On a Conjecture of Butler and Graham

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1 On a Conjecture of Butler and Graham Tengyu Ma 1, Xiaoming Sun 2, and Huacheng Yu 1 1 Institute for Interdisciplinary Information Sciences, Tsinghua University 2 Institute of Computing Technology, Chinese Academy of Sciences Abstract. Motivated by a hat guessing problem proposed by Iwasawa [6], Butler and Graham [2] made the following conjecture on the existence of certain way of maring the coordinate lines in [] n : there exists a way to mar one point on each coordinate line in [] n, so that every point in [] n is mared exactly a or b times as long as the parameters (a, b, n, ) satisfies that there are non-negative integers s and t such that s+t = n and as+bt = n n 1. In this paper we prove this conjecture for any prime number. Moreover, we prove the conjecture for the case when a = 0 for general. Keywords: hat guessing games, maring coordinate lines, characteristic function MSC classes: 00A08 97A20 94B05 1 Introduction In [2] Butler and Graham considered the problem of the existence of certain way of maring coordinate lines in [] n. A coordinate line in [] n is the set of points in which all but one coordinate are fixed and the unfixed coordinate varies over all possibilities. Maring a line 1 means designating a point on that line. They conjectured that Conjecture 1 (Butler, Graham [2]). There is a maring of the lines in [] n so that on each line exactly one point is mared and each point is mared either a or b times if and only if there are nonnegative integers s and t satisfying the linear equations s + t = n and as + bt = n n 1. The only if part of the conjecture is straightforward, leaving to be crucial the construction of a maring of lines in [] n with the desired properties. Buhler, Butler, Graham, and Tressler [1] proved the conjecture for = 2. Butler and Graham [2] proved the conjecture when n 5. The main contributions of this paper are i) we prove all the cases when is an odd prime; ii) we prove the case when a = 0 (without any assumption on ). Theorem 1. For any prime and 0 a < b n, there exists a maring of lines in [] n so that each point is mared either a times or b times if and only if there are nonnegative integers s and t so that s + t = n and as + bt = n n 1. Theorem 2. For 0 < b n, there exists a maring of lines in [] n so that each point is either unmared or mared b times if and only if there are nonnegative integers s and t so that s + t = n and bt = n n 1. As in [2], we use the notation [a, b] n as a shorthand for a realization of a maring of the lines in [] n where each point is mared either a times or b times. Then Theorem 1 provides a sufficient and necessary condition for the existence of [a, b] n for any prime, and Theorem 2 considers the existence of [a, b] n when a = 0. In the proof of Theorem 1 we reduce the existence of [a, b] n to the existence of [a 1, b 1]n when a > 0 and b n + 1 (see Proposition 2 in Section 2). It turns out that the most 1 for convenience, we use line to indicate coordinate line throughout this paper.

2 2 Tengyu Ma, Xiaoming Sun, and Huacheng Yu complicated part of this inductive argument is the construction of the base cases, that is, [0, b] n and [a, n t] n (where t < ). We provide two theorems (Theorem 3 and 4) giving a direct realizations for these two inds of marings. The proofs of these theorems use similar approaches, though different in many details, that we partition the whole grid [] n according to certain number theory based characteristic function, which has a nice symmetrical property: Informally speaing, for any fixed s, any point x [] n, and any value v in the range of this function, there exists a unique direction along which by moving from x with distance s, we can reach a point with value v. By a sophisticated utilizing this property, we accomplish the design of marings for these two base cases. Furthermore, we prove the case [0, b] n for general by generalizing the characteristic function in a delicate way. Related Wor The motivation of investigating this maring line problem