# Diameter of paired domination edge-critical graphs

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1 AUSTRALASIAN JOURNAL OF COMBINATORICS Volume 40 (2008), Pages Diameter of paired domination edge-critical graphs M. Edwards R.G. Gibson Department of Mathematics and Statistics University of Victoria Victoria, BC V8W 3P4 Canada M.A. Henning School of Mathematical Sciences University of KwaZulu-Natal Pietermaritzburg, 3209 South Africa C.M. Mynhardt Department of Mathematics and Statistics University of Victoria Victoria, BC V8W 3P4 Canada Abstract A paired dominating set of a graph G without isolated vertices is a dominating set of G whose induced subgraph has a perfect matching. The paired domination number γ pr (G) of G is the minimum cardinality amongst all paired dominating sets of G. The graph G is paired domination edge-critical (γ pr EC) if for every e E(G), γ pr (G + e) < γ pr (G). We investigate the diameter of γ pr EC graphs. To this effect we characterize γ pr EC trees. We show that for arbitrary even k 4 there exists a k pr EC graph with diameter two. We provide an example which shows that the maximum diameter of a k pr EC graph is at least k 2 and prove that it is at most min{2k 6, 3k/2 + 3}. Recipient of an Undergraduate Student Research Award from the Canadian National Science and Engineering Research Council, Summer Current address: Department of Mathematics, Simon Fraser University, Burnaby, BC, Canada V5A1S6. Research supported in part by the South African National Research Foundation and the University of KwaZulu-Natal. Research supported by the Natural Sciences and Engineering Research Council of Canada.

2 280 EDWARDS, GIBSON, HENNING AND MYNHARDT 1 Introduction Criticality is a fundamental concept for many graph parameters. Much has been written about graphs where a parameter increases or decreases whenever an edge or vertex is removed or added. Sumner and Blitch [28] began the study of those graphs, called domination edge-critical graphs, where the (ordinary) domination number decreases on the addition of any edge. This concept was further investigated in [4, 8, 9, 11, 27, 29, 30, 31] and elsewhere. The study of total domination edge-critical graphs, defined analogously, was initiated by Van der Merwe [32] and continued in [12, 15, 16, 17, 18, 19, 32] and elsewhere. We investigate paired domination edge-critical graphs, first studied by Edwards [6]; in particular, we obtain results on the diameter of these graphs. 2 Definitions For notation and graph theory terminology we generally follow [13]. Specifically, for a graph G = (V, E) and v V, the open and closed neighbourhoods of v are, respectively, N(v) = {u V : uv E} and N[v] = {v} N(v). For a set S V, N(S) = v S N(v) and N[S] = N(S) S. For sets S, R V, we say that S dominates R (abbreviated S R) if R N[S], S totally dominates R if R N(S), and S pairwise dominates R (abbreviated S pr R) if S dominates R and G[S] contains a perfect matching. If R = V in the above sentence, then S is, respectively, a dominating set, a total dominating set (TDS), and a paired dominating set (PDS) of G. We then write S G and S pr G, respectively. If S = {u} and R = {v} we also write u v, S v, u R, etc. Two vertices of a PDS S with perfect matching M are said to be paired (by M), or partners in S, if they are joined by an edge of M. Every graph without isolated vertices has a PDS since the end-vertices of any maximal matching form such a set. The total domination number γ t (G) (paired domination number γ pr (G), respectively) of G is the minimum cardinality of a TDS (a PDS, respectively). Note that these parameters are defined if and only if G has no isolated vertices. A PDS of cardinality γ pr (G) is called a γ pr -set of G; a γ t -set is defined similarly. Since every PDS of G is a TDS, 2 γ t (G) γ pr (G) for all graphs G. Paired domination was introduced by Haynes and Slater [20] as a model for assigning backups to guards for security purposes, and is studied, for example, in [1, 2, 3, 6, 7, 10, 14, 21, 22, 23, 25, 26]. The graph G is paired domination edge-critical, or γ pr EC, if for every edge e E(G), γ pr (G+e) < γ pr (G). If G is γ pr EC and γ pr (G) = k, we say that G is k pr EC. A total domination edge-critical (γ t EC) graph and a k t EC graph are defined similarly. For example, the 5-cycle is 3 t EC and 4 pr EC. Note that since γ t (G), γ pr (G) 2, the complete graphs are the only 2 t EC graphs and also the only 2 pr EC graphs. If diamg = k and the distance d(u, v) = k, then we say that u and v are peripheral vertices, and a shortest u v path is called a diametrical path of G. A vertex adjacent

