# ABOUT UNIQUELY COLORABLE MIXED HYPERTREES

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1 Discussiones Mathematicae Graph Theory 20 (2000 ) ABOUT UNIQUELY COLORABLE MIXED HYPERTREES Angela Niculitsa Department of Mathematical Cybernetics Moldova State University Mateevici 60, Chişinău, MD-2009, Moldova and Vitaly Voloshin Institute of Mathematics and Informatics Moldovan Academy of Sciences Academiei, 5, Chişinău, MD-2028, Moldova Abstract A mixed hypergraph is a triple H = (X, C, D) where X is the vertex set and each of C, D is a family of subsets of X, the C-edges and D-edges, respectively. A k-coloring of H is a mapping c : X [k] such that each C-edge has two vertices with the same color and each D-edge has two vertices with distinct colors. H = (X, C, D) is called a mixed hypertree if there exists a tree T = (X, E) such that every D-edge and every C-edge induces a subtree of T. A mixed hypergraph H is called uniquely colorable if it has precisely one coloring apart from permutations of colors. We give the characterization of uniquely colorable mixed hypertrees. Keywords: colorings of graphs and hypergraphs, mixed hypergraphs, unique colorability, trees, hypertrees, elimination ordering Mathematics Subject Classification: 05C15. Partially supported by DAAD, TU-Dresden. Partially supported by DFG, TU-Dresden.

2 82 A. Niculitsa and V. Voloshin 1 Preliminaries We use the standard concepts of graphs and hypergraphs from [1, 2] and updated terminology on mixed hypergraphs from [4, 5, 6, 7]. A mixed hypergraph is a triple H = (X, C, D) where X is the vertex set, X = n, and each of C, D is a family of subsets of X, the C-edges and D-edges, respectively. A proper k-coloring of a mixed hypergraph is a mapping c : X [k] from the vertex set X into a set of k colors so that each C-edge has two vertices with the same color and each D-edge has two vertices with different colors. The chromatic polynomial P (H, k) gives the number of different proper k-colorings of H. A strict k-coloring is a proper coloring using all k colors. By c(x) we denote the color of vertex x X in the coloring c. The maximum number of colors in a strict coloring of H is the upper chromatic number χ(h); the minimum number is the lower chromatic number χ(h). If for a mixed hypergraph H there exists at least one coloring, then it is called colorable. Otherwise H is called uncolorable. Throughout the paper we consider colorable mixed hypergraphs. If H = (X, C, D) is a mixed hypergraph, then the subhypergraph induced by X X is the mixed hypergraph H = (X, C, D ) defined by setting C = {C C : C X }, D = {D D : D X } and denoted by H = H/X. The mixed hypergraph H = (X,, D) (H = (X, C, )) is called Dhypergraph ( C-hypergraph ) and denoted by H D (H C ). If H D contains only D-edges of size 2 then from the coloring point of view it coincides with classical graph ([2]). We call it D-graph. For each k, let r k be the number of partitions of the vertex set into k nonempty parts (color classes) such that the coloring constraint is satisfied on each C- and D- edge. In fact r k equals the number of different strict k-colorings of H if we disregard permutations of colors. The vector R(H) = (r 1,..., r n ) = (0,..., 0, r χ(h),..., r χ(h), 0,..., 0) is the chromatic spectrum of H. For the simplicity we assume that two strict k-colorings are considered the same if they can be obtained from each other by permutation of colors. In this case the number of different strict k-colorings coincides with r k (H). A mixed hypergraph H is called a uniquely colorable (uc for short) [5] if it has just one strict coloring.

