Maximum oriented forcing number for complete graphs

The maximum oriented $k$-forcing number of a simple graph $G$, written $\MOF_k(G)$, is the maximum directed $k$-forcing number among all orientations of $G$. This invariant was recently introduced by Caro, Davila and Pepper in [CaroDavilaPepper], and in the current paper we study the special case where $G$ is the complete graph with order $n$, denoted $K_n$. While $\MOF_k(G)$ is an invariant for the underlying simple graph $G$, $\MOF_k(K_n)$ can also be interpreted as an interesting property for tournaments. Our main results further focus on the case when $k=1$. These include a lower bound on $\MOF(K_n)$ of roughly $\frac{3}{4}n$, and for $n\ge 2$, a lower bound of $n - \frac{2n}{\log_2(n)}$. Along the way, we also consider various lower bounds on the maximum oriented $k$-forcing number for the closely related complete $q$-partite graphs.


Introduction
In this paper we discuss the maximum k-forcing number over all orientations of a complete graph, which is an interesting case of a more general concept recently introduced by Caro, Davila and Pepper [6]. These concepts generalize both the directed zero forcing number, first introduced in [10] and studied in [3], while also expanding recent work on the k-forcing number introduced in [2] and studied further in [5]. The idea of zero forcing (for simple graphs) was introduced independently in [1] and [4]. In [1], zero forcing was used to bound from below the minimum rank of a graph, or equivalently, to bound from above the maximum nullity of a graph. In [4], it is indirectly introduced in relation to a study of control of quantum systems. Additionally, the problem of zero forcing number is closely related to the Power Dominating Set problem, which is motivated by monitoring electric power networks using Kirchoff's Law [16]. Many other papers have been written about zero forcing in recent years (for example [7,11,15]). While most of the first papers written were from a linear algebra point of view ( [3,9,13]), a fruitful change to a graph theoretic approach, and connection to basic graph parameters such as degree and connectivity, as well as the more general notion of k-forcing, was introduced and developed in [2], [5] and [6]. The main point of this paper is to focus the attention on complete graphs and complete q-partite graphs, where we already get some interesting results.
Let G be a finite and simple undirected graph with vertex set V = V (G) and edge set E = E(G). We say that G is oriented by assigning to each edge {u, v} ∈ E exactly one of the ordered pairs (u, v) and (v, u) -which we call arcs. We call the resulting digraph D an orientation of G, and say that D is an oriented graph with underlying graph G. Let D be an oriented graph with underlying simple graph G. If (u, v) is an arc of D, then we say that u is directed towards v, that v is an out-neighbor of u, and that u is an in-neighbor of v. Following standard notation: we use n = n(G), δ = δ(G) and ∆ = ∆(G) to denote the order of G, the minimum degree of G and the maximum degree of G respectively. A graph with n = 1 is called a trivial graph.
, and is the number of out-neighbors of v (resp. inneighbors of v). The minimum out-degree (resp. in-degree) is denoted δ + = δ + (D) (resp. δ − = δ − (D)), and the maximum out-degree (resp. in-degree) is denoted ∆ + = ∆ + (D) (resp. ∆ − = ∆ − (G)). If every vertex has the same out-degree (resp. in-degree), then D is said to be out-regular (resp. in-regular ). A directed path in D is a sequence of vertices u 1 , u 2 , . . . , u p of D such that (u i , u i+1 ) is an arc of D, 1 ≤ i ≤ p − 1. For terms not defined here, the reader is referred to [14]. Now we will describe the k-forcing process for oriented graphs. Suppose that D is an orientation of G, and S is some subset of colored vertices in D, all vertices not in S being non-colored. For each positive integer k, we define the k-color change rule as follows: any colored vertex that is directed towards at most k non-colored vertices (has at most k non-colored out-neighbors) forces each of these non-colored vertices to become colored. A colored vertex that forces a non-colored vertex to become colored is said to k-force that vertex to change color. By the oriented k-forcing process starting from S ⊆ V , we mean the process of first coloring the vertices of S, and then iteratively applying the k-color change rule as many times as possible. During each step (or iteration) of the oriented k-forcing process, all vertices that k-force do so simultaneously. If, after termination of the oriented k-forcing process, every vertex of D is colored, we say that S is an oriented k-forcing set (or simply a k-forcing set) for D. The cardinality of a smallest oriented k-forcing set for D is called the oriented k-forcing number of D and is denoted F k (D). When k = 1, we will drop the subscript from our notation and write F (D) instead of F 1 (D), and this case corresponds to the directed zero forcing number. The maximum oriented k-forcing number, over all orientations of G, is denoted MOF k (G). The minimum oriented k-forcing number, over all orientations of G, which is denoted mof k (G) was also introduced and studied in [6]. These graph invariants turn out to be related to some other well studied graph parameters. For instance, in [6] it is shown that MOF k (G) ≥ α(G) and, when k = 1, mof 1 (G) = mof(G) = ρ(G), where α(G) is the independence number and ρ(G) is the path covering number.
The remainder of the paper is organized as follows. In Section 2, we present our results about MOF k (K n ). In Section 3, we consider MOF k (G) when G is a complete q-partite graph. In Section 4, we offer some concluding remarks and acknowledgments.
As a notational convenience, we will use log(n) (in place of log 2 (n)) to denote the base 2 logarithm of n, and k will always denote a positive integer.

