# The positive minimum degree game on sparse graphs

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1 The positive minimum degree game on sparse graphs József Balogh Department of Mathematical Sciences University of Illinois, USA András Pluhár Department of Computer Science University of Szeged, Hungary Submitted: Aug 17, 2011; Accepted: Jan 12, 2011; Published: Jan 21, 2012 Mathematics Subject Classifications: 05C65, 91A43, 91A46 Abstract In this note we investigate a special form of degree games defined by D. Hefetz, M. Krivelevich, M. Stojaković and T. Szabó [4]. Usually the board of a graph game is the edge set of K n, the complete graph on n vertices. Maker and Breaker alternately claim an edge, and Maker wins if his edges form a subgraph with prescribed properties; here a certain minimum degree. In the special form the board is no longer the whole edge set of K n, Maker first selects as few edges of K n as possible in order to win, and our goal is to compute the necessary size of that board. Solving a question of [4], we show, using the discharging method, that the sharp bound is around 10n/7 for the positive minimum degree game. 1 Introduction We briefly recall the necessary definitions about positional games, for a deeper introduction see J. Beck [3] or specifically D. Hefetz, M. Krivelevich, M. Stojaković and T. Szabó [4]. The game H = (V (H), E(H)) is played by two players, called Maker and Breaker, on the hypergraph H = (V (H), E(H)). The players take turns in claiming one previously unclaimed vertex of V (H); here we assume Breaker starts the game and the game ends if all vertices are taken. Maker wins by taking every element of some A E(H), while Breaker wins if he can take at least one vertex of every edge in E(H). Clearly, exactly one of the players wins this game. We consider graph games; in those V (H) E(K n ), and E(H) 2 E(Kn). The set E(H) is a global property if for all A E(H) the subgraph spanned by A has Ω(n) vertices. A The research is partially supported by NSF CAREER Grant DMS , UIUC Campus Research Board Grant 11067, OTKA Grant K76099 and TÁMOP-4.2.1/B-09/1/KONV The second author s research is partially supported by OTKA grant K76099 and TÁMOP-4.2.1/B- 09/1/KONV the electronic journal of combinatorics 19 (2012), #P22 1

2 well-known example for this is the Shannon s switching game, in which E(H) consists of the edge sets of the spanning trees in V (H), [5]. Here Maker wins if and only if V (H) hosts two edge-disjoint spanning trees. That is 2n 2 appropriately selected edges results in a Maker s win, while Breaker wins if V (H) has fewer than 2n 2 edges. In general, for a property P = E(H), we look for ˆm(P), the smallest size of V (H) that is sufficient to guarantee Maker s win. In this note we investigate a global property, achieving in a graph a given minimum degree. A possible way to define degree games is to say D k := D k (n) consist of the subgraphs of K n having minimum degree k. For other degree games see [1, 2, 3, 7]. D. Hefetz, M. Krivelevich, M. Stojaković and T. Szabó [4] showed that 11n/8 ˆm(D 1 ) 10n/7 + 4, and ˆm(D 1 ) 10n/7 if n = 7l. Our main result is the exact determination of ˆm(D 1 ). Theorem 1 For n n = ˆm(D 1 ), for n 2 (mod 7), and 10 7 n + 1 = ˆm(D 1 ), for n 2 (mod 7). It means that there exists graphs with 10 n (or 10 n + 1, if n = 7l + 2) edges on 7 7 n vertices on which Maker wins, and on graphs with fewer edges always Breaker wins. Similar, but much weaker results might be spelled out for ˆm(D 2 ) and ˆm(D k ). Proposition 2 For all k N, ˆm(D 2 ) 20n 7 for n = 14k, and ˆm(D k ) 10 7 kn for n = 7k. L. Székely [7] and J. Beck [2] studied degree games on the edges of K n,n, but their methods also apply to K n. Briefly Maker wins if k < n/2 n log n, and Breaker wins if k > n/2 n/12. To put it differently, if k is about n/2 n log n, then ˆD k = (k+f(k))n, where f(k) = O( k log k). It would be interesting to know if the function f behaves similarly for smaller values, too. A quick calculation shows that adding copies of K l,l and using a weight function strategy on each K l,l gives a better bound on ˆm(D k ) than 10kn/7, provided k > Proofs Proof of Theorem 1. Observe that for n 3 Breaker always wins, so we may assume that n 4. First we construct graphs on which Maker can win, establishing the upper bounds. It is easy to check that Maker wins on K 4, K 3,3, and D 7, where D 7 is a double diamond, that is we take two copies of K 4 \ v 1 v 4, and identify the two vertices called v 4. (Note that this was observed already in [4].) the electronic journal of combinatorics 19 (2012), #P22 2