is to reformulate and solve a hat guessing question proposed by Iwasawa [6]. In that game there are several players sitting around a table, each of which is assigned a hat with one of colors. Each player can see all the colors of others hat but his/her own. The players try to coordinate a strategy before the game starts, and guess the colors of their own hats simultaneously and independently after the hats are placed on their heads. Their goal is to design a strategy that guarantees exactly either a or b correct guesses. For example, one special case is that either everybody guesses the color correctly or nobody guesses correctly, i.e. a = 0 and b = n. Several variations of hat guessing game have been considered in the literature. Ebert [4] considered the model that players are allowed to answer unnown. He showed that in this model there is a perfect strategy for players when n is of the form 2 m 1. Lenstra and Seroussi [7] studied the case that n is not of such form. Butler, Hajiaghayi, Kleinberg and Leighton [3] considered the worst case that each player can see only part of the others hats with respect to a sight graph. Feige [5] investigated the average case with a sight graph. Peterson and Stinson [9] investigated the case that each player can see hats in front of him and they guess one by one. Recently Ma, Sun and Yu [8] proposed a variation which allow to answer unnown and require at least correct guesses for winning condition. Notations and Preliminaries [] = {1, 2,..., }, [] n = [] []. [a, b] n }{{} : maring of lines in []n in which each point is n mared either a times or b times. We assume that a < b as well. Throughout the paper we always use boldface type letters for vectors and vector-valued functions. For a vector x = (x 1,..., x n ), define the -modulo parity function (x) = x 1 + x x n mod. We denote by x i the line (x 1,..., x i 1,, x i+1,..., x n ), i.e. x i = {(x 1,..., x i 1, y, x i+1,..., x n ) y []}. The necessary condition in the conjecture is straightforward. Proposition 1 ([1]). If we have [a, b] n, then the following equations system has nonnegative integer solution. { s + t = n, as + bt = n 1 n. More specifically, s = n 1 (b n) b a is the number of points that are mared a times, and t = is the number of points that are mared b times. n 1 (n a) b a

3 On a Conjecture of Butler and Graham 3 The rest of the paper is organized as follows: In Section 2 we show that the necessary condition is sufficient when the number of colors is a prime. Section 3 considers the case for general when a = 0. Finally we conclude the paper in Section 4 with some open problems. 2 [a, b] n for Prime In this section we prove the conjecture when is an odd prime number (the case = 2 has been proved by Buhler et al. [1]). The first step is to reduce [a, b] n to [a 1, b 1]n as in Butler and Graham [2]. Proposition 2 ([2]). Given [a, b] n, we have [a + 1, b + 1]n+. By repeatedly using this Proposition, we can reduce the problem [a, b] n to two possible cases: (1) [a, b ] n, where b > n ; (2) [0, b ] n (recall that a < b). During this procedure, the divisibility is unchanged (see the proof of Theorem 1). The following two theorems (Theorem 3 and Theorem 4) give the constructions of two base cases, respectively. Theorem 3. If is a prime and 0 t 1, 1 a < n t, and ( n a n t a )n 1 is a nonnegative integer, then we have [a, n t] n. Proof. Firstly we do some elementary number theory substitution to mae the parameters more manageable. Suppose that ( n a n t a )n 1 is a nonnegative integer. Since is a prime, there exists m, r N so that n t a = m r, where m n 1, r n a, and n a. (n t a implicitly holds.) Observe that r n a and r n t a implies that r ( 1)a t. Let ( 1)a t = ra, where a ( 1)a t (a a t)m (n a t)m = = = m. r n t a n t a Thus, n = t + a + m r and ( 1)a = t + ra, where a, m and r are all nonnegative integers. Now we construct a maring of lines in [] n so that each point is mared either (n t) times or a times. For convenience, we partition the n = a + t + m r dimensions into three groups, each of which contains m r, t, and a coordinates respectively 