Main results
In this section, we study the maximum oriented k-forcing number for complete graphs.
In what follows, we will need to recall that a transitive orientation of D, with vertices labeled {v 1 , v 2 , . . . , v n }, is an orientation which satisfies: v i is directed towards v j if and only if i < j. Also, a balanced orientation of D is an orientation satisfying the inequality Note that while a complete graph has only one transitive orientation, up to isomorphism, it can have many different non-isomorphic balanced orientations. It turns out that while the forcing number for the transitive orientation is about half the order (seen below), certain kinds of balanced orientations (or nearly balanced) can have forcing numbers that are quite high.
Proof. Let D be the transitive orientation of G, suppose G has n = q(k + 1)+ r vertices where 0 ≤ r < k + 1, n ≥ 2 and k < n. Label the vertices so that v i has in-degree n − i and out-degree i − 1. So, for illustration, v 1 has in-degree n − 1 and out-degree 0, v 2 has in-degree n − 2 and out-degree 1 and v n has in-degree 0and out-degree n − 1. First Due to the transitive orientation, no vertex in S with higher label can k-force before vertices in S with lower labels. Since v k+1 is the lowest labeled vertex in S, we start by coloring that vertex. The vertex v k+1 has exactly (k + 1) − 1 = k out-neighbors and can k-force all of them to change color on the first step of the k-forcing process. Once the k out-neighbors of v k+1 are colored, the vertex v 2(k+1) with out-degree i − 1 = 2(k + 1) − 1 = 2k + 1 can color its k non-colored out-neighbors (k = 1 of its out-neighbors are already colored). This process continues, with v j(k+1) coloring its remaining non-colored out-neighbors only after v (j−1)(k+1) colors its non-colored out-neighbors until j = q. At the last step, if r = 0 everything is colored, and if r > 0, the vertex v n will color the remaining non-colored vertices since there will be at most (n − 1) − q(k + 1) = r − 1 < k + 1 − 1 = k of them. This shows that S is a k-forcing set with q = ⌈ n k+1 ⌉ vertices if r = 0 and Proceeding by contradiction, assume F k (D) < ⌈ n k+1 ⌉ and let S be a smallest k-forcing set with |S| < ⌈ n k+1 ⌉. After coloring each vertex of S, and observing that each vertex could k-force at most k others, the total number of vertices that end up colored is |S| + k|S| = |S|(k + 1). Since S was an oriented k-forcing set, every vertex must have been colored so |S|(k + 1) ≥ n = q(k + 1) + r. Thus, |S| ≥ q + r k+1 . However, since |S| < ⌈ n k+1 ⌉ = q +⌈ r k+1 ⌉, and since |S| is an integer, we reach a contradiction and proves the theorem. ✷ The result above leads to the following corollary, which partially supports a conjecture in [6], namely that MOF k (G) ≥ ⌈ n k+1 ⌉, and in particular, MOF(G) ≥ ⌈ n 2 ⌉.
Corollary 2 For all positive integers n, Proof. This follows because MOF k (K n ) is at least as much as the oriented k-forcing number of the transitive orientation which is ⌈ n k+1 ⌉, as seen above. ✷ We next recall two results from [6]. Applying these results to complete graphs, we get the following corollary.
Proof. The lower bound comes from Proposition 4. For the upper bound, let H be an induced K n inside of a K n+1 . From Theorem 3, 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Table 1: Values of M OF (K n ) found from a computer program employing various theorems from [6], as well as Corollary 5, and with many processors running in parallel.
Since MOF(K n ) can grow by at most one as n grows by one, Corollary 5 considerably speeds up any attempt to find exactly the values of MOF(K n ). Further speed ups to any computation of MOF(K n ) come from other theorems in [6], in particular the Reversal Theorem. Namely, that the forcing number of an orientation of a graph is equal to the forcing number of its reversal. These ideas, and others, were used to write a computer program to find the exact value of MOF(K n ) for n < 20. The results of this can be seen in Table 1. We are now ready to present our main results, which are lower bounds for MOF(K n ).
Theorem 6 If G is a graph with order n, then Proof. As can be seen from inspection of Table 1 (below), this theorem is true for all values of n ≤ 10 (achieving equality when n = 6 and n = 10 if we consider ⌈ 3n−9 4 ⌉ since MOF(K n ) is an integer). Let us then assume, without loss of generality, that n ≥ 10.
Let q be the largest odd integer such that n = 2q + r where 0 ≤ r < q. It can be readily seen then that r ∈ {0, 1, 2, 3}, since if r ≥ 4, q was not the largest odd integer satisfying the equation. Hence, it is implied that 0 ≤ r ≤ 3.
We consider the following orientation D of K n . Partition the vertices into q − r sets of order 2 and r sets of order 3. Label the q − r sets of order 2 as {A 1 , A 2 , . . . , A q−r } and label the r sets of order 3 as {A q−r+1 , A q−r+2 , . . . , A q }. Now split these sets into two nearly equal halves, with one having q−1 2 parts and the other having q+1 2 parts. Let the lowered labeled sets be in the smaller of these groups, so that each of the sets {A 1 , A 2 , . . . , A q−1 2 } has order 2. Since n ≥ 10, we know that q ≥ 5, and this implies that all r extra vertices are in the higher labeled group. To ease the notation, let X denote the lowest q−1 2 labeled sets and let Y denote the highest q+1 2 labeled sets. Now, give each of these sets A i the transitive orientation with respect to the other vertices in that set. Consider