# Every tree is 3-equitable

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1 Discrete Mathematics 220 (2000) Note Every tree is 3-equitable David E. Speyer a, Zsuzsanna Szaniszlo b; a Harvard University, USA b Department of Mathematical Sciences, University of South Dakota,Vermillion, SD 57069, USA Received 11 May 1999; revised 29 October 1999; accepted 29 November 1999 Abstract A labeling of a graph is a function f from the vertex set to some subset of the natural numbers. The image of a vertex is called its label. We assign the label f(u) f(v) to the edge incident with vertices u and v: In a k-equitable labeling the image of f is the set {0; 1; 2;:::;k 1}: We require both the vertex labels and the edge labels to be as equally distributed as possible, i.e., if v i denotes the number of vertices labeled i and e i denotes the number of edges labeled i; we require v i v j 61 and e i e j 61 for every i; j in {0; 1; 2;:::;k 1}: Equitable graph labelings were introduced by I. Cahit as a generalization for graceful labeling. We prove that every tree is 3-equitable. c 2000 Elsevier Science B.V. All rights reserved. 1. Introduction In 1964, Ringel conjectured that K 2n+1 ; the complete graph on 2n + 1 vertices, can be decomposed into 2n + 1 isomorphic copies of a given tree with n vertices [4]. In 1967, Rosa introduced -labelings as a tool to attack Ringel s conjecture [1]. This labeling was called graceful by Golomb, and now this is the term most widely used. A graceful labeling of a graph with e edges and vertex set V is an injective function f : V {0; 1; 2;:::;e} with the property that the resulting edge labels are also distinct where an edge incident with vertices u and v is assigned the label f(u) f(v) : A graph that admits a graceful labeling is called a graceful graph. If a tree on n vertices is graceful then K 2n+1 can be decomposed to isomorphic copies of this tree. The Ringel Kotzig conjecture that states that every tree is graceful has been around for three decades. Many papers focused on the problem, but very few general results are known. For an excellent survey on this and other graph labeling problems see [3]. Corresponding author. addresses: (D.E. Speyer), (Z. Szaniszlo) X/00/\$ - see front matter c 2000 Elsevier Science B.V. All rights reserved. PII: S X(00)

2 284 D.E. Speyer, Z. Szaniszlo / Discrete Mathematics 220 (2000) In 1995, Cahit introduced the idea of k-equitable labeling, a generalization of graceful labeling [5]. In a k-equitable labeling we distribute the labels {0; 1; 2;:::;k 1} as evenly as possible among both the vertices and the edges. More precisely, the number of vertices labeled i and the number of vertices labeled j dier by at most one and the same holds for the edge labels. A graph that can be labeled k-equitably is called a k-equitable graph. Notice that a graph on e edges is graceful if and only if it is (e + 1)-equitable. Cahit proved that every tree is 2-equitable and that every tree with fewer than ve end vertices is 3-equitable [5]. It seems to be widely believed that Cahit also proved that every caterpillar is 3-equitable, but the proof has never been published. (A caterpillar is a tree with the property that the removal of the end vertices results in a path.) Szaniszlo proved that every path is k-equitable for any k [6]. No other general result is known about equitability of trees, though Cahit conjectured that every tree is k-equitable for any k. In this paper we prove that every tree is 3-equitable. 2. Some useful facts The rst easy observation we make is that in a 3-equitable labeling the vertex labels 0 and 2 play the same role. If a labeling is 3-equitable then switching every 0 vertex label to a 2 label and vice versa will result in another equitable labeling. In the proof we will construct a labeling through several steps. We will use the fact that in a rst approximation we can choose the vertices labeled one and then decide on the distribution of the even labels later. This permits the use of proper 2-colorings along the way. Lemma 1. If the vertices of a tree are properly 2-colored black and white and there are more black vertices than white vertices then there is at least one end vertex colored black. Proof. We ll prove the equivalent statement: If there are only white end vertices then at least half of the vertices are white. Suppose this statement is not true. Let T be a minimal counter example. Then T has only white end vertices. Remove from T every end vertex and every vertex that is adjacent to an end vertex. The remaining forest will have only white end vertices, and since T was minimal, each tree in the forest will have at least half the vertices colored white. We only need to show that we removed at least as many white vertices as black vertices. But this is clearly true since every removed black vertex was adjacent to at least one white end vertex in T: Hence T had at least as many white vertices as black vertices. Denition 1. A tree is balanced if in a proper 2-coloring the number of vertices of dierent colors dier by at most one. We will use the fact that in many trees there are large balanced subtrees. We will label such a subtree rst and then the rest of the tree. But rst we need to establish the existence of these subtrees.