2, and represent each point in [] n by (x, y, z) where x [] mr, y [] t, and z [] a. Furthermore, we index x by a pair (i, j) Z m [r], i.e. x i,j [] are the coordinates of x (where i Z m, j [r])3. For a point (x, y, z) [] n and i Z m, j [r], denote by (x (i,j), y, z) the lines of [] n for which all the coordinates are fixed except the coordinate (i, j) of x, i.e. (x (i,j), y, z) = {( x, ỹ, z) [] n ỹ = y, z = z, (i, j ) (i, j) x i,j = x i,j, x i,j []} Similarly (x, y i, z) (i [t]) is a line of [] n which the i-th coordinate of y is unfixed, (x, y, z i ) (i [a]) is a line which the i-th coordinate of z is unfixed. For each x [] mr, define the characteristic function q : [] mr [] m as follows: q(x) = r i (1) i=(i 1,...,i m) Z m where i is -based representation of i in Z m, and the operations + and are over Z. According to the characteristic value q(x), we can group the points in [] n into equivalence classes. Specifically, 2 When t = 0, then there is only two groups, the proof still holds. 3 Since we need to tae some ring operations on the index i, we index i by Z m here instead of [] m. j=1 x i,j

4 4 Tengyu Ma, Xiaoming Sun, and Huacheng Yu for any w [] m, let Q(w) be the collection of points in [] n which have characteristic value w, i.e. Q(w) = {(x, y, z) [] n q(x) = w}. Arbitrarily choose a different values w 1,..., w a from [] m, for example the first a elements in the lexicographical order (recall that 0 a m ), and let M be the collections of points which have one of these a values as characteristic value and 0 as -modulo parity, i.e., M = (Q(w 1 ) Q(w a )) {(x, y, z) [] n (x, y, z) = 0}. We arbitrarily partition the set [a ] [r] (recall that [a ] [r] [ m ] [r] is the indices set of x) into ( 1) subsets L 1 L 2 L 1 with the requirement that the cardinality L 1 = = L t = a 1 and L t+1 = = L 1 = a. (Notice that t(a 1) + ( 1 t)a = ( 1)a t = a r.) Based on these preparations, now we give the construction of the desired maring of [] n. There are three different types of lines: (x (i,j), y, z), (x, y i, z), and (x, y, z i ), we mar them as follows: 1. for the line (x (i,j), y, z), there are two sub-cases: (x (i,j), y, z) M, i.e. on line (x (i,j), y, z) there exists some point belongs to set M. (by the condition that (x, y, z) = 0, this point is unique, if exists). Suppose the point is ( x, y, z) (x (i,j), y, z) M, and suppose that ( x, y, z) Q(w i0 ) for some i 0 [a ], and (i 0, j) L s for some s [ 1] (recall that L 1,..., L 1 is a partition of [a ] [r]). Then we mar the point ( x + s e i,j, y, z) (x (i,j), y, z), where e i,j = (0,..., 0, 1, 0,..., 0) is the unit vector in [] mr for which only x i,j = 1, and all other coordinates equal 0. The addition and multiplication are over Z. This is also the unique point on this line which has ( ) = s; otherwise (x (i,j), y, z) M =, mar the unique point ( x, y, z) on the line such that ( x, y, z) = for line (x, y i, z) (i [t]), mar the unique point (x, ỹ, z) which satisfies (x, ỹ, z) = i. (recall that 1 i t 1) 3. for line (x, y, z i ) (i [a]), mar the point (x, y, z) which satisfies (x, y, z) = 0. We claim that the construction above is indeed a [a, n t] n. We need to chec that each point (x, y, z) is mared either a times or (n t) times. 1. if (x, y, z) = 0, then for each line (x, y i, z), we never mar the point (x, y, z) (recall that we mar some point which has ( ) = i 0). On the contrary, for each line (x, y, z i ), we always mar (x, y, z). For lines of the form (x (i,j), y, z), there are two sub-cases: if (x, y, z) M, then on the line (x (i,j), y, z) we mar the point (x+s e i,j, y, z) for some s > 0 by the construction, which is not (x, y, z). Thus in this case, (x, y, z) is mared 0, 0, and a times in x, y and z s directions respectively, hence a times in total; if (x, y, z) M, then on the line (x (i,j), y, z) there is no point in M. Thus we mared the point with ( ) = 0, which is exactly point (x, y, z) itself. In this case, (x, y, z) is mared m r, 0, and a times in three groups of directions respectively, and m r a = n t times in total. Therefore, (x, y, z) is mared either a or (n t) times.