the sets themselves as vertices in larger odd order graph, and give that graph a balanced orientation in the following way. Each vertex in A i is joined to each vertex in each of the next q−1 2 highest labeled sets (wrapping around again when we get past A q ). So for example, if q = 7, then each vertex of A 2 is joined to each vertex of A 3 , A 4 , and A 5 . Now we are ready to consider how many vertices need to be colored to have a chance at forcing the whole graph. In order for any vertex in Y to force any other vertex to change color, the initial set of colored vertices must be at least as large as |X| − 1. Considering the set Y as a separate oriented complete graph, we discover that it has the transitive orientation. Thus, according to Theorem 1, in order for that set to be colored, once X is colored, we need at least ⌈ |Y | 2 ⌉ many vertices to be initially colored. Taken together, Finally, since r ≤ 3, the result follows and the theorem is proven. ✷ The graph K 9 x 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 The graph K 11 x 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x 10 x 11 Figure 1: Orientations of K 9 and K 11 , with minimum forcing sets highlighted.
Proof. Assume n = 3p + r ≥ 9 with r ∈ {0, 1, 2}. Partition K n into three parts whose orders are as close to equal as possible. Label these three parts as V 1 , V 2 and V 3 . Let D denote the orientation of K p which realizes MOF(K p ), and let D i denote the orientation of V i which realizes M OF (K |V i | ) for i ∈ {1, 2, 3}. Now, orient all other edges of the graph as follows. Each vertex in V 1 is directed toward each vertex of V 2 , each vertex of V 2 is directed towards each vertex of V 3 , and each vertex of V 3 is directed towards each vertex of V 1 . Call the completed orientation of K n thus created D * , and let S be a minimum oriented forcing set of D * . Finally, for each i ∈ {1, 2, 3}, let S i = S ∩ V i .
Not all vertices are originally colored, so there must be a vertex which forces at the first step of the forcing process. Let v be such a vertex and assume that v ∈ V 1 . There are two main cases to consider, that v forces a vertex in V 2 or that v forces a vertex in V 1 .
First, suppose v and forces a vertex in V 2 . This is only possible if all but one of the vertices of V 2 are already colored, implying |S 2 | = |V 2 | − 1. Next, in order for V 1 itself to be fully colored, either S 1 itself is an oriented forcing set of V 1 , which implies |S 1 | ≥ F (D 1 ), or the last non-colored vertex from V 1 is forced by a vertex in V 3 . This later situation is only possible if all but one vertex in V 1 is already colored, which implies S 1 was able to color all but one vertex of V 1 so that |S 1 | ≥ F (D 1 ) − 1. Finally, in order now for V 3 to be fully colored, either S 3 itself was a forcing set of V 3 , which implies |S 3 | ≥ F (D 3 ), or the last non-colored vertex of V 3 is forced by a vertex in V 2 . This later situation is only possible if all but one vertex of V 3 is already colored, which implies S 3 was able to color all but one vertex in V 3 so that |S 3 | ≥ F (D 3 ) − 1. Hence, summing the parts, we get: Second, suppose v and forces a vertex in V 1 . This is clearly only possible if all of V 2 is already colored, which implies |S 2 | = |V 2 |. Now, the argument repeats as in the preceding paragraph. In order for V 1 to be fully colored, |S 1 | ≥ F (D 1 ) − 1 and in order for V 3 to be fully colored, |S 3 | ≥ F (D 3 ) − 1. Hence, summing the parts, we get: Thus, in either case we have, (1) If we assume v ∈ V 2 instead of V 1 , the argument above could be repeated and we would arrive at the inequality, If we assume v ∈ V 3 instead of V 1 , the argument above could be repeated and we would arrive at the inequality, To conclude, we make use of the facts that for each i ∈ {1, 2, 3}, |V i | ≥ p and F (D i ) ≥ F (D) = MOF(K p ), to bound from below each of Inequalities 1, 2 and 3 by p + 2M OF (K p ) − 3. Therefore, MOF(K n ) ≥ F (D * ) ≥ p + 2MOF(K p ) − 3 as claimed. ✷ Theorem 6 and Lemma 7 can be used together to help us prove the following main result, namely that MOF(K n ) is asymptotically equal to n.
Theorem 8 For all positive integers n ≥ 2, Proof. Proceeding by mathematical induction, notice that for all values of n in the range, 2 ≤ n ≤ 202, the results follows from Theorem 6 since, when 2 ≤ n ≤ 202 as seen from calculation and inspection. This settles our base case. Assume now that the theorem is true for all complete graphs with smaller orders than K n , with n ≥ 203. We will show this implies it is also true for K n . Let n = 3p + r, with r ∈ {1, 2, 3}. Now, from Lemma 7 we know that, Since p < n, from our inductive assumption we know that MOF(K p ) ≥ p − 2p log(p) . Together, this implies, It remains to show that 3p − 4p log(p) − 3 ≥ n − 2n log(n) . Replacing p by n−r 3 , and rearranging the terms, this is equivalent to showing that, Finally, since this last inequality is true for all n ≥ 117, since the function, is never negative for r and n in the ranges given, as can be seen using standard techniques from calculus and algebra, and we already assumed n ≥ 202. The general result now follows by induction and the theorem is proven. ✷ From this we easily deduce that MOF(Kn) n → 1 as n → ∞. Furthermore, when combined with Proposition 4, we get the following corollary. Recall that the clique number of G, written ω(G), is the cardinality of a largest induced complete graph in G.