3 DOMINATION EDGE-CRITICAL GRAPHS 281 to an end-vertex is called a support vertex. It is intuitively clear that graphs with fixed paired domination number cannot have arbitrary diameter. This idea also suggests that k pr EC graphs have smaller diameter than the maximum diameter realized by graphs with γ pr = k. We shall show that these notions are indeed correct. After giving some preliminary results in Section 3, we characterize γ pr EC trees in Section 4. It will follow that γ pr EC trees have diameter four. In Section 5 we provide an example which shows that the maximum diameter of a k pr EC graph is at least k pr 2, and in Section 6 we obtain upper bounds for the diameter of γ pr EC graphs. Some open problems are given in Section 7. 3 Preliminary results We first present some preliminary results. A set S is a minimal PDS if S is a PDS and no proper subset of S is a PDS. Observation 1 [3] A PDS S of a graph G is a minimal PDS if and only if any two vertices x, y S satisfy one of the following conditions: (i) G [S {x, y}] does not contain a perfect matching, (ii) without loss of generality, x is an end-vertex in G[S] adjacent to y, (iii) there exists a vertex u V S such that N(u) S {x, y}. Observation 2 of G. (i) Each support vertex in a graph G is contained in every PDS (ii) Every vertex in a γ pr EC graph is adjacent to at most one end-vertex. Observation 3 If G is γ pr EC and uv E(G), then every γ pr -set S of G + uv contains at least one of u and v, and if {u, v} S, then u and v are paired in S. Observation 4 e E(G). (i) If G is a γ pr EC graph, then γ pr (G + e) = γ pr (G) 2 for every (ii) [16] If G is a γ t EC graph, then γ t (G) 2 γ t (G + e) γ t (G) 1 for every e E(G). Moreover, the lower bound holds if and only if G is the disjoint union of two or more complete graphs. If the lower bound holds in Observation 4(ii), then G is called γ t -super-edgecritical or γ t SEC. As mentioned in Section 2, C 5 is 3 t EC and 4 pr EC. That this is no coincidence was shown in [6]. Theorem 1 [6, Theorem 3.7] If 2k < γ t (G) 2(k + 1) = γ pr (G) and G is γ pr EC, then G is γ t EC or γ t SEC.

4 282 EDWARDS, GIBSON, HENNING AND MYNHARDT Corollary 2 [6, Corollary 3.9] The class of 4 pr EC graphs is the union of the classes of 3 t EC graphs and 4 t SEC graphs. We show in the next section that Corollary 2 does not extend to k pr EC graphs where k 6 (see the remark following the proof of Theorem 5). In preparation for results on the diameter of γ pr EC graphs we now bound the diameter of graphs with γ pr = k. Let x be a peripheral vertex of a graph G with diam G = m. Define the levels V 0, V 1,..., V m of G with respect to x by V i = {v G : d(x, v) = i}. Notice that V 0 = {x} and V m φ. Lemma 3 If S is a PDS of a graph G and a and b are paired in S, then {a, b} dominates at most four levels of G. Proof. Suppose {a, b} dominates at least five levels V j, V j+1,, V j+l (l 4) of G. Then every vertex in W = V j V j+1 V j+l is adjacent to at least one of a or b. For u V j and v V j+l, d(u, v) l 4. But each of u and v is adjacent to a or b, hence d(u, v) 3, a contradiction. Proposition 4 If G is connected and γ pr (G) = k, then diamg 2k 1. Proof. Let m = diamg and V 0, V 1,..., V m be the levels of G with respect to a peripheral vertex x. Let S be a γ pr -set of G. By Lemma 3, every set of partners {a, b} S dominates at most four levels of G. Since there are k/2 such pairs in S, at most 2k levels of G are dominated. Hence if diam G 2k, at least one level of G is undominated. Therefore diamg 2k 1. 4 Trees We begin the study of the diameter of γ pr EC graphs by characterizing γ pr EC trees. The subdivided star S 2n+1 is obtained from K 1,n by subdividing each edge exactly once. Theorem 5 A tree T K 2 is γ pr EC if and only if T = S 2n+1, n 3. Proof. The sufficiency is straightforward to verify. To prove the necessity, let T be a γ pr EC tree. If diam T 3, then T is a star or a double star. But then γ pr (T) = 2. Since the complete graphs are the only 2 pr EC graphs, T = K 2. Thus we may assume that diamt 4. Let P : v 0, v 1, v 2,..., v d be a diametrical path in T and suppose firstly that diam T = d 5. By Observation 2(ii), deg v 1 = deg v d 1 = 2. Let e = v 2 v d 1 E(T) and consider the tree T = T +e. Since T is γ pr EC, γ pr (T ) = γ pr (T) 2. Let S be a γ pr -set of T. By Observation 2(i), S contains the two support vertices v 1 and v d 1.