3 About Uniquely Colorable Mixed Hypertrees 83 A mixed hypergraph H = (X, C, D) is called uc-orderable [5] if there exists the ordering of the vertex set X, say X = {x 1, x 2,..., x n }, with the following property: each subhypergraph H i = H/X i induced by the vertex set X i = {x i, x i+1,..., x n } is uniquely colorable. The corresponding sequence x 1,..., x n will be called a uc-ordering of H. A sequence x 0, x 1,..., x t+1 of vertices is called a D-path if (x i, x i+1 ) D, 0 i t. A mixed hypergraph H = (X, C, D) is called reduced if C 3 for each C C, and D 2 for each D D, and moreover, no one C-edge (D-edge) is included in another C-edge (D-edge). As it follows from the splitting-contraction algorithm [6, 7] colorings properties of arbitrary mixed hypergraph may be obtained from some reduced mixed hypergraph. Therefore, throughout the paper we consider reduced mixed hypergraphs. Let C(x)(D(x)) denote the set of C-edges (D-edges) containing vertex x X. Call the set N(x) = {y : y X, y x, C(x) C(y), or D(x) D(y) } the neighbourhood of the vertex x in a mixed hypergraph H. In other words, the neighbourhood of a vertex x consists of those vertices which are contained in common C-edges or D-edges with x except x itself. A vertex x is called simplicial [8] in a mixed hypergraph if its neighbourhood induces a uniquely colorable mixed subhypergraph. A mixed hypergraph H = (X, C, D) is called pseudo-chordal [8] if there exists an ordering σ of the vertex set X, σ = (x 1, x 2,..., x n ), such that the vertex x j is simplicial in the subhypergraph induced by the set {x j, x j+1,..., x n } for each j = 1, 2,..., n 1. Definition [8]. A mixed hypergraph H = (X, C, D) is called a mixed hypertree if there exists a tree T = (X, E) such that every C-edge induces a subtree of T and every D-edge induces a subtree of T. Such a tree T is called further a host tree. The edge set of a host tree T is denoted by E = {e 1, e 2,..., e n 1 }, e i = (x, y), x, y X, i = 1, 2,..., n 1. 2 Uniquely Colorable Mixed Hypertrees Let H = (X, C, D) be an arbitrary mixed hypergraph. Definition. A sequence of vertices of H, x = x 0, x 1,..., x k = y, k 1, is called (x, y)-invertor iff:

4 84 A. Niculitsa and V. Voloshin (1) x i x i+1, i = 0, 1,..., k 1; (2) (x i, x i+1 ) D, i = 0, 1,..., k 1; (3) if x j x j+2 then (x j, x j+1, x j+2 ) C, j = 0, 1,..., k 2. In H for two vertices x, y X there may exist many (x, y)-invertors. The shortest (x, y)-invertor contains minimal number of vertices. Two (x, y)-invertors are different if they have at least one distinct vertex. A (x, y)-invertor with x = y is called cyclic invertor. Definition. In a mixed hypertree, a cyclic invertor is called simple if all C-edges are different and every D-edge appears consecutively precisely two times. Let µ = (z 0, z 1,..., z k = z 0 ), k 6, be some simple cyclic invertor in a mixed hypertree. Without loss of generality assume that z 0 z 1 z 2 z 0. From the definition of simple cyclic invertor it follows that z 0 z 2... z k 2 and z 1 = z 3 =... = z k 1 = y, where y is the center of some star in the host tree T. Theorem 1. If H = (X, C, D) is a mixed hypertree then (1) χ(h) 2; (2) if, in addition, D n 2 then r 2 (H) 2. P roof. (1) It follows from the possibility to start at any vertex and to color H alternatively by the colors 1 and 2 along the host tree T. (2) Let T = (X, E) be a host tree of the mixed hypertree H. Since D n 2 in T there exists an edge e = (x, y) D. Starting with the vertices x, y we can construct two different colorings with two colors in the following way. First, put c(x) = c(y) = 1 and color all the other vertices alternatively along the tree T with the colors 2, 1, 2,.... Second, apply the same procedure starting with c(x) = 1 and c(y) = 2. Theorem 2. A mixed hypertree H = (X, C, D) is uniquely colorable if and only if for every two vertices x, y X there exists an (x, y)-invertor. P roof. Let c be the unique strict coloring of the mixed hypertree H. We show that for any two vertices x, y X there exists an (x, y)-invertor. Suppose H has two vertices u, v X such that there is no (u, v)-invertor in H. Consider the unique (u, v)-path in the host tree T of H. The assumption implies that either in H there is no D-path connecting u and v or in