3 v 3 v 3 v 4 v 1 v 1 v 4 v 2 v 2 The graph D 7. Here v 4 = v 4. For n = 7k, Maker wins on k vertex disjoint copies of D 7. For n = 7k + 4, Maker wins on k vertex disjoint copies of D 7 and one copy of K 4. Similarly, for n = 7k + 6, Maker wins on k vertex disjoint copies of D 7 and one copy of K 3,3. For the rest of the cases, the following observation is sufficient: Assume that Maker wins on G. Form G by adding a vertex to G, with edges to arbitrarily chosen two other vertices. Then by playing the winning strategy on the edges of G and a simple pairing on the two newly selected edges, Maker wins on G as well. For n = 7k + 1 Maker wins on (k D 7 ), for n = 7k + 2 on ((k D 7 ) ), for n = 7k + 3 on (k 1) D 7 + K 4 + K 3,3, and for n = 7k + 5 on (k D 7 + K 4 ). Now we prove the lower bounds. Let G be such a graph on n vertices with as few edges as possible, that Maker, as a second player, can achieve degree at least one at every vertex. We first observe some properties of G. A path xz 1... z m y is a (k, l)-(x, y)-path, if d(x) = k, d(z 1 ) =... = d(z m ) = 3 and d(y) = l. Note that m can be 0, that is the path consisting of the edge xy. Lemma 3 The graph G has the following properties. (i) For every x V (G) we have d(x) 2. (ii) There is no (2, 2)-(x, y)-path in G. (iii) For any k 3, there are no vertices x, y 1,..., y k 1 with a (k, 2)-(x, y i )-path for every i. (iv) There is a vertex x V (G) with d(x) = 2. (v) In a component of G if there is a vertex x V (G) with d(x) = 2 then either there is a vertex y V (G) with d(y) 4 or the component consists of at least 7 vertices. Proof of Lemma 3. If (i) was false then Breaker would trivially win instantly. To prove (ii), assume for a contradiction that G contains a (2, 2)-(x, y)-path xz 1... z m y, where m is chosen to be minimum possible. The minimality of m implies that this path is an induced path. Now Breaker easily wins, claiming edges xz 1, z 1 z 2,..., z m y. In order to avoid instant loss, Maker has to claim the last unclaimed edge at x, z 1,..., z m, and finally Breaker could claim the last unclaimed edge at y. For (iii), assume that for some k 3 such a path system exists. We might choose one, with minimum number of vertices. Then using (ii) and the minimality, any edge spanned by the vertex set of this path system is also an edge of some of those paths. Now as in the electronic journal of combinatorics 19 (2012), #P22 3

