Coloring Vertices and Edges of a Path by Nonempty Subsets of a Set

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1 Coloring Vertices and Edges of a Path by Nonempty Subsets of a Set P.N. Balister E. Győri R.H. Schelp April 28, 28 Abstract A graph G is strongly set colorable if V (G) E(G) can be assigned distinct nonempty subsets of a set of order n, where V (G) + E(G) = 2 n, such that each edge is assigned the symmetric difference of its end vertices. The principal result is that the path P 2 n is strongly set colorable for n 5, disproving a conjecture of S.M. Hegde. We also prove another conjecture of S.M. Hegde on a related type of set coloring of complete bipartite graphs. Introduction In a recent article by Hedge [], he considered coloring certain graphs G, where V (G) + E(G) = 2 n for some integer n, by nonempty subsets of an n-element set. One primary assignment considered was to assign distinct subsets to the vertices and edges such that each edge is assigned the symmetric difference of its end vertices. Since V (G) + E(G) = 2 n this means all nonempty subsets are used in the assignment. When such an assignment exists it is called a strong set coloring of the graph. One interesting conjecture made in this article was that paths of order 2 n where n > 2 are not strongly set colorable. The primary purpose of this article is to disprove this conjecture for n 5. We prove the following theorem. Theorem The paths P 4 and P 8 are not strongly set colorable while all other paths of the form P 2 n are strongly set colorable. An equivalent formulation for a graph to be strongly set colorable and one used throughout this article is the following one. Let G be a connected graph such that V (G) + E(G) = 2 n for some n. We say that G is strongly set colorable if there is a bijection f from V (G) E(G) to the set of nonzero vectors in F n 2, where F 2 is the field with two elements,

2 such that for every edge xy E(G), f(xy)+f(x)+f(y) =. We regard vectors in F n 2 as - sequences of length n, addition in F n 2 corresponding to componentwise addition modulo 2. The vector v then corresponds to the subset S v {, 2,..., n}, where S v consists of all i such that the ith coordinate of v is. In this language the Hedge conjecture says there is no permutation v, v 2, v 3,..., v 2 n of the n-dimensional nonzero vectors of F n 2 such that v i + v i+ + v i+2 = for i =, 3, 5,..., 2 n 3. Here v i represents the set coloring of a vertex when i is odd and of an edge when i is even. These conditions will be referred to as sum conditions. When a permutation satisfying the sum conditions exists we will refer to it as representing vectors in a good permutation or more briefly as a good permutation. In the sequel we also briefly discuss some other trees which are strongly set colorable and give a related condition which is of interest in its own right. 2 Results Lemma 2 The paths P 4 and P 8 of orders 4 and 8 are not strongly set colorable. Proof. First consider the path P 4 and suppose there is a good permutation of its representing vectors v,..., v 7. Since for all n > the sum of all the non-zero vectors in F n 2 is zero, v + v 2 + v 3 + v 4 + v 5 + v 6 + v 7 =. By the sum condition both v + v 2 + v 3 = and v 5 + v 6 + v 7 =, so we have v 4 =, a contradiction. Therefore P 4 is not strongly set colorable. Next consider the path P 8. Suppose that there is a good permutation v,..., v 5 of the representing vectors satisfying the sum conditions. First notice that 5 v 4 + v 8 + v 2 = v i (v +v 2 +v 3 ) (v 5 +v 6 +v 7 ) (v 9 +v +v ) (v 3 +v 4 +v 5 ) i= = =. Claim: The vectors v 5, v 6, v 7, v 9, v, v, v 3, v 4, v 5 can be written in the form v i + v 4j where i, j =, 2, 3. We call the forms of these 9 vectors canonical. Proof of Claim. First we show that these 9 vectors are different from v, v 2, v 3, v 4, v 8, v 2. Suppose that v i = v i2 + v 4j2. Then adding v i2 to this vector, we obtain that either v 4j2 = if i = i 2 or v 4j2 is the third vector out of v, v 2, v 3 if i i 2, a contradiction in both cases. Likewise v 4i v i2 + v 4j2, otherwise v i2 = or the third vector of v 4, v 8, v 2. Then, by pigeonhole principle, it is sufficient to prove that these 9 vectors are pairwise distinct. Suppose that v i + v 4j = v i2 + v 4j2. If i = i 2, j j 2, then it follows that v 4j = v 4j2, a contradiction. If i i 2 then let i 3 be the third index out of, 2, 3. Then v i3 = v i + v i2 = v 4j + v 4j2, which is either if j = j 2 or the third vector out of v 4, v 8, v 2 if j j 2, a contradiction, completing the proof of this claim. 2