5 On a Conjecture of Butler and Graham 5 2. if (x, y, z) = s for some 1 s t. Among lines (x, y i, z) (1 i t), only on the line (x, y s, z) we mared point (x, y, z). On each line (x, y, z i ) (i [a]), we never mar (x, y, z). By the definition of the characteristic function, q(x s e i,j ) are different for different i s (here we use the fact that is a prime, hence s is coprime to and has an inverse in Z ). Therefore there are exactly a r different pairs of (i, j) such that line (x (i,j), y, z) contains a point (x s e i,j, y, z) M. On exactly L s = a 1 lines of these a r lines, point (x, y, z) is mared. On all other lines, there is no point belong to M, thus we only mar the point with ( ) = 0. Thus the point (x, y, z) is mared (a 1), 1 and 0 times in x, y, and z s directions, respectively, and in total a times. 3. if (x, y, z) = s for some s > t. It is similar to the case above, except that we never mar point (x, y, z) on lines of form (x, y i, z), and on L s = a lines of form (x (i,j), y, z) we mar (x, y, z). Therefore in this case (x, y, z) is also mared exactly a times. Theorem 4. If is a prime, 0 < b n, and ( ) b n b n 1 is an integer, then we have [0, b] n. Proof. Suppose gcd(b, n) = r, since is a prime number and ( ) b n b n 1 Z, we have that b = r m, n = rn for some non-negative integer m and n. By the following Proposition, it suffices to prove the r = 1 case. Proposition 3 ([2]). Given [0, b] n, then for every r, we have [0, br]rn. Since b n b, let n = tb + h, where 1 t < and 0 h b. We partition the n coordinates into three groups, each of which contains m, (t 1)b, and h coordinates (note that now b = m ), respectively 4. We represent each point in [] n by (x, y, z), where x [] m, y [] (t 1)b, and z [] h. Notations (x i, y, z), (x, y (i,j), z) and (x, y, z i ) are similarly defined as in the previous proof (note that for (x, y (i,j), z), the indexes i [t 1], j [b]). Similarly we define the characteristic function q : [] m [] m as follows: q(x) = i=(i 1,...,i m) Z m x i i. Let Q(w) = {(x, y, z) [] n q(x) = w} as usual, and the notation of i and, + are the same as in the proof of Theorem 3. We arbitrarily choose h different values w 1, w 2,..., w h from [] m, and let M = (Q(w 1 ) Q(w h )) {(x, y, z) [] n (x, y, z) = 0}. Now we describe the maring of the line (for some fixed point (x, y, z)): 1. For the line (x i, y, z), if there exists a point ( x, y, z) M on it, then we mar this point. Otherwise we mar the unique point ( x, y, z) with -modulo parity ( x, y, z) = For line (x, y (i,j), z) (i [t 1], j [b]), we mar the point (x, ỹ, z) on the line with parity (x, ỹ, z) = i On line (x, y, z i ) (i [h]), we mar the unique point (x, y, z) with parity (x, y, z) = 1. We claim that the construction above is a [0, b] n. We need to verify that each point (x, y, z) is mared either b = m times or unmared. There are three cases: 1. (x, y, z) = 0. 4 If t = 1 or h = 0, one of the group probably vanishes, while the proof still holds.

6 6 Tengyu Ma, Xiaoming Sun, and Huacheng Yu if (x, y, z) M. The point is only mared by lines of the form (x i, y, z). There are b such lines; otherwise, the point is never mared. 2. (x, y, z) = 1. q(x e i ) are pairwise different, thus on exactly (b h) lines (x i, y, z) there is no point in M. On these lines (x, y, z) is mared. All the lines of form (x, y i, z) will not mar the point (x, y, z), and all h lines of form (x, y, z i ) will mar this point. Thus (x, y, z) is mared b times in total (x, y, z) t. The point is mared on all the line of the form (x, y (i,j), z) where i = (x, y, z) 1 and j [b]. There are b such lines. 4. (x, y, z) > t. The point is not mared. Now we are ready to present our main theorem. Theorem 1 (Restated) For prime and 0 a < b n, there exists a maring of lines in [] n so that each point is mared either a times or b times if and only if there are nonnegative integers s and t so that s + t = n and as + bt = n n 1. Proof. The necessary part of the theorem is trivial and has been shown in the preliminary section of the introduction. The proof of sufficiency is an induction on n by essentially using Theorem 3 and 4 as base steps. The n = 1 case is obvious. Assume that for any n m 1, the theorem holds. Now we prove the theorem for n = m. There are three possible occasions: 1. If a = 0, then by Theorem 4 the theorem holds. 2. If b > m, by Theorem 3 the theorem holds. 3. If a > 0 and b m. Assume s = m 1 (b m), t = m 1 (m a) b a b a are both nonnegative integers. We claim that both s = m 1 [(b 1) (m )] b a and t = m 1 [(m ) (a 1)] b a are nonnegative integers. Suppose that b a = r d, where gcd(r, ) = 1. Thus we have that r m a, since r m 1 (m a) and gcd(r, m 1 ) = 1. Since d b a m 1, we have that d m 1 and d m 1. Then r d m 1 (m a), that is Similar argument shows that b a m 1 ((m ) (a 1)). b a m 1 ((b 1) (m )). Since a > 0 and b m, we still have 0 a 1 b 1 m. By invoing inductive hypothesis, we have that [a 1, b 1] m exists. Therefore, by applying Proposition 2 we have that [a, b] m exists.

