Journal of Combinatorial Theory Series B ( IF 1.2 ) Pub Date : 2020-05-27 , DOI: 10.1016/j.jctb.2020.05.005 Dániel Gerbner , Ervin Győri , Abhishek Methuku , Máté Vizer
Given a graph H and a set of graphs , let denote the maximum possible number of copies of H in an -free graph on n vertices. We investigate the function , when H and members of are cycles. Let denote the cycle of length k and let . We highlight the main results below.
- (i)
We show that for any . Moreover, in some cases we determine it asymptotically: We show that and that the maximum possible number of 's in a -free bipartite graph is .
- (ii)
Erdős's Girth Conjecture states that for any positive integer k, there exist a constant depending only on k, and a family of graphs such that , with girth more than 2k.
Solymosi and Wong [37] proved that if this conjecture holds, then for any we have . We prove that their result is sharp in the sense that forbidding any other even cycle decreases the number of 's significantly: For any , we have . More generally, we show that for any and such that , we have .
- (iii)
We prove , provided a stronger version of Erdős's Girth Conjecture holds (which is known to be true when ). This result is also sharp in the sense that forbidding one more cycle decreases the number of 's significantly: More precisely, we have , and for .
- (iv)
We also study the maximum number of paths of given length in a -free graph, and prove asymptotically sharp bounds in some cases.
中文翻译:
偶数周期的广义图兰问题
给定一个图H和一组图,让 表示一个H中H的最大可能副本数n个顶点上的免费图形。我们调查功能,当H和的成员是周期。让表示长度为k的周期,令。我们在下面突出显示主要结果。
- (一世)
我们证明 对于任何 。此外,在某些情况下,我们渐近确定: 并且最大可能数量 在 无二部图是 。
- (ii)
Erdős的周长猜想指出,对于任何正整数k,存在一个常数仅取决于k和一系列图 这样 , 周长超过2 k。
Solymosi和Wong [37]证明,如果这个猜想成立,那么对于任何 我们有 。我们证明,在禁止其他偶数周期会减少次数的意义上,它们的结果是很清晰的重要的是:对于任何 , 我们有 。更普遍地,我们表明对于任何 和 这样 , 我们有 。
- (iii)
我们证明 ,提供了Erdős的周长猜想更强的版本(这在以下情况下是正确的 )。在禁止再增加一个循环会减少循环次数的意义上,此结果也很明显。重要的是:更确切地说,我们有 和 对于 。
- (iv)
我们还研究了给定长度的最大路径数 自由图,并在某些情况下证明渐近尖锐的边界。