5 DOMINATION EDGE-CRITICAL GRAPHS 283 If v 2 / S, let S = S. If v 2 S, then (Observation 3) v 2 is paired with v d 1 S, hence v 1 is paired with v 0 in S, and we let S = (S \ {v 0 }) {v d }. In both cases S is a PDS of T with S = S = γ pr (T ). (In the latter case observe that v 1 is paired with v 2, v d 1 is paired with v d while all other pairings in S remain unchanged from those in S.) Thus γ pr (T) γ pr (T ), a contradiction. Hence diamt 4 and so d = diamt = 4. If T = P 5, then T + v 0 v 4 = C 5. But γ pr (P 5 ) = γ pr (C 5 ) = 4, contradicting the fact that T is γ pr EC. Hence (T) 3. Thus, by Observation 2(ii), T is obtained from K 1,n, n 3, by subdividing every edge, except for possibly one edge. Let v be the central vertex of K 1,n and N(v) = {l 1,..., l n }. Suppose vl i has been subdivided by the vertex u i, i = 1,..., n 1, but not vl n. Then n 1 i=2 {u i, l i } {v, u 1 } is a γ pr -set of T and of T + u 1 l n, a contradiction. Therefore each edge vl i has been subdivided by u i, so that T = S 2n+1 and n i=1 {u i, l i } is a γ pr -set of T. Thus all γ pr EC trees have diameter 4. As shown in the proof of Theorem 5, γ pr (S 2n+1 ) = 2n and it is easy to see that S 2n+1 is 2n pr EC. It is also easy to see that γ t (S 2n+1 ) = n + 1 and that S 2n+1 is not γ t EC. Thus Corollary 2 does not extend to k pr EC graphs where k 6. 5 γ pr -Edge critical graphs with small/large diameter The only graphs with diameter 1 are the complete graphs, and, except for K 1, they are vacuously 2 pr EC. In this section we provide constructions of, firstly, a k pr EC graph with diameter two for each k 4, and secondly, γ pr EC graphs with large diameter. The latter result shows that the maximum diameter of a γ pr EC graph is at least γ pr (G) 2. Proposition 6 For every even k 4 there exists a k pr EC graph of diameter 2. Proof. For k = 2l, l 2, consider the Cartesian product of the graph K k with itself, i.e. the graph G k = K k K k. We can think of G k as having k disjoint copies of K k in rows and k disjoint copies of K k in columns. In other words, we consider the vertices of G k as a matrix, where vertex v ij is in the i th row (copy of K k ) and the j th column (copy of K k ). For ease of discussion we shall use the words row and column to mean a copy of K k. We show first that γ pr (G k ) = k. Since {v 11, v 21,..., v k1 } is a dominating set with a perfect matching, γ pr (G k ) k. Suppose γ pr (G k ) k 2. Then for any γ pr -set S there exists an i such that S does not have a vertex in row i. Any vertex in S dominates only one vertex of row i, implying that at most k 2 of the k vertices of row i are dominated, a contradiction. Thus γ pr (G k ) = k. We now show that diamg = 2. Clearly, for k 2, G k is not complete and so diam G 2. Consider distinct vertices x, y V (G k ). If x and y are in the same row, i.e. x = v ij and y = v ik, then d(x, y) = 1; this is also true if x and y are in the same column. If x and y are not in the same row or column, i.e. x = v hi and y = v jk where