5 About Uniquely Colorable Mixed Hypertrees 85 the sequence u = x 1, x 2,..., x p = v there exists a triple of pairwise different vertices x j, x j+1, x j+2 not belonging to C. If there is no D-path connecting u and v then by Theorem 1(2) H has two different colorings with two colors. This contradicts to the unique colorability of mixed hypertree H. Assume that in the sequence u = x 1, x 2,..., x p = v there exists a triple of pairwise different vertices x j, x j+1, x j+2 such that (x j, x j+1, x j+2 ) C. Evidently, x j+1 is not pendant in T. Let T 1 and T 2 be two connected components obtained after deletion of vertex x j+1 from the host tree T. There are two cases. (1) c(x j ) = c(x j+2 ). From Theorem 1(1) it follows that the number of colors in the unique coloring c of H is 2. Recolor the vertex x j+2 and all vertices on even distance from x j+2 in the component T 2 with the new color. The obtained coloring is a proper coloring of H different from c, a contradiction. (2) c(x j ) c(x j+2 ). Since (x j, x j+1 ), (x j+1, x j+2 ) D we have that c(x j ) c(x j+1 ) c(x j+2 ). Consequently, H is colored with at least three colors. But according to Theorem 1 every mixed hypertree can be colored with two colors, a contradiction. Assume that any two vertices x, y X are joined by an (x, y)- invertor. Suppose H has at least two strict colorings c 1 and c 2. Then there exist two vertices, say x, y, such that c 1 (x ) = c 1 (y ) but c 2 (x ) c 2 (y ). Consider (x, y )-invertor x = x 0, x 1,..., x k = y. From the definition of invertor follows that if k is even then in all possible colorings the vertices x and y have the same color. If k is odd then in all possible colorings the vertices x and y have distinct colors. Consider the unique (x, y )-path connecting the vertices x, y on the host tree T. One can see that the parity of k coincides with the parity of length of the path. Moreover, it is true for any other (x, y )-invertor. Therefore, in all colorings either c(x ) = c(y ) or c(x ) c(y ), a contradiction. Corollary 1. If H is a uniquely colorable mixed hypertree then D = E. Definition. Let H = (X, C, D) be a mixed hypergraph. The C-edge C C is called redundant if R(H) = R(H 1 ), where H 1 = (X, C \ {C}, D). In a uniquely colorable mixed hypertree H = (X, C, D) any C-edge of size 4 is redundant because there is no invertor containing such C-edge. Theorem 3. In a uniquely colorable mixed hypertree H = (X, C, D) a C-edge Cof size 3 is redundant if and only if there exists a simple cyclic invertor containing C.

6 86 A. Niculitsa and V. Voloshin P roof. Let C = (x 1, x 2, x 3 ) be the redundant C-edge. By definition H = (X, C, D) where C = C \ {C} is a uniquely colorable mixed hypertree. Then for the vertices x 1 and x 3 in H there exisits an (x 1, x 3 )-invertor: x 1 = z 0, z 1,..., z k = x 3. Construct the (x 1, x 1 )-invertor in the following way: x 1 = z 0, z 1,..., z k = x 3, x 2, x 1. This invertor is a simple cyclic invertor of H containing C. Conversely, suppose that C-edge, C = (x 1, x 2, x 3 ) is contained in some simple cyclic invertor x 1 = z 0, z 1,..., z k = x 3, x 2, x 1. Then the vertices x 1 and x 3 are joined by two different (x 1, x 3 )-invertors: (x 1, x 2, x 3 ) = C and (x 1 = z 0, z 1,..., z k = x 3 ) = (x 1, x 3 ) -invertor. In each (x, y)-invertor containing C replace this C-edge by (x 1, x 3 ) -invertor. Thus, H = (X, C \ {C}, D) is uniquely colorable, i.e., the C-edge C is redundant. Let us have a mixed hypergraph H = (X, C, D). Consider X = X 1 X 2... X i any i-coloring of H, χ(h) i χ(h). Choose any X j and construct touching graph L j = (X j, V j ) in the following way: if some C C satisfies C X j = {x, y} and C X k 1, k j, for some x, y X j, then (x, y) V j (cf. pair graphs [3]). Theorem 4. If a mixed hypertree H = (X, C, D) is uniquely colorable then in its 2-coloring the touching graphs L 1 and L 2 are connected. P roof. By Theorem 1(2), Corollary 1 we obtain D = n 1, χ = 2 for each uniquely colorable mixed hypertree. If at least one touching graph is disconnected, then we can construct a new coloring of H with 3 colors by assigning new color to the vertices of one component. This assures the proper coloring also of any C-edge of size 4. Corollary 2. The minimal number of C-edges in any uniquely colorable mixed hypertree H = (X, C, D) is n 2. P roof. Let H be a uniquely colorable mixed hypertree. Consider its unique 2-coloring, say X = X 1 X 2, and construct the touching graphs L 1 = (X 1, V 1 ), L 2 = (X 2, V 2 ). The minimal number of edges in L i to be connected is X i 1, and in this case each of L i is a tree, i = 1, 2. Since every edge in L i corresponds to some C-edge of H, we obtain that the minimal number of C-edges is: X X 2 1 = X 2.