4 the proof of part (ii), Breaker can claim all path edges, starting from the vertices y i, and then could claim the last edge at x. Part (iv) follows from that if G was a counterexample for Theorem 1, then its density was smaller than 3/2. For (v) assume that G is a counterexample. Then by (ii), each component of G contains at most one vertex of degree 2. Each such component should have odd number of vertices. There is one possible graph with 5 vertices, and it is easy to see that Breaker wins on it. We now apply the discharging method, see e. g. [6]: In the charging phase a vertex v V (G) is assigned a weight or charge w(v) := d(v). In the discharging phase the vertices send some of their charges to other, not necessarily neighboring vertices. The rules of the discharging are as follows: 1. A vertex of degree 2 sends no charge. 2. Only vertices of degree 2 receive any charge. 3. A 3-degree vertex x sends charge 1/7 to a 2-degree vertex y if there is a (3, 2)-(x, y)- path. 4. If for a k > 3 there is a (k, 2)-(x, y)-path, then x sends a charge of 4/7 to y. In the beginning the sum of the charges is the sum of the degrees. The sum of the charges does not change during the discharging phase, so the following claim completes the proof for the case when n 7l + 2. Claim 1 After the discharging phase every vertex has charge at least 20/7. Proof of Claim 1. Observe that charges are staying within components. If every vertex of a component is of degree at least 3, then the charges do not change, and the claim is trivially true. If a component has a degree-2 vertex but has no vertex of degree at least 4, then by Lemma 3 (v) it consists of at least 7 vertices. By Lemma 3 (ii) such a component contains exactly one vertex of degree 2. Each vertex of degree 3 sends a charge of 1/7 to the 2-degree vertex, which will have charge at least /7 = 20/7. The 3-degree vertices will have charge 3 1/7 = 20/7. Now consider a component, which contains vertices both of degree 2 and at least 4. By Lemma 3 (iii), a vertex x with d(x) = 3 sends to at most one 2-vertex a charge of 1/7, so it will have charge at least 3 1/7 = 20/7. For any k > 3, by Lemma 3 (iii), a vertex x with d(x) = k will have charge at least k 4(k 2)/7 = 3k/7 + 8/7 20/7. (1) Now assume that d(x) = 2. For some k > 3 there is a (2, k)-(x, y) path for some k-vertex y. So x will receive from y a charge of 4/7. If xy E(G) then x has two neighbors of degree 3, and receives charges of 1/7 from both. So the charge of x will be at least the electronic journal of combinatorics 19 (2012), #P22 4

5 2 + 4/7 + 1/7 + 1/7 = 20/7. Otherwise, let z be the other neighbor of x. Observe, that d(z) 3, because of Lemma 3 (ii), and z should send a charge of at least 1/7 to x. Observe also that the vertex z must have a neighbor w that differs both from x and y, and, by Lemma 3 (ii) the degree of w is at least 3. But then w sends a charge at least 1/7 to x, which is sufficient to achieve charge of at least 20/7 at x. The proof of Theorem 1 is completed when n 2 (mod 7). Assume that n = 7k + 2 for some positive integer k. Then the sum of the charges in G is at least (7k + 2) 20/7 = 20k + 40/7, yielding that e(g) 10k + 3. If we were able to find extra 3/7 charges, then this would be sufficient to imply e(g) 10k + 4, and then the proof of Theorem 1 would have been completed. Trivially, each component contains at least four vertices. If there is a component, with each of its vertices having degree at least 3, then each will have extra charge 1/7 and the proof is completed. Now we can assume that each component contains a degree-2 vertex. If there is a vertex of degree at least 5, then (1) implies that this vertex will have at least extra 3k/7 + 8/8 20/7 + 3/7 charge. So now we can assume that the maximum degree is at most 4. If there is a component which contains only vertices of degree 2 and 3, then Lemma 3 (iii) implies that it contains only one degree-2 vertex, and by parity reasons 2s degree-3 vertices for some s. One can check (we omit the details) that Breaker wins if s 4. On the other hand, if s 5 then we have extra charge at least 2s/7 6/7 4/7. So now we can assume that each component contains vertices of both degree 2 and 4. Since n 2 (mod 7), G must have a component C, such that C 0 (mod 7). Assume now that in such a component C there are l 1 vertices of degree 4. Then there should be exactly 2l vertices of degree 2. It cannot be fewer, otherwise either a degree-4 vertex retains charge 4/7 or at least two degree-2 vertices overcharged by 2/7, or a degree-2 vertex is overcharged by 6/7 and we are done. It cannot be more, as now every degree-2 vertex receives charge from a degree-4 vertex, and a degree-4 vertex can send charges to at most two degree-2 vertices. The proof of Claim 1 implies that each degree-2 vertex receives charges from one degree-4 vertex and at least two degree-3 vertices. So the number of degree-3 vertices is at least 4l. Because C 0 (mod 7), it is more than 4l, but it must be less than 4l + 4, otherwise there would be 4/7 extra charge, So the number of degree-3 vertices is 4l + 2. Denote {x 1,..., x l } the set of degree-4 vertices in C. Observe that C {x 1,..., x l } has at least 2l components, and 2l of them contain a degree-2 vertex and at least two degree-3 vertices. Assume that 2l i components contain exactly two degree-3 vertices among those, where i 2. There are at least 4l i edges between these 2l components and the vertex set {x 1,..., x l }, and because C is connected, the number of additional edge endpoints of vertices in {x 1,..., x l } is at least l, yielding 4l i + l 4l, i.e. l 2. Note that when there is a cut-edge, then Breaker in his first step can claim it, and can easily win in the component in which Maker does not occupy an edge. One can check that when l = 2 then there is always a cut-edge. When l = 1, then C has nine vertices, with degree sequence 2, 2, 3, 3, 3, 3, 3, 3, 4, and the degree-4 vertex is a cut-vertex. It can be checked that Breaker has a winning strategy, we omit the details. the electronic journal of combinatorics 19 (2012), #P22 5