3 Define the sets B = {v 5, v 6, v 7 }, B 2 = {v 9, v, v }, B 3 = {v 3, v 4, v 5 } as blocks. Claim: For each block, the vectors v, v 2, v 3, v 4, v 8, v 2 appear exactly once in canonical form of the vectors in the block. Proof of Claim. Let w, w 2, w 3 be the vectors in the block, and suppose that, say, v i appears in the canonical form of w and w 2, i.e., w = v i + v 4j and w 2 = v i + v 4j2 where j j 2 since w w 2. Then w 3 = w + w 2 = v 4j + v 4j2 = v 4j3 where v 4j3 is the third vector in {v 4, v 8, v 2 }, a contradiction. The case when v 4j appears twice can be settled similarly. Having established the above claims we complete the proof of the theorem. Observe that v 5 = v 3 + v 4 by the sum condition. If the canonical form of v 7 contains v 8 then the sum condition implies that v 9 is an element of v, v 2, v 3, a contradiction. So v 7 has v 2 in its canonical form, and since we have symmetry in v and v 2, we may assume that v 7 = v + v 2 (and so v 6 = v 2 + v 8 ). Then v 9 = v + v 4 by the sum condition. As above, v cannot have v 2 in its canonical form, and of course, cannot be v 6 = v 2 + v 8. Thus v = v 3 + v 8, since if v = v + v 8 then v 2 + v = v + v 4 = v 9 = v 3, a contradiction. Then by the sum condition, v 3 = v 3 + v 4 = v 5, a final contradiction, completing the proof of the theorem. Lemma 3 The path P 2 n of order 2 n is strongly set colorable for n = 2 and n 5. Proof. The result is clear for n = 2 so we assume n 5. The proof is inductive applying the induction separately for n odd and for n even. The idea of the proof is to take four copies of a path P which has been strongly set colored and extend this fixed coloring to a path of order 4 P. This involves three basic operations. First, each of the - vectors of the fixed path using n coordinates is increased to one using n + 2 coordinates by adding two new - coordinates such that the resulting vector is distinguished in each of the four copies of this path. Secondly, the four copies are laid end to end with the order of the second and fourth copies reversed from the first and third. Lastly, three new vectors each with zeros in the first n coordinates and differing only in at least one of the last two - coordinates are inserted between the first path and the second, the second and the third, and the third and the fourth. In order to accomplish what has just been described in a precise formal way we first introduce some notation. If v is an n-dimensional vector (- sequence) then v and v are the (n+)- dimensional vectors obtained from v by extending it with a coordinate and, respectively. Similarly, say, v is the (n + 2)-dimensional vector obtained from v by extending it with two coordinates. To avoid confusion, the n-dimensional zero vector will be denoted by n. Figure shows that there are good permutations for both n = 5 and n = 6. It is easy to verify the correctness of these examples. These examples will anchor the inductive proof that we give separately for n odd and for n even. 3

