Local resilience of an almost spanning kk-cycle in random graphs

The famous P\'{o}sa-Seymour conjecture, confirmed in 1998 by Koml\'{o}s, S\'{a}rk\"{o}zy, and Szemer\'{e}di, states that for any $k \geq 2$, every graph on $n$ vertices with minimum degree $kn/(k + 1)$ contains the $k$-th power of a Hamilton cycle. We extend this result to a sparse random setting. We show that for every $k \geq 2$ there exists $C > 0$ such that if $p \geq C(\log n/n)^{1/k}$ then w.h.p. every subgraph of a random graph $G_{n, p}$ with minimum degree at least $(k/(k + 1) + o(1))np$, contains the $k$-th power of a cycle on at least $(1 - o(1))n$ vertices, improving upon the recent results of Noever and Steger for $k = 2$, as well as Allen et al. for $k \geq 3$. Our result is almost best possible in three ways: for $p \ll n^{-1/k}$ the random graph $G_{n, p}$ w.h.p. does not contain the $k$-th power of any long cycle; there exist subgraphs of $G_{n, p}$ with minimum degree $(k/(k + 1) + o(1))np$ and $\Omega(p^{-2})$ vertices not belonging to triangles; there exist subgraphs of $G_{n, p}$ with minimum degree $(k/(k + 1) - o(1))np$ which do not contain the $k$-th power of a cycle on $(1 - o(1))n$ vertices.Comment: 24 pages; small updates to the paper after anonymous reviewers' report

On resilience of connectivity in the evolution of random graphs

In this note we establish a resilience version of the classical hitting time result of Bollob\'{a}s and Thomason regarding connectivity. A graph $G$ is said to be $\alpha$-resilient with respect to a monotone increasing graph property $\mathcal{P}$ if for every spanning subgraph $H \subseteq G$ satisfying $\mathrm{deg}_H(v) \leq \alpha \cdot \mathrm{deg}_G(v)$ for all $v \in V(G)$, the graph $G - H$ still possesses $\mathcal{P}$. Let $\{G_i\}$ be the random graph process, that is a process where, starting with an empty graph on $n$ vertices $G_0$, in each step $i \geq 1$ an edge $e$ is chosen uniformly at random among the missing ones and added to the graph $G_{i - 1}$. We show that the random graph process is almost surely such that starting from $m \geq (\tfrac{1}{6} + o(1)) n \log n$, the largest connected component of $G_m$ is $(\tfrac{1}{2} - o(1))$-resilient with respect to connectivity. The result is optimal in the sense that the constants $1/6$ in the number of edges and $1/2$ in the resilience cannot be improved upon. We obtain similar results for $k$-connectivity.Comment: 13 pages; update after reviewers' report

Ramsey numbers for multiple copies of sparse graphs

For a graph H and an integer n, we let nH denote the disjoint union of n copies of H. In 1975, Burr, Erdős and Spencer initiated the study of Ramsey numbers for nH, one of few instances for which Ramsey numbers are now known precisely. They showed that there is a constant c = c(H) such that r(nH) = (2 | H | - α (H))n + c, provided n is sufficiently large. Subsequently, Burr gave an implicit way of computing c and noted that this long-term behaviour occurs when n is triply exponential in | H |. Very recently, Bucić and Sudakov revived the problem and established an essentially tight bound on n by showing r(nH) follows this behaviour already when the number of copies is just a single exponential. We provide significantly stronger bounds on n in case H is a sparse graph, most notably of bounded maximum degree. These are relatable to the current state-of-the-art bounds on r(H) and (in a way) tight. Our methods rely on a beautiful classic proof of Graham, Rödl and Ruciński, with an emphasis on developing an efficient absorbing method for bounded degree graphs.ISSN:0364-9024ISSN:1097-011