounded by polynomials and the whole reduction can be done in polynomial time This finishes the proof of Theorem 1 5

7 p 1 F 1 F 2 B F n p 0 v 0 p 2 v 2 p 3 Figure 4: Part of G embedded onto P The edges with solid segments are embedded at the end of the first step We can have either p 1 v 1 and p 3 v 3, or p 1 v 3 and p 3 v 1 The edges with dotted segments are the separators placed during the second step They induce the faces F 1,F n 3 Concluding remarks The point set P that we have constructed has many collinear points However, in the proof we have not used this fact, and so it is easy to modify the reduction in such a way that no three points of P are collinear Probably, the easiest way for keeping integer coordinates is replacing each of the points lying in a vertical line by points lying in a parabola, and adjusting the value K accordingly Therefore, the result remains valid even if P is in general position, meaning that no 3 points are collinear In the proof, the graph G that we constructed is 2-outerplanar, as shown in Figure 1 k-outerplanarity is a generalization of outerplanarity that is defined inductively A planar embedding of a graph is k-outerplanar if removing the vertices of the outer face produces a (k 1)-outerplanar embedding, where 1-outerplanar stands for an outerplanar embedding A graph is k-outerplanar if it admits a k-outerplanar embedding For (1-)outerplanar graphs, the embedding problem is polynomially solvable [2], but for 2-outerplanar we showed that it is NP-complete Therefore, regarding outerplanarity, our result is tight The graph G that we constructed in the proof is 2-connected; removing the vertices v 1,v 3 disconnects the graph As mentioned in the introduction, we have strongly used this fact in the proof because in a 2-connected graph the maximal 3-connected blocks can flip from one face to another one Therefore, it would be interesting to find out the complexity of the problem when the graph G is 3-connected, and more generally, the complexity when the topology of the embedding is specified beforehand 6

8 Acknowledgements I would like to thank Marc van Kreveld for careful reading of the manuscript and pointing out that the reduction can use a 2-outerplanar graph References [1] M Abellanas, J García-López, G Hernández, M Noy, and P A Ramos Bipartite embeddings of trees in the plane Discrete Applied Mathematics, 93: , 1999 A preliminary version appeared in Graph Drawing (Proc GD 96), LNCS 1190, pg 1 10 [2] P Bose On embedding an outer-planar graph in a point set Comput Geom Theory Appl, 23: , November 2002 A preliminary version appeared in Graph Drawing (Proc GD 97), LNCS 1353, pg [3] P Bose, M McAllister, and J Snoeyink Optimal algorithms to embed trees in a point set Journal of Graph Algorithms and Applications, 1(2):1 15, 1997 A preliminary version appeared in Graph Drawing (Proc GD 95), LNCS 1027, pg [4] F Brandenberg, D Eppstein, MT Goodrich, SG Kobourov, G Liotta, and P Mutzel Selected open problems in graph drawing In Graph Drawing (Proc GD 03), LNCS, 2003 To appear [5] N Castaneda and J Urrutia Straight line embeddings of planar graphs on point sets In Proc 8th Canad Conf Comput Geom, pages , 1996 [6] M Chrobak and H Karloff A lower bound on the size of universal sets for planar graphs SIGACT News, 20:83 86, 1989 [7] H de Fraysseix, J Pach, and R Pollack How to draw a planar graph on a grid Combinatorica, 10(1):41 51, 1990 [8] E D Demaine, J S B Mitchell, and J O Rourke (eds) The Open Problems Project [9] R Diestel Graph Theory Springer-Verlag, New York, 2nd edition, 2000 [10] M R Garey and D S Johnson Computers and Intractability: A Guide to the Theory of NP-Completeness W H Freeman, New York, NY, 1979 [11] P Gritzmann, B Mohar, J Pach, and R Pollack Embedding a planar triangulation with vertices at specified points Amer Math Monthly, 98(2): , 1991 [12] Y Ikebe, M A Perles, A Tamura, and S Tokunaga The rooted tree embedding problem into points in the plane Discrete Comput Geom, 11:51 63, 1994 [13] M Kaufmann and R Wiese Embedding vertices at points: Few bends suffice for planar graphs Journal of Graph Algorithms and Applications, 6(1): , 2002 A preliminary version appeared in Graph Drawing (Proc GD 99), LNCS 1731, pg

9 [14] J Pach Geometric graph theory In J D Lamb and D A Preece, editors, Surveys in Combinatorics, number 267 in London Mathematical Society Lecture Note Series, pages Cambridge University Press, 1999 [15] J Pach and J Töumlrőcsik Layout of rooted trees In WT Trotter, editor, Planar Graphs, volume 9 of DIMACS Series, pages Amer Math Soc, Providence, 1993 [16] J Pach and R Wenger Embedding planar graphs at fixed vertex locations Graphs Combin, 17: , 2001 A preliminary version appeared in Graph Drawing (Proc GD 98), LNCS 1547, pg [17] W Schnyder Embedding planar graphs on the grid In Proc 1st ACM-SIAM Sympos Discrete Algorithms, pages ,

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