4 .... Figure : Example set colorings of path P 2 n for n = 5 and n = 6, sets given as - sequences for each edge and vertex. To finish the proof of the theorem, we prove for that if P 2 n is strongly set colorable for n 5 then P 2 n+ is strongly set colorable. Thus assume that v, v 2,..., v 2 n is a desired good permutation of the n-dimensional nonzero - vectors. Then it is a somewhat tedious, but straightforward to verify that the following sequence is a desired permutation of the (n + 2)-dimensional nonzero vectors satisfying the sum conditions. (The extension is 4-periodic in the blocks of 2 n vectors but requires certain interchanges of the first or last two vectors to fulfill the sum conditions). We provide the lengthy sequence to clarify the desired resulting good permutation. (Vectors written vertically, bold entries indicate entries that do not fit into the 4-periodic pattern.) v v 2 v 3 v 4 v 5 v 6 v 7... v 2 n 8 v 2 n 7 v 2 n 6 v 2 n 5 v 2 n 4 v 2 n 3 v 2 n 2 v 2 n n v 2 n v 2 n 2 v 2 n 3 v 2 n 4 v 2 n 5 v 2 n 6 v 2 n 7... v 8 v 7 v 6 v 5 v 4 v 3 v v 2 n v 2 v v 3 v 4 v 5 v 6 v 7... v 2 n 8 v 2 n 7 v 2 n 6 v 2 n 5 v 2 n 4 v 2 n 3v 2 n v 2 n 2 n v 2 n 2v 2 n v 2 n 3 v 2 n 4 v 2 n 5 v 2 n 6 v 2 n 7... v 8 v 7 v 6 v 5 v 4 v 3 v 2 v This completes the inductive step and the proof of the theorem. The reader should note that the interchanges required in the constructed sequence in the proof just given makes this construction fail for n = 2, so that P 2 strongly set colorable does not imply P 8 is strongly set colorable. The general strategy used in the proof of the last theorem can be applied beginning with a strongly set colorable bipartite graph in place of a path. The interchange idea is not needed in this case. 4

5 Theorem 4 Let G be a strongly set colorable bipartite graph with color classes X, Y and edge set E. Let G, G 2, G 3, G 4 be four disjoint copies of G with color classes X, Y, X 2, Y 2, X 3, Y 3, X 4, Y 4 and edge sets E, E 2, E 3, E 3, respectively. Let G denote the graph obtained from the disjoint union of the graphs G, G 2, G 3, G 4 by adding edges e, e 2, e 2 with the following properties:. each e i joins two copies of the same vertex; 2. one of the following three possibilities occurs: (a) the edges join X and X 2, X and X 3, X and X 4, respectively; or (b) the edges join X and X 2, X 2 and X 3, Y 3 and Y 4, respectively; or (c) the edges join X and X 2, Y 2 and Y 4, Y and Y 3, respectively. Then G is strongly set colorable. Proof. Consider the same vector coloring of G, G 2, G 3 and G 4. Then extend the vectors in X, Y, and E with, the vectors in X 3, Y 4, and E 2 with, the vectors in X 2, Y 3, and E 4 with, the vectors in X 4, Y 2, and E 3 with. If we color the edges e, e 2, e 3 according to the sum condition, then it is easy to verify that we obtain a desired vector coloring of G, completing the proof. Symmetric possibilities of those listed in the last theorem are clearly strongly set colorable as well. 3 Other Trees The binary tree has a particularly nice set coloring. To ensure that we have exactly 2 n vertices, we add one additional leaf to the root of the tree. The coloring is then given at each level by assigning vertices the binary numbers from 2 l+ to 3.2 l in turn, see Figure 2. The edges the are colored with the numbers 3.2 l to 2 l+2 in the order of a grey code. Another interesting possible construction is to take a strong set coloring of G and extend it to a new graph G obtained from G by adding 2 n pending edges. The color of each of the original edges and vertices of G is obtained by appending to the original color. New edges e i get color p i and new vertices u i get color q i where the pendent edge e i = u i v i is joined to a vertex v i of color v i. We need v i = p i q i for this to give a strong coloring of G, and the set of all p i and q i needs to exhaust F n 2. Note that some of the v i s may be the same since we can attach several pendent edges to the same vertex. We need v i = for such p i and q i to exist (since = p i + q i = v i ). It appears that this is sufficient, so we make the following conjecture. 5