Corollary 9
If G is a graph with order n, then Proof. Let H be a largest complete subgraph of G. From Proposition 4, MOF(G) ≥ MOF(H). Now, since H is a complete graph of order ω(G), the inequality follows from Theorem 8. ✷ To conclude this section, we recall one more result from [6].

Corollary 10 [6]
If G is a graph with order n, then Thus, taken together with the observation that ω(K n ) = n, we find the following.

Corollary 11
For all positive integers n, 3 The maximum oriented k-forcing number for complete q-partite graphs In this section, we extend our investigation to from complete graphs to complete qpartite graphs. Recall that a graph G is q-partite if its vertex set can be partitioned into q ≥ 2 independent sets. The independent sets are called parts and, if G is qpartite, every edge in G has its two incident vertices in different parts. We say that G is a complete q-partite graph if G is q-partite with every possible edge between vertices in different parts.
Theorem 12 If G is a complete q-partite graph and n 1 ≥ n 2 ≥ . . . ≥ n q denote the orders of the partite sets, then Proof. Label the partite sets as A 1 , A 2 , . . . , A q , labeled so that larger parts have smaller labels. That is, if i < j, then |A i | ≥ |A j |. Moreover, let n i = |A i |. Create the orientation D of the edges of G by directing vertices from parts with smaller labels towards parts with larger labels. That is, if u ∈ A i and v ∈ A j , with i < j, then (u, v) is an arc in D (this is called the transitive orientation). Now, in D, all vertices from A 1 must be in any oriented k-forcing set. Moreover, with i < j, vertices from A i could only k-force vertices from A j if max {n i − k, 0} vertices from A j were already colored. Since this is true for all pairs i and j, we get the following lower bound on the oriented k-forcing number of D, Finally, since MOF k (G) ≥ F k (D), the proof is complete. ✷

Corollary 13
If G is a complete q-partite graph, with q ≥ 2, and k is a positive integer, then Proof. Let G be a complete q-partite graph with parts A 1 , A 2 , . . . , A q . Set |A i | = n i and without loss of generality, assume n 1 ≥ n 2 ≥ . . . ≥ n q . From Theorem 12 above, together with the fact that max {n i − k, 0} ≥ n i − k, we get the following chain of inequalities; which completes the proof. ✷ Specifying that k = 1, we arrive at the result below.