3 D.E. Speyer, Z. Szaniszlo / Discrete Mathematics 220 (2000) Lemma 2. Let T be a tree on n vertices with p end vertices. Let b denote the number of vertices in the largest balanced subtree of T. Then p + b n. Proof. If the tree is balanced, then we are done. If T is not balanced, then we have at least one end vertex of the majority color, say black. Delete all the black end vertices of T one by one. In the resulting tree every end vertex is white, so now the white vertices are in majority. But then during the process we have to have a balanced subtree at some point. Lemma 3. A balanced tree has a balanced subtree of any smaller size. Proof. Let T be a balanced tree properly colored black and white. It is sucient to show that we can remove an end vertex to get a smaller balanced tree. If the number of black and white vertices is the same in T then remove any end vertex. If there is a majority color then remove an end vertex of this color. The resulting graph in both cases is clearly balanced. We plan to show that a tree can be labeled 3-equitably if it has either many end vertices or a large balanced subtree. If the tree has many end vertices, the proof relies on the fact that every tree is 2-equitable [2]. Lemma 4. If in a tree with n vertices; there are at least n=3 end vertices; then it is 3-equitable. Proof. Let T be a tree with n vertices and at least n=3 end vertices. Label n=3 of the end vertices 1. The rest of T is a tree, hence it has a 2-equitable labeling. In the 2-equitable labeling change every 1 vertex label to a 2 label. This labeling is clearly a 3-equitable labeling of T. 3. Labeling algorithms If a tree does not have many end vertices then it has to have a large balanced subtree. We construct labelings for such trees. The construction will be dierent depending on the number of vertices mod 3. Let n denote the number of vertices. Case 1: n =3m If the number of vertices is divisible by 3 then each vertex label has to be used the same number of times. Among the edge labels one of the labels have to appear one less time than the other two. We start with some colorings of certain vertices. Note that if the tree does not have many end vertices then it has a balanced subtree of order m +1: Let H denote such a balanced subtree. Color every vertex in H blue. (Blue will stand for vertices that we want to label either 0 or 2 later on.)

4 286 D.E. Speyer, Z. Szaniszlo / Discrete Mathematics 220 (2000) We want to label the remaining 2m 1 vertices. Before we decide on labels, rst color the remaining vertices blue and red in the following fashion: At each step we color two vertices. If an unlabeled 2-path is connected to a red vertex, color the adjacent vertex red, color the other blue. (Red will stand for the vertices that we want to label 1 later.) If an unlabeled 2-path is connected to a blue vertex, color the adjacent vertex blue, color the other vertex red. If two end vertices are each connected to a vertex of the same color (not necessarily the same vertex), color one of them red, the other blue. If this is not possible any more then there is only one uncolored end vertex left. This vertex might be connected to a red or a blue vertex. At each coloring step we create a red blue edge and another edge that is either red red or blue blue. Make the last edge blue blue or red red. (We will refer to this step later in the algorithm, we might have to come back and revise our choice here.) So far we created m or m 1 red vertices depending on the last color used. We also created m 1 edges between blue and red vertices. Denote by r the number of red red edges. Note that 06r6m 1. Label every red vertex 1. This will create m 1 edges labeled 1, since every blue red edge will turn into a 1 edge label. It also means that every red red edge will have 0 as a label, and we have freedom among the blue blue edges, they can turn into edges labeled either 0 or 2. In the next step we create r edges labeled 2. To do this, nd a balanced subtree of H of size r: Properly color the vertices of this small tree with colors 0 and 2, these colors will correspond to the required labels for these vertices. This coloring creates r edges that have 2 as labels. We already decided the label for r + r +(m 1) edges and (r + 1)+ m or (r +1)+(m 1) vertices, depending on whether we colored the last vertex in the rst part of the algorithm red or blue. We need to decide the label for the remaining 3m 1 (2r + m 1)=2m 2r edges. Now the part of the graph that has no label yet is a rooted forest where every vertex is colored blue. The roots are connected to vertices that are already labeled. Now label those roots that are not connected to vertices labeled 0, or 2. The only thing we need to be careful about in this process is to keep the balance between the 0 and 2 vertex labels. The rest of the tree can be labeled similarly to the rst part of the algorithm. I.e., if an unlabeled 2-path is connected to a vertex labeled 2, label the adjacent vertex 2, label the other vertex 0. If an unlabeled 2-path is connected to a vertex labeled 0, label the adjacent vertex 0, label the other vertex 2. If two end vertices are each connected to a vertex of the same label (not necessarily the same vertex), label one of them 0, the other 2. At each step we created two new edge labels, one of them is a 0; the other is a 2. Hence the number of edges labeled 0 and the number of edges labeled 2 are the same at each step of the algorithm after the labeling of the balanced subtree.