6 Figure 2: Coloring of the binary tree. All - sequences should be padded on the left by the appropriate number of zeros. Conjecture Given 2 n non-zero (not necessarily distinct) vectors v,..., v 2 n F n 2, n 2, with 2 n i= v i =, there exists a partition of F n 2 into pairs of vectors {p i, q i }, i =,..., 2 n such that for all i, v i = p i q i. This conjecture, which appears to be of interest in its own right, is true for n 5. It is also true if at least half of all the vectors v i are the same and all vectors occur an even number of times. Theorem 5 Given 2 n non-zero vectors v,..., v 2 n F n 2, n 2, with v = v 2 = = v 2 n 2, v 2i = v 2i+ for all i and 2 n i= v i =, there exists a partition of F n 2 into pairs of vectors {p i, q i }, i =,..., 2 n such that for all i, v i = p i q i. Proof. 4 Proper set colorings of complete bipartite graphs In [], a proper set coloring of a graph G is defined as an assignment of subsets of an n-element set to the vertices and edges of G so that. each edge receives the symmetric difference of the sets assigned to its end vertices; 2. distinct vertices receive distinct sets; 3. distinct edges receive distinct non-empty sets; and 4. every non-empty set appears on some edge. 6

7 Note that the same set can occur on both an edge and a vertex. Clearly, if a proper set coloring of G is to exists we need E(G) = 2 n. The following theorem appears as a conjecture in []. Theorem 6 If the complete bipartite graph K s,t has a proper set coloring then either s = or t =. Writing S,..., S s and T,..., T t as the sets assigned to the vertices of K s,t, this theorem is an immediate consequence of the following. Theorem 7 Assume S,..., S s and T,..., T t are subsets of {,..., n} and the symmetric differences S i T j, i =,..., s, j =,..., t, are non-empty and represent every non-empty subset of {,..., n} exactly once (so in particular st = 2 n ). Then either s = or t =. Proof. Consider for any X {,..., n} the quantities ŜX = s i= ( ) X S i and ˆT X = t j= ( ) X Tj. (These are Fourier transforms over F n 2 of the indicator functions giving the sets S i and T j respectively.) Now Ŝ X ˆTX = i,j ( ) X S i + X T j = Y ( ) X Y since X S i + X T j X (S i T j ) mod 2 and the S i T j range over all nonempty subsets of {,..., n}. Now if X, then the number of (possibly empty) subsets Y {,..., n} with X Y is even is the same as the number with X Y odd. Indeed, if k X we can pair up the sets Y with k / Y with Y = Y {k} and then X Y and X Y have different parities. Thus Ŝ X ˆTX = ( ) Y ( ) X Y = ( ) X Y ( ) X = =. But ŜX, ˆT X Z, so ŜX, ˆT X {, } for X. Now consider ŜX 2 = Ŝ2 + ŜX 2 = s 2 + (2 n ).. X X Y We can also write this as ŜX 2 = X X s s ( ) X Si ( ) X Sj = ( ) X (S i S j ). i,j= i,j= X But the last sum over X is zero unless S i S j =, i.e., unless S i = S j. But if S i = S j then S i T = S j T, so i = j since the S k T l are distinct. Hence ŜX 2 = X s ( ) X = 2 n s. i= X 7

8 (This is just a special case of Parseval s theorem for Fourier transforms.) Thus 2 n s = s 2 + (2 n ) and so (s )(s (2 n )) = s 2 2 n s + (2 n ) =. Hence either s =, or s = 2 n. But if s = 2 n then t = since st = 2 n. References [] S.M. Hegde, Set Colorings of Graphs, preprint. 8