6 284 EDWARDS, GIBSON, HENNING AND MYNHARDT u x u x u u x u 0i 1i 1i 2i 2i l -1i l l v y v y v v y v 0i 1i 1i 2i 2i l -1i Figure 1: A 2(l + 1) pr -edge critical graph with diameter 2l G l -1i l l G l h j and i k, let z = v hk. Then d(x, z) = 1 and d(z, y) = 1 and so d(x, y) = 2. It follows that diamg k = 2. If G k is k pr EC, then we are finished. Otherwise, as can be seen by adding edges to G k without changing γ pr, G k is a spanning subgraph of some k pr EC graph G and, since G is not complete, diamg = 2. We next construct a (2l + 2) pr -EC graph with diameter 2l, where l 1. For each i = 1,..., l, let H i = P4 with vertex sequence x i, u i, v i, y i. Construct G l recursively as follows. Let G 0 = K 2 with V (G 0 ) = {u 0, v 0 }, and once G i 1 has been constructed, let G i be the graph with V (G i ) = V (G i 1 ) V (H i ) and E(G i ) = E(G i 1 ) E(H i ) {u i 1 x i, u i 1 y i, v i 1 x i, v i 1 y i }. See Figure 1. Proposition 7 For any l 1, G l is a (2l + 2) pr -EC graph with diameter 2l. Proof. It is obvious that diamg l = 2l. We first prove by induction that γ pr (G l ) = 2(l + 1). Since D = l i=0 {u i, v i } is a PDS of G l with D = 2(l + 1), it remains to show that γ pr (G l ) 2(l + 1). This is easy to verify for G 1. For l 2, assume γ pr (G l 1 ) 2l and let S be any minimal PDS of G l. Suppose firstly that S {x l, y l } = φ. Then S = S V (G l 1 ) is a PDS of G l 1, hence by assumption S 2l. Since S does not dominate u l or v l, {u l, v l } S to pairwise dominate {u l, v l }, hence S 2(l + 1). Now assume without loss of generality that x l S. Then x l is paired in S with w {u l 1, v l 1, u l }. Suppose w u l. By symmetry we may then assume that w = u l 1. To dominate v l, S {u l, v l, y l } φ. If v l 1 / S, or if v l 1 S and v l 1 is not partnered by y l, then S {u l, v l, y l } 2. In the latter case, v l 1 is partnered by y l 1 ; moreover x l 1 / S, otherwise {u l 1, x l } satisfies none of the conditions of Observation 1. Define S by { (S S {xl, u = l, v l, y l }) {x l 1 } if v l 1 S is partnered by y l 1 (S {x l, u l, v l, y l }) {v l 1 } otherwise. In the first instance of the definition of S, each of the sets {u l 1, x l 1 } and {v l 1, y l 1 } is a pair, and in the second instance {u l 1, v l 1 } is a pair; in either case S S 2.

7 DOMINATION EDGE-CRITICAL GRAPHS 285 Also, S is a PDS of G l 1, hence by the induction hypothesis S 2l and so S 2(l + 1). Suppose w = u l. To pairwise dominate y l, S {u l 1, v l 1, y l, v l } 1. If S {u l 1, v l 1 } φ, then S = S {x l, u l } pr G l 1, hence S 2l and S 2(l + 1). If S {u l 1, v l 1 } = φ, then {y l, v l } S. In this case (S {u l, v l, x l, y l }) {u l 1, v l 1 } pr G l 1 and again S 2(l + 1). It follows that γ pr (G l ) = 2(l + 1). We next prove by induction that G l is γ pr EC, the case l = 1 being easy to verify. For l 2, assume that G l 1 is γ pr EC and let e = ab E(G l ). If e E(G l 1 ), let S be any γ pr -set of G l 1 + e and S = S {u l, v l }. Then S pr G l + e and S = S + 2 = 2l by the induction hypothesis. If e E(H l ) = {x l y l, x l v l, y l u l }, let S = ( l 2 i=0 {u i, v i }) {a, b}. Then S pr G l +e and S = 2l. Hence assume a V (G l 1 ) and b {x l, u l, v l, y l }. We may assume without loss of generality that a {u 0, u 1,..., u l 1 } {x 1, x 2,..., x l 1 }. By symmetry we may also assume without loss of generality that b {y l, v l }. If b = v l, then regardless of a, let S = l 1 i=0 {u ix i+1 }, and if b = y l, let S = ( l 2 i=0 {u ix i+1 }) {x l, u l }. In either case S pr G l + e and S = 2l. Therefore G l is γ pr EC. 6 Bounds on the diameter In general, the diameter of a k pr EC graph is smaller than the general upper bound established in Proposition 4 for graphs with γ pr = k. In this section we establish upper bounds on the diameter of connected k pr EC graphs. In Section 5 we exhibited a 4 pr EC graph with diameter 2. However, this is not the maximum diameter amongst 4 pr EC graphs, because, as shown in [16], 2 diam G 3 whenever G is a connected 3 t EC graph, and the bounds are