7 On a Conjecture of Butler and Graham 7 3 [0, b] n for General In this section we prove the conjecture when a = 0 for general. Before proving the theorem, we provide a crucial building bloc of the proof, which can be viewed as a generalization of the characteristic function defined in Theorem 4. Proposition 4. If b and are two integers so that each prime factor of b is also a prime factor of, that is, and b can be decomposed as = p α1 1 pα l l and b = p β1 1 pβ l l, for integers α j > 0 and β j 0, (thus [] and [b] can be viewed as Z α1 Z α l and Z β1 Z β l, respectively), then there exists a linear characteristic function q : [] b [b] with the following property: There exists s [], so that for any x [] b, there is a unique index i [b] satisfying that q(x s e i ) = 0, where s e i is the vector in [] b with all entries 0 except that the i-th is s. Proof. Define the a linear operator : [] [b] [b], as a generalization of multiplication of scalar and vector, as follows: Suppose that u = (u 1,..., u l ) Z α1 Z α l ( = []), where each u j Z αj p j can be represented as u j = (u j α,..., j 1 uj 0 ). Similarly, suppose v = (v1,..., v l ) Z β1 Z β l ( = [b]). Define u v = (u 1 0 v 1, u 2 0 v 2,..., u l 0 v l ), where u j 0 vj is the multiplication of a scalar and a vector over Z pj. Based on this operator, define q : [] b [b] as follows: 5 q(x) = x i i. α i [b] =Z 1 Z α l Now we prove that q(x) indeed has the desired property. Let s = (1,..., 1) Z α1... Z α l ( = []) be the element with each entry 1 6, since is a linear operator with addition over Z α1... Z α l, we have that q(x s e i ) = q(x) q(s e i ) = q(x) s i. Since s is a vector with every entry 1, s i = i, hence the equation q(x s e i ) = 0 only holds for i = q(x). Now we prove Theorem 2 by using the above characteristic function q( ). Proof of Theorem 2. We only need to prove the sufficient part. Suppose that s = ( ) b n b n 1 is a nonnegative integer. We factor b = dr, where r is coprime with and each prime factor of d is a prime factor of. Thus we have r b n, and since d n, we have d n 1. By Proposition 3 it is sufficient to prove the r = 1 case. Suppose n = tb + h, where 1 t < and 0 h b. Similar to the approach of proving Theorem 4, we partition the n coordinates into three groups, each of which contains b, (t 1)b and h coordinates, respectively. We represent a point in [] n by (x, y, z), where x [] b, y [] (t 1)b, and z [] h. Let q(x) be the characteristic function which satisfies the condition in Proposition 4. Let Q(w) = {(x, y, z) [] n q(x) = w} as before. We arbitrarily choose h different values w 1,..., w h from [b] = Z β1 Z β l (since 0 h b), and let M = (Q(w 1 ) Q(w h )) {(x, y, z) (x, y, z) = 0}. 7 5 Here we view x i a vector in [] = Z α 1... Z α l as in the definition of. 6 Since s could be viewed both as an element in [] and a vector in Z α 1... Z α l, we use s and s correspondingly. 7 We redefine (a) as a 1 + a a n, the addition is over the group Z α 1 Z α l ( = []).