7 About Uniquely Colorable Mixed Hypertrees 87 Corollary 3. In a uniquely colorable mixed hypertree H = (X, C, D) the number of redundant C-edges is C n + 2. P roof. Indeed, consider touching graphs L i, and construct a spanning trees T i, i = 1, 2. Each elementary cycle in L i generates some simple cyclic invertor in H. Therefore, each C-edge of H which has a size 4 or corresponds to some edge of L i which is a chord with respect to T i, is redundant. Hence, the assertion follows. Remark. Redundant C-edge may become not redundant after deleting from C some another redundant C-edges. Definition. A mixed hypertree H = (X, C, D) is called complete if every edge of the host tree T forms a D-edge of H, and every path on three vertices of T forms a C-edge in H. Therefore, having the host tree T for the complete mixed hypertree H = (X, C, D) we obtain that D = E. Denote by M the number of C-edges of a complete mixed hypertree H = (X, C, D). Then ( ) d(x) M =, 2 x T d(x) 2 where d(x) is the degree of vertex x in the host tree T. Examples show that for any k > 1 one can construct a mixed hypertree H = (X, C, D) with D = n 1, n 2 C M and χ(h) = k. Therefore these bounds on D and C are not sufficient for the mixed hypertrees to be uniquely colorable. Proposition 1. A uniquely colorable mixed hypertree with C = n 2 is a pseudo-chordal mixed hypergraph. P roof. Since H is uniquely colorable mixed hypertree and C = n 2 then it contains no redundant C-edges and, moreover, all C-edges have the size 3. It follows that there exists a pendant vertex, say x, of the host tree T = (X, E) which belongs to precisely one C-edge, say (x, y, z). The neighbourhood of x induces a complete D-graph on 2 vertices, which itself is uniquely colorable. Consequently, the vertex x is simplicial in H. Deleting the vertex x and C-edge and D-edge containing it, obtain H which

8 88 A. Niculitsa and V. Voloshin is uniquely colorable mixed hypertree with minimal number of C-edges. Indeed, if H would be not uniquely colorable, then two distinct colorings of H formed different colorings of H because c(x) = c(z), a contradiction. Remark. Redundant C-edges enlarge the neighbourhood of some vertices without affecting any coloring. Therefore, to recognise the pseudo-chordality we need to delete the redundant C-edges. From the Theorem 4, Corollaries 2 4 and Proposition 1 we conclude that a uc-orderable mixed hypertree H can be recognised by consecutive elemination of pendant vertices of D-graph H D in special ordering by applying the following Algorithm (uc-ordering). Input: A mixed hypertree H = (X, C, D), σ n-dimensional empty vector. Idea: Simultanious decomposition of H D, spanning trees T 1 and T 2 of touching graphs L 1, L 2, respectively, by pendant vertices. Iterations: 1. If there is a vertex x X belonging to none C-edge of size 3 or D-edge of size 2 then return NON UC. Otherwise remove from C all elements of size Color D-graph H D with two colors. 3. Construct touching graphs L 1 and L If L i, i = 1, 2, is not connected then return NON UC. 5. For L i construct spanning tree T i, i = 1, i := While in T i there exists a vertex x pendant in both T i and H D then delete it from T i and H D and include x in σ. 8. If at least one of T 1 and T 2 is not empty then go to 9. Otherwise return UC, σ-uc-ordering. 9. If i = 1 then assign i := i + 1, otherwise i := i 1. Go to 7. Remark. All chords of graph L i with respect to spanning tree T i, i = 1, 2, correspond to redundant C-edges in H. The trees T 1 and T 2 provide existence of unique (x, y)-invertor for any x, y X. The last assures at any step of the algorithm the existence of a vertex, say x, pendant in both H D and one