sharp. The corresponding result for connected 4 pr EC graphs follows from Observation 4(ii) and Corollary 2. We henceforth consider connected k pr EC graphs with k 6. Theorem 8 If G is a connected k pr EC graph with k 6, then diamg 2k 6. Proof. Suppose to the contrary that G is a k pr EC graph, k 6, such that diamg 2k 5. Since G is k pr EC, γ pr (G + e) = k 2 for any e E(G). Let k = 2l and consider the nonempty levels V 0, V 1,..., V 2k 5 with respect to a peripheral vertex v. Let u V 4. Then uv E(G), γ pr (G + uv) = 2l 2 and, by Observation 3, any γ pr -set D of G + uv contains at least one of u and v, where u and v are paired in D if D contains both. Suppose first that {u, v} D. Then the pair {u, v} dominates only vertices in V 0 V 1 V 3 V 4 V 5. Let u, v be paired in D such that {u, v } dominates vertices in V 2. Then by Lemma 3, {u, v } does not dominate any vertices in V t, t 6. Hence the remaining 2(l 3) vertices in D dominate all of 2k 5 t=6 V t, a total of 4l 10 levels

8 286 EDWARDS, GIBSON, HENNING AND MYNHARDT of G. (Note that 2k 10 > 0 because k 6.) Using Lemma 3 and the pigeonhole principle, we see that this is impossible. If {u, v} D = {u}, then u is paired with a vertex u V 3 V 4 V 5. By Lemma 3, {u, u } dominates only (some) vertices in V 0 V 2 V 3 V 4 V 5 V 6. At least one more pair of vertices in D is needed to dominate V 1, and by Lemma 3, this pair does not dominate any vertex in V t, t 7. Hence the remaining 2(l 3) vertices in D dominate at least 4l 11 levels. By the pigeonhole principle, at least one pair dominates at least five levels, contradicting Lemma 3. Hence we conclude that u / D and so v D. Necessarily, v is paired with the vertex v V 1. Then the pair {v, v } dominates only vertices in V 0 V 1 V 2 V 4. At least one more pair is needed to dominate V 3, and by Lemma 3, no such pair dominates any vertices in V t, t 7. This leads to a contradiction as before. Therefore diam G 2k 6. It is not known whether the bound in Theorem 8 is the best possible if G is 6 pr EC; the graph constructed in Section 5 for k = 6 has diameter 4, and we have not found a 6 pr EC graph with diameter 5 or 6. The bound can be improved to 2k 7 and 2k 8 for k = 8 and k = 10, respectively, which suggests that the coefficient of k in the bound in Theorem 8 is incorrect. We show next that this coefficient can be decreased from 2 to 3 ; the bound in Theorem 10 is better than the one in Theorem 2 8 if k > 18. We start with a lemma. Lemma 9 Let G be a connected graph with S 1, S 2 V. If there exist sets D 1, D 2 V such that D i pr S i, i = 1, 2, then there exists a set D V such that D D 1 + D 2 and D pr S 1 S 2. Proof. Let M i be a perfect matching in D i, i = 1, 2. Partition M 2 into three sets P 1, P 2, P 3 as follows. Let P 1 = {ab M 2 : a, b / D 1 }, P 2 = {ab M 2 : without loss of generality, a D 1 and b / D 1 } and P 3 = {ab M 2 : a, b D 1 }. Say P 2 = {a 1 b 1, a 2 b 2,..., a t b t } for some 0 t D 1 D 2. If t = 0, let D = D 1 D 2 and M = M 1 P 1. Then D = D 1 + D 2 D 1 D 2 D 1 + D 2 and M is a perfect matching in D ; thus D pr S 1 S 2 and we are done. So assume t 1. Let T 0 = D 1 D 2 {b 1,..., b t }. Then M 1 P 1 is a perfect matching in T 0 and so T 0 pr S 1 S 2 t i=1 N[b i]. For 1 i t, we inductively define T i as follows. If N(b i ) T i 1, then let T i = T i 1. Note that since G is connected, b i has at least one neighbour and that neighbour is in T i. Otherwise, there exists a vertex c i V T i 1 such that b i c i E(G). In this case, let T i = T i 1 {b i, c i }.