8 8 Tengyu Ma, Xiaoming Sun, and Huacheng Yu Now we describe the maring of lines. Considering the set [], 0 and s are two special elements in it. There are 2 t 1 elements left. Let τ be an arbitrary injective function that maps [t 1] to [] \ {0, s }. Thus τ([t 1]) = t For line (x i, y, z) (i [b]), if there exists a point ( x, y, z) M on it, then we mar this point. Otherwise we mar the unique point ( x, y, z) so that ( x, y, z) = s. 2. For line (x, y (i,j), z) (i [t 1], j [b]), we mar the point ( x, y, z) on the line so that ( x, y, z) = τ(i). 3. On line (x, y, z i ) (i [h]), we always mar the unique point ( x, y, z) with ( x, y, z) = s. Next we prove that each point (x, y, z) is mared either b times or a = 0 time. There are three cases: 1. (x, y, z) = 0. On lines of form (x, y (i,j), z) or (x, y, z i ), this point will never get mared. if (x, y, z) M. The point is mared by all lines of form (x i, y, z). There are exactly b such lines; otherwise, the point is never mared. 2. (x, y, z) = s. By Proposition 4, q(x s e i ) are pairwise different. On exactly h lines of form (x i, y, z), there is a point in M. Therefore on (b h) lines of such form, (x, y, z) will be mared. All the lines of form (x, y, z i ) will mar this point as well. On lines of form (x, y (i,j), z), this point will not be mared. Thus it is mared (b h) + h = b times in total. 3. (x, y, z) τ([t 1]). The point is mared on all the line of the form (x, y (i,j), z) where i = τ 1 ( (x, y, z)). Thus it is mared b times. 4. (x, y, z) [] \ (τ([t 1]) {0, s }). The point is never mared. 4 Conclusion and Remars In this wor we investigate a conjecture of Butler and Graham on maring lines of [] n. We proved the necessary and sufficient condition of the existence of [a, b] n for the case when is a prime and the case when a = 0 with general. A natural open question is how to settle the remaining case of the conjecture, [a, n t] n (t < ) for general. The proof of Theorem 3 actually can be generalized to the following case when is a prime power (with an additional constrain that (n t a) contains more prime factors than ). Theorem 5. [a, n t] n exists if = pm, n = t + a + rp s, n a = ru and s m. The proof utilized the property of field F p s. It is essentially similar to the proof of Theorem 2 and we will put it in appendix. It is an interesting question to now whether the method here can be further generalized. The difficulty is that during this generalization, the strong symmetric property cannot be maintained. References 1. Joe Buhler, Steve Butler, Ron Graham, and Eric Tressler. Hypercube orientations with only two indegrees. Journal of Combinatorial Theory, Series A, 118: , Steve Butler and Ron Graham. A note on maring lines in [] n. Designs, Codes and Cryptography, 2011.

9 On a Conjecture of Butler and Graham 9 3. Steve Butler, Mohammad T. Hajiaghayi, Robert D. Kleinberg, and Tom Leighton. Hat guessing games. SIAM J. Discrete Math., 22(2): , Todd T. Ebert. Applications of recursive operators to randomness and complexity. PhD thesis, University of California at Santa Barbara, Uriel Feige. On optimal strategies for a hat game on graphs. SIAM Journal of Discrete Mathematics, 24(3): , H. Iwasawa. Presentation given at the ninth gathering 4 gardner (g4g9). March Hendri W. Lenstra and Gadiel Seroussi. On hats and other covers. In Proceedings of IEEE International Symposium on Information Theory, page 342, Tengyu Ma, Xiaoming Sun, and Huacheng Yu. A new variation of hat guessing games. In Proceedings of 17th Annual International Computing and Combinatorics Conference, pages , Maura B. Peterson and Douglas R. Stinson. Yet another hat game. The Electronic Journal of Combinatorics, 17(1):#R86, Appendix Proof (Theorem 5(setch)). We construct a maring of lines in [] n so that each point is mared either (n t) times or a times. First, similar to in the proof of Theorem 3, we partition the n = a + t + p s r dimensions into three groups, each of which contains p s r, t, and a coordinates respectively, and represent each point in [] n by (x, y, z) where x [] psr, y [] t, and z [] a. Then, we index x by a pair (i, j) F s p [r], i.e. x i,j [] are the coordinates of x (where i F s p, j [r]) 8. We define lines (x (i,j), y, z), (x, y i, z) and (x, y, z i ) in a similar way. For each x [] psr, we define the characteristic function q : [] psr F p s in a slightly different way: q(x) = r q i (2) i F p s q is an arbitrary function that maps F p m to F p s. q ( r j=1 x i,j) is an element in F p s. The way of maring lines and the proof of correctness is the same as that in the proof of Theorem 3. j=1 x i,j 8 Here we use a different arithmetic system for i.