5 D.E. Speyer, Z. Szaniszlo / Discrete Mathematics 220 (2000) If we run out of the vertices after repeating these steps then we have the same number of edges labeled 0 and 2, m 1 edges labeled 1; and the number of 0 and 2 vertex labels are within one of each other. Unfortunately, we might have m 1 vertices labeled 1 and m + 1 vertices labeled 0 or 2; say 2: If this is the case then we created m 1 red vertices at the rst part of the algorithm. This means that the last vertex we colored blue instead of red creating a blue blue edge. Now go back to this step and create a blue red edge instead by using a red vertex. This way the number of red red edges does not change and we can continue the algorithm by choosing the same balanced subtree to be labeled as before. The only dierence is that the number of edges with label after the labeling of the balanced subtree is (3m 1) (r + r + m)=2m 2r 1; an odd number. Now when we get stuck at the end of the algorithm we have only one vertex to label, and the number of vertices labeled 0 and 2 are not the same. We can use the minority label for the last vertex. If we do not run out of vertices then we have the following situation: We needed to label an even number of edges, so if we still have any blue vertices left, we must have two blue vertices, one of them connected to a vertex labeled 0, the other to a vertex labeled 2. Recall again, that at the beginning of the labeling we made a choice for the last red or blue vertex. We created an edge dierent from 1 there. If we used a blue vertex to do that, then we still need to use a 1 for a vertex label. We also have only m 1 edges labeled 1, so adding another edge labeled 1 does not cause any problem. Before labeling the last two vertices, we have m 1 of any of the three edge labels, so we can choose any two to be used at this point. Also, the number of 0 and 2 vertex labels is balanced, one of them used one less than the other. Label one of the last two vertices using the minority label, label the other vertex 1. If we are stuck with two blue vertices and used a red vertex to create a red red edge in the rst part of the algorithm then we used m 1 s for vertex labels which means that we also labeled m 1 vertices 2 and m 1 vertices 0: We need to use both vertex labels at the end creating two dierent edge labels. This cannot be done. To solve this problem we need to have a closer look at the rst coloring of the tree with blue and red colors. We will show that after this step we have one of the following two cases: a blue end vertex connected to a red vertex or a degree 2 blue vertex connected to both a red and a blue vertex. Once we have one of these two cases, this is how we can use them to nish the labeling: If a degree 2 red vertex is connected to both a red and a blue vertex then after the rst red blue coloring change the middle vertex to a blue vertex. Instead of a red red and a red blue edge we ll have a blue blue edge and a blue red edge. We also labeled only m 1 vertices 1. Hence at the end we can assign the label 1 to the last end vertex we need to label. If we have a blue end vertex connected to a red vertex, we note the label the blue vertex gets. Before labeling the last two vertices we can change the label of this vertex leaving a vertex label in minority and assign the minority label to the last two vertices.