9 DOMINATION EDGE-CRITICAL GRAPHS 287 In either case T i N[b i ]. Let D = T t. Then D D 1 + D 2 D 1 D 2 {b 1,..., b t } + 2t D 1 + D 2 t t + 2t = D 1 + D 2. Since T 0 D, we have D S 1 S 2 t i=1 N[b i] and since T i D for all 1 i t, we have D N[b i ] for all 1 i t; thus D S 1 S 2. Let M = M 1 P 1 {b i c i : N(b i ) T i 1, 1 i t}. Then M is a perfect matching in D and so D pr S 1 S 2. Theorem 10 If G is k pr EC, then diam G 3k Proof. Assume k 8 (we already have this for small k). For m = diamg, let V 0, V 1,..., V m be the levels of G with respect to a peripheral vertex v 0. Suppose m 3k ( 16). Let v 4 V 4. Then e = v 0 v 4 E(G) and thus by the criticality of G, γ pr (G + e) = k 2. Let A be a γ pr -set of G + e. Then (Observation 3) {v 0, v 4 } A φ. If {v 0, v 4 } A, then v 0, v 4 are paired in A. Note that {v 0, v 4 } only dominates vertices in V 0 V 1 V 3 V 4 V 5 and so there exists another pair of vertices in A that dominates vertices in V 2. This pair does not dominate any vertices in V i for all i 7 (Lemma 3). If {v 0, v 4 } A = {v 0 }, then v 0 is necessarily paired with a vertex v 1 V 1. Note that {v 0, v 1 } only dominates vertices in V 0 V 1 V 2 V 4 and so there exists another pair of vertices in A that dominates vertices in V 3. This pair does not dominate any vertices in V i for all i 7. If {v 0, v 4 } A = {v 4 }, then v 4 is paired with a vertex w V 3 V 4 V 5. Hence {v 4, w} does not dominate any vertices in V 1 and so there exists another pair of vertices that dominates vertices in V 1. Note that neither pair dominates any vertices in V i for all i 7. Thus in any of the three possibilities listed above, there exist two pairs of vertices in A that do not dominate any vertices in V i for all i 7. Thus there exists a set D 1 A such that D 1 k 6 and D 1 pr m i=7 V i in G + e and thus in G. We generalize this result as follows. For each j {1,..., k } there exists a set D 6 j V (G) such that m D j k 6j and D j pr V i in G. i=9(j 1)+7 (1) We prove (1) by induction on j. The base case holds for j = 1 as shown above. Thus let j {2,..., k } and assume that (1) holds for j 1; i.e. there exists a set 6

10 288 EDWARDS, GIBSON, HENNING AND MYNHARDT D j 1 V (G) such that D j 1 k 6(j 1) and D j 1 pr m i=9(j 2)+7 V i in G. Since k is even, j k 6 k m Thus 9(j 1) + 7 m. (2) Consider vertices w 1 V 9(j 1) and w 2 V 9(j 1)+4. Then e = w 1 w 2 E(G) and thus by the criticality of G, γ pr (G + e) = k 2. Let B j be a γ pr -set of G + e. By Observation 3, {w 1, w 2 } B j φ. We show, in each of three cases, that there are two pairs of vertices in B j that do not dominate any vertices of G in levels V i, for all integers i in the intervals I 1 = [0,9(j 2) + 6] and I 2 = [9(j 1) + 7, m]. The endpoints of I 2 have been chosen to match those of the union in (1), while the endpoints of I 1 have been chosen not necessarily to maximise the length of the interval, but to facilitate the proof of (1). If {w 1, w 2 } B j, then w 1, w 2 are paired in B j. Note that {w 1, w 2 } only dominates (some) vertices in ( 5 i= 1 V 9(j 1)+i ) V 9(j 1)+2 and so there exists another pair of vertices in B j that dominates vertices in V 9(j 1)+2. This pair dominates at most four levels of G and hence the two pairs of