Corollary 14
If G is a complete q-partite graph, with q ≥ 2, then This inequality is sharp when q = 2 or when q = 3 and each part has at least two vertices.
Proof. The inequality comes from substituting k = 1 into the above corollary. To see that equality holds when q = 2, the complete bipartite case, we first note that MOF(G) ≤ n − 1 for any non-empty graph G. This is because there is always a vertex v with in-degree at least one in such cases, and the set of all vertices other than v is a forcing set. To see that MOF(G) ≥ n − 1, let A and B be the two parts and direct all edges from A to B. Now each vertex of A is necessarily in any forcing set and nothing can be forced unless at least |B| − 1 vertices from B are included. Therefore, MOF(G) = n − 1 = n − q + 1, as claimed.
Next we show that the inequality is sharp when q = 3 and each part has at least two vertices. Let A, B, and C be the three parts with cardinalities a ≥ b ≥ c ≥ 2 respectively. It is sufficient to show that MOF(G) ≤ n − q + 1 = n − 2 = a + b + c − 2, since the same lower bound is already established (using transitive orientation). To this end, let D be any orientation of G. We first show that there must be two vertices, u and v, in two different parts, with in-degree at least one. To see this, suppose there was a vertex w such that d + (w) = d(w). In this case, each vertex in the two parts not containing w have in-degree at least one and we are done. On the other hand, if no such w exists, then all vertices have d + (w) < d(w) and consequently have in-degree at least one, and we are done. So, let u and v be vertices in different parts such that d − (u) ≥ 1 and d − (v) ≥ 1. Without loss of generality, assume u ∈ B and v ∈ C. Let u * ∈ B and v * ∈ C be vertices from B and C respectively, different from u and v (since each part has at least two vertices). Suppose, without loss of generality, (v, u) is an arc. This implies there is a vertex w such that (w, v) is an arc since d − (v) ≥ 1. If both (v, u * ) and (u, v * ) are arcs, the set V \ {v * , u * } is a forcing set of order n − 2 since on the first step of the forcing process, u forces v * and v forces u * . Otherwise, either (v * , u) or (u * , v) is an arc. Suppose (v * , u) is an arc, and consider the set V \ {v, u}. This is a forcing set of order n − 2, since v * forces u and then w forces v. Suppose (u * , v) is an arc, and consider the set V \ {v, u}. This is a forcing set of order n − 2, since u * forces v and then v forces u. Therefore, F (D) ≤ n − 2 for any orientation D. Consequently, MOF(G) ≤ n − q + 1 = n − 2 = a + b + c − 2, completing the proof. ✷ For q ≥ 9, and each of the q parts has at least three vertices, we have examples where MOF(G) > n − q + 1, so the inequality is not sharp for large values of q. We do not know the situation for 4 ≤ q ≤ 8.
If G is a complete q-partite graph, then G contains a subgraph isomorphic to K q . This observation together with Theorem 3 shows that the maximum oriented forcing number of a q-partite graph can be bounded from above in terms of MOF(K q ). In particular, together with Corollary 14, we have the following.

Corollary 15
If G is a complete q-partite graph with order n, and q ≥ 2, then n − q + 1 ≤ MOF(G) ≤ n − q + MOF(K q ).

Concluding remarks and acknowledgments
In this paper we have given a detailed study of the maximum k-forcing number over all orientations of complete graphs and complete q-partite graphs. However, our focus was primarily on the interesting case when k = 1. We highlight that in Corollary 11, we have the lower bound MOF(K n ) ≥ n − 2n log (2) , and the upper bound MOF(K n ) ≤ n − log(n) 2 . It remains to be seen which of these bounds is closer to the truth, and we pose this formally with the following problem.
Problem 1 Which of the bounds presented in Corollary 11 is a better approximation to MOF(K n )?
Finally, we would like to thank several individuals -and former students of the second author -for their help in working on this paper: David Amos for many fruitful conversations about early results; Randy Davila for his great help in preparing and reviewing the manuscript; and Mobeen Azhar for his work in writing and developing the computer program which generated the results from Table 1. Their contributions and enthusiasm helped provide the necessary motivation to make this paper possible.