6 288 D.E. Speyer, Z. Szaniszlo / Discrete Mathematics 220 (2000) This will leave the 0 and 2 vertex labels balanced and will create a 0 and a 2 edge label at the last step. Here is the promised argument that will nish up the case of a tree with 3m vertices. Claim 1. After coloring every vertex of H blue at least one of the remaining rooted trees will have a vertex at least distance 3 away from the root. Proof. Note that if every vertex is at most distance 2 away from the root in a rooted tree then at least half of the non root vertices are end vertices. Counting the vertices shows that 2m of the vertices are non root vertices and there are less than m end vertices. Claim 2. After the rst use of the algorithm when every vertex is colored red or blue we have one of the following cases: (i) a blue end vertex connected to a red vertex or (ii) a degree 2 blue vertex connected to both a red and a blue vertex. Proof. Let us look at a rooted tree with the required property. The root is colored blue and it has at least a 3-path connected to the root. Color the vertex of this 3-path that is adjacent to the root blue, color the next adjacent vertex red. (The interesting part of the graph without all the incident edges is now blue blue red?) If this red vertex is of degree 2 then we can color the remaining vertex of the 3-path red during the algorithm. (If it is an end vertex then we can color it with red. If it is connected to something else, then it is the rst vertex on a 2-path connected to a red vertex and it will be colored red.) If this red vertex is not of degree 2 then it is at least of degree 3 and it is adjacent to at least two other vertices. If both of these vertices are end vertices then our algorithm colors one of them blue the other one red, and we have a blue end vertex connected to a red vertex. We need to consider what happens if one of the 2 adjacent vertices is not an end vertex. (The interesting part of the graph without all the incident edges is? red?? where the red vertex is connected to at least one blue vertex from the path.) Look at the red?? part of this picture. If the rst not colored vertex is of degree two or the second is of degree one then we are done by the coloring red red blue. Thus the only trouble we can have is if there is at least a 3-path connected to the red vertex where the rst vertex of the path is of at least degree 3. Color the rst vertex of the path red and one of the adjacent vertices that is not part of the 3-path blue. This is a legal step in our coloring algorithm which leaves us with a red?? picture again. We can continue this process and eventually one of these not colored vertices will have the required degree. Case 2: n =3m +1 If the tree we plan to label 3-equitably has 3m + 1 vertices then every edge label has to appear m times. On the other hand we have some exibility with the vertex labels. We can freely choose a label that will appear m + 1 times while the other two

7 D.E. Speyer, Z. Szaniszlo / Discrete Mathematics 220 (2000) labels will appear m times. The method of the labeling is very similar to the previous case. We again start with nding a balanced subtree of order m +1: We color all its vertices blue and follow with our algorithm to color the rest of the tree red and blue. In this case we need to color then label the remaining 2m vertices. This process is either successful leaving the same amount of red blue and other edges or we are left with two end vertices connected to two dierently colored vertices at the end. If this is the case then create a blue blue and a red red edge in the last step. This means that we colored m vertices red and created m 1 red blue edges. This will give us the exibility to use a 1 label at the end of the labeling process which will assure that neither of the 0 or 2 vertex labels becomes majority. The dicult case occurs when we color m + 1 vertices red in this rst part of the algorithm. Fortunately the same argument as at the end of Case 1 enables us to change a label and be able to use an extra 1 for a vertex label at the last step. Case 3: n =3m +2 Remove an end vertex of the graph and label the remaining tree on 3m + 1 vertices 3-equitably. Note which vertex label is in majority in this labeling. Now reattach the removed vertex and assign one of the other two labels to it. This will clearly leave the vertex labels evenly distributed. Also, there was an equal distribution of edge labels in the 3-equitable graph on 3m + 1 vertices, so any new edge label can be introduced leaving the new graph 3-equitable. 4. Open questions The conjecture that every tree is k-equitable for any k seems to be very dicult. Unfortunately our argument very much depends on the symmetry of the labels in 3-equitable labelings, so it does not seem to be likely that it would be easy to generalize for higher k values. Probably a new approach is needed for deciding the 4-equitability of trees. Of course, the most interesting question is whether every tree is e+1 equitable, where e is the number of edges in the tree. As we mentioned before this question is equivalent to the question of gracefulness of trees. Though the conjecture that every tree is graceful has been widely accepted, no signicant progress has been made towards the solution in the last three decades. References [1] I. Cahit, Cordial graphs: a weaker version of graceful and harmonious graphs, Ars Combin. 23 (1987) [2] I. Cahit, Equitable tree labellings, Ars Combin. 40 (1995) [3] J. Gallian, A dynamic survey of graph labeling, Dynamical Surveys, DS6, Electron. J. Combin. (1998). [4] G. Ringel, Problem 25, in: Theory of Graphs and its Applications, Proceedings of the Symposium Smolenice 1963, Prague, 1964, p [5] A. Rosa, On certain valuations of the vertices of a graph, Theory of Graphs, International Symposium, Rome, July 1966, Dunod, Paris, 1967, pp [6] Z. Szaniszlo, k-equitable labelings of cycles and some other graphs, Ars Combin. 37 (1994)

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