vertices do not dominate any vertices in V i for all i I 1 I 2. If {w 1, w 2 } B j = {w 1 }, then w 1 is paired with a vertex w V 9(j 1) 1 V 9(j 1) V 9(j 1)+1. Then {w 1, w} does not dominate any vertices in V 9(j 1)+3 and so there exists another pair of vertices in B j that dominates vertices in V 9(j 1)+3. These two pairs of vertices do not dominate any vertices in V i, i I 1 I 2. If {w 1, w 2 } B j = {w 2 }, then w 2 is paired with u V 9(j 1)+3 V 9(j 1)+4 V 9(j 1)+5. In any case, {w 2, u} does not dominate any vertices in V 9(j 1)+1 and so there exists another pair of vertices that dominates vertices in V 9(j 1)+1. Again these two pairs of vertices do not dominate any vertices in V i, i I 1 I 2. Thus in any of the three possibilities listed above, there exist two pairs of vertices in B j that do not dominate any vertices in V i for all i I 1 I 2. Therefore there exists a set C j B j such that C j k 6 and C j pr i I 1 I 2 V i in G + e and thus in G. Suppose there exists a set D C j such that D 6(j 1) 2 and D pr i I 1 V i. Then by Lemma 9 and the induction hypothesis, there exists a set D V (G) such that D D + D j 1 k 2 and 9(j 2)+6 m D pr i=0 V i i=9(j 2)+7 V i = V (G);

11 DOMINATION EDGE-CRITICAL GRAPHS 289 a contradiction since γ pr (G) = k. Thus at least 6(j 1) vertices in C j are required to pairwise dominate the vertices in i I 1 V i. Since none of these vertices dominates any vertices in i I 2 V i (Lemma 3), at most k 6 6(j 1) = k 6j vertices in C j remain to dominate the vertices in i I 2 V i. It follows that there exists a set D j C j such that D j k 6j and D j pr i I 2 V i, and thus (1) holds. Now for j = k, 9(j 1) + 7 m by (2) and thus 6 i I 2 V i φ. However, by (1), D j k 6j = k 6 k 0 and D 6 j pr i I 2 V i, which is absurd. Thus diam G 3k k 2 If k 0 (mod 6), then the same proof shows that if G is k pr EC, then diam G 3, which generalizes the bound in Theorem 8 for the case k = 6. 7 Open Problems We conclude with a few open problems. 1. As remarked above it is not known whether the bound in Theorem 8 is the best possible if G is 6 pr EC, and the graph constructed in Section 5 for k = 6 has diameter 4. Find a 6 pr EC graph with diameter 5 or 6, or improve this bound. 2. In general, let d k be the maximum value of the diameter for a k pr EC graph. Find a sharp upper bound for d k, or at least improve the bound in Theorem We showed in Section 5 that the minimum value for the diameter of a noncomplete γ pr EC graph is 2, and that k pr EC graphs satisfying this diameter exist for all even k 4. What is the spectrum of diameters for k pr EC graphs? In particular, is it true that there exists a k pr EC graph of diameter l for every 2 l d k? 4. In Section 4 we characterized γ pr EC trees. It is evident that they have diameter 4, regardless of the value of γ pr. Characterize bipartite γ pr EC graphs and determine or bound their diameter. 5. All the above questions may be also be asked (with obvious modifications) for paired domination vertex-critical graphs, i.e. graphs G for which γ pr (G v) < γ pr (G) for all v V. See [6, 24] for results on these graphs. References [1] M. Chellali and T. W. Haynes, Trees with unique minimum paired-dominating sets, Ars Combin. 73 (2004), 3 12.