6 Sketch of the proof of Proposition 2. We use the graph D 7 that was shown to be a Maker s win in the positive minimum degree game in [4]. From two copies of D 7 one can make a graph D 14 such that v(d 14 ) = 14, e(d 14 ) = 40 and Maker wins the game (E(D 14 ), D 2 ). The construction is made in two steps. First we take two copies of D 7, and glue them together in three vertices. The degree two and degree four vertices are associated to the same vertices in the other copy, one to each other. The resulting graph H has 11 vertices and 20 edges. Note that playing Maker s winning strategy for the positive minimum degree game separately on the edges of the D 7 s, Maker gets degrees at least two at the glued vertices, and at least one at the others. In the second step we glue together two copies of H, this time at the degree three vertices, taking care not creating parallel edges, this we call D 14. (Simply the vertices of a diamond should be glued to vertices that are not in a diamond in the other copy.) Again, playing separately on the edges of the two copies of H, Maker gets at least two degrees at all vertices of D 14. The graph D 14. Let D7 2 = D 7 D 7 be the Cartesian product of D 7 with itself, and D7 k := D 7 D7 k 1 be kth power of D 7. To play the (E(D7), k D k )-game, let Maker play the winning strategy for a (E(D 7 ), D 1 )-game in the same projection in that Breaker has just played. This clearly gives a winning strategy for the (E(D7), k D k )-game, and e(d 7 )/v(d 7 ) = 10(7) k 1 k/7 k = 10k/7. Remarks. The discharging method also gives lower bound on ˆm(D 2 ), alas, it is not matching with ˆm(D 2 ) 20n/7, the upper bound of Proposition 2. In fact, we think the upper bound on ˆm(D 2 ) is not tight, but we cannot improve on it. References [1] J. Balogh, R. Martin and A. Pluhár, The diameter game. Random Structures and Algorithms Vol. 35 (2009) [2] J. Beck, Deterministic graph games and a probabilistic intuition. Combinatorics, Probability and Computing 3 (1994), the electronic journal of combinatorics 19 (2012), #P22 6

7 [3] J. Beck, Combinatorial games. Tic-Tac-Toe Theory. Encyclopedia of Mathematics and its Applications, 114. Cambridge University Press, Cambridge, xiv+732 pp. [4] D. Hefetz, M. Krivelevich, M. Stojaković and T. Szabó, Global Maker-Breaker games on sparse graphs. European Journal of Combinatorics 32 (2011), [5] A. Lehman, A solution of the Shannon switching game. J. Soc. Indust. Appl. Math. 12 (1964) [6] R. Radoičić and G. Tóth, The discharging method in combinatorial geometry and the Pach-Sharir conjecture. Contemporary Math. AMS 453 (2008) [7] L. A. Székely, On two concepts of discrepancy in a class of combinatorial games. Finite and Infinite Sets, Colloq. Math. Soc. János Bolyai, Vol. 37, North-Holland, 1984, the electronic journal of combinatorics 19 (2012), #P22 7

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