9 About Uniquely Colorable Mixed Hypertrees 89 of T 1 or T 2. Notice that not every elimination of pendant vertices generates a uc- ordering in H D. Example. Given the mixed hypertree H with X = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}, C = {(0, 1, 2); (0, 1, 3); (0, 2, 3); (0, 3, 4); (0, 4, 5); (0, 5, 6); (0, 4, 6); (0, 2, 7); (0, 7, 8); (7, 8, 9); (9, 8, 10); (9, 10, 11); (9, 11, 12); (9, 10, 12); (9, 12, 13); (9, 13, 14); (9, 14, 15); (9, 13, 15)}, and D = {(0, 1); (0, 2); (0, 3); (0, 4); (0, 5); (0, 6); (0, 7); (7, 8); (8, 9); (9, 10); (9, 11); (9, 12); (9, 13); (9, 14); (9, 15)}, see the figures 1 and 2 (the C-edges are depicted by triangles). Figure 1 Apply the algorithm. Each vertex of H belongs to at least one D-edge of size 2 and at least one C-edge of size 3. Color H D with 2 colors. Denote by X 1 = {0, 8, 10, 11, 12, 13, 14, 15} and X 2 = {1, 2, 3, 4, 5, 6, 7, 9} two color classes of H D. Construct the following touching graphs L 1 = (X 1, V 1 ) and L 2 = (X 2, V 2 ), where V 1 = {(0, 8); (8, 10); (10, 11); (10, 12); (11, 12); (12, 13); (13, 14); (13, 15); (14, 15)} and V 2 = {(1, 2); (1, 3); (2, 3); (2, 7); (3, 4); (4, 5); (4, 6); (5, 6); (7, 9)}. Choose the respective trees T 1 and T 2 (Figure 3). Consecutively applying the algorithm we obtain one of uc-orderings of H: σ = {15, 14, 13, 11, 12, 10, 1, 5, 9, 6, 4, 3, 2, 8, 0, 7}. At the 7-th step of the algorithm, after including of vertex 10 in σ, we alternate the trees because T 1 has no pendant vertex which is also pendant in H D. The next alternations of trees are made after adding to σ of vertices 2 and 0. From the above algorithm we have

10 90 A. Niculitsa and V. Voloshin Figure 3

11 About Uniquely Colorable Mixed Hypertrees 91 Theorem 5. A mixed hypertree is uniquely colorable if and only if it is uc-orderable. Therefore, combining the Theorems 2, 5 and relation between chromatic polynomial and chromatic spectrum [6, 7], we obtain the following Theorem 6. Let H = (X, C, D) be a mixed hypertree. Then the following five statements are equivalent: (1) R(H) = (0, 1, 0,..., 0); (2) P (H, k) = k(k 1); (3) H is uniquely colorable; (4) Every two vertices x, y X are joined by an (x, y)-invertor; (5) H is uc-orderable. References [1] C. Berge, Hypergraphs: combinatorics of finite sets (North Holland, 1989). [2] C. Berge, Graphs and Hypergraphs (North Holland, 1973). [3] K. Diao, P. Zhao and H. Zhou, About the upper chromatic number of a co-hypergraph, submitted. [4] Zs. Tuza and V. Voloshin, Uncolorable mixed hypergraphs, Discrete Appl. Math. 99 (2000) [5] Zs. Tuza, V. Voloshin and H. Zhou, Uniquely colorable mixed hypergraphs, submitted. [6] V. Voloshin, The mixed hypergraphs, Computer Science J. Moldova, 1 (1993) [7] V. Voloshin, On the upper chromatic number of a hypergraph, Australasian J. Combin. 11 (1995) [8] V. Voloshin and H. Zhou, Pseudo-chordal mixed hypergraphs, Discrete Math. 202 (1999) Received 16 April 1999 Revised 24 March 2000

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