12 290 EDWARDS, GIBSON, HENNING AND MYNHARDT [2] M. Chellali and T.W. Haynes, Total and paired-domination numbers of a tree, AKCE Int. J. Graphs Comb. 1 (2004), [3] M. Chellali and T.W. Haynes, On paired and double domination in graphs, Utilitas Math. 67 (2005), [4] Y. Chen, F. Tian and B. Wei, The 3-domination-critical graphs with toughness one, Utilitas Math. 61 (2002) [5] P. Dorbec, M.A. Henning and J. McCoy, Upper total domination versus upper paired-domination, submitted. [6] M. Edwards, Criticality concepts for paired domination in graphs, Masters Thesis, University of Victoria, [7] O. Favaron and M. A. Henning, Paired domination in claw-free cubic graphs, Graphs Combin. 20 (2004), [8] O. Favaron, D. Sumner and E. Wojcicka, The diameter of domination-critical graphs, J. Graph Theory 18 (1994), [9] O. Favaron, F. Tian and L. Zhang, Independence and hamiltonicity in 3- domination-critical graphs, J. Graph Theory 25 (1997), [10] S. Fitzpatrick and B. Hartnell, Paired-domination, Discuss. Math. Graph Theory 18 (1998), [11] E. Flandrin, F. Tian, B. Wei and L. Zhang, Some properties of 3-dominationcritical graphs, Discrete Math. 205 (1999), [12] D. Hanson and P. Wang, A note on extremal total domination edge critical graphs, Utilitas Math. 63 (2003), [13] T. W. Haynes, S. T. Hedetniemi and P. J. Slater, Fundamentals of Domination in Graphs, Marcel Dekker, New York, [14] T.W. Haynes and M.A. Henning, Trees with large paired-domination number, Utilitas Math. 71 (2006, 3 12). [15] T.W. Haynes, M.A. Henning and L.C. van der Merwe, Total domination critical graphs with respect to relative complements, Ars Combin. 64 (2002), [16] T.W. Haynes, C.M. Mynhardt and L.C. van der Merwe, Total domination edge critical graphs, Util. Math. 54 (1998), [17] T.W. Haynes, C.M. Mynhardt and L.C. van der Merwe, 3-domination critical graphs with arbitrary independent domination numbers, Bull. Inst. Combin. Appl. 27 (1999),

13 DOMINATION EDGE-CRITICAL GRAPHS 291 [18] T.W. Haynes, C.M. Mynhardt and L.C. van der Merwe, Total domination edge critical graphs with maximum diameter, Discuss. Math. Graph Theory 21 (2001), [19] T.W. Haynes, C.M. Mynhardt and L.C. van der Merwe, Total domination edge critical graphs with minimum diameter, Ars Combin. 66 (2003), [20] T. W. Haynes and P. J. Slater, Paired-domination and the paired-domatic number, Congr. Numer. 109 (1995), [21] T. W. Haynes and P. J. Slater, Paired-domination in graphs, Networks 32 (1998), [22] M. A. Henning, Trees with equal total domination and paired-domination numbers, Utilitas Math. 69 (2006), [23] M. A. Henning, Graphs with large paired-domination number, J. Combin. Optimization. 13 (2007), [24] M. A. Henning and C. M. Mynhardt, The diameter of paired-domination vertex critical graphs, submitted. [25] M.A. Henning and M.D. Plummer, Vertices contained in all or in no minimum paired-dominating set of a tree, J. Combin. Optimization 10 (2005), [26] H. Qiao, L. Kang, M. Cardei and Ding-Zhu, Paired-domination of trees, J. Global Optimization 25 (2003), [27] D. P. Sumner, Critical concepts in domination, Discrete Math. 86 (1990) [28] D. P. Sumner and P. Blitch, Domination critical graphs, J. Combin. Theory Ser. B 34 (1983) [29] D. P. Sumner and E. Wojcicka, Graphs critical with respect to the domination number, Domination in Graphs: Advanced Topics (Chapter 16), T. W. Haynes, S.T. Hedetniemi and P.J. Slater, eds. Marcel Dekker, Inc., New York (1998). [30] F. Tian, B. Wei and L. Zhang, Hamiltonicity in 3-domination-critical graphs with α = δ + 2, Discrete Appl. Math. 92 (1999), [31] E. Wojcicka, Hamiltonian properties of domination-critical graphs, J. Graph Theory 14 (1990) [32] L.C. van der Merwe, Total domination edge critical graphs, Ph.D. Thesis, University of South Africa, (Received 1 May 2007)

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