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تسجيل مستخدم جديدNear-perfect clique-factors in sparse pseudorandom graphs
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We prove that, for any $tge 3$, there exists a constant $c=c(t)>0$ such that any $d$-regular $n$-vertex graph with the second largest eigenvalue in absolute value~$lambda$ satisfying $lambdale c d^{t-1}/n^{t-2}$ contains vertex-disjoint copies of $K_t$ covering all but at most $n^{1-1/(8t^4)}$ vertices. This provides further support for the conjecture of Krivelevich, Sudakov and Szabo [emph{Triangle factors in sparse pseudo-random graphs}, Combinatorica textbf{24} (2004), pp.~403--426] that $(n,d,lambda)$-graphs with $nin 3mathbb{N}$ and $lambdaleq cd^{2}/n$ for a suitably small absolute constant~$c>0$ contain triangle-factors.
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We prove that for any $tge 3$ there exist constants $c>0$ and $n_0$ such that any $d$-regular $n$-vertex graph $G$ with $tmid ngeq n_0$ and second largest eigenvalue in absolute value $lambda$ satisfying $lambdale c d^{t}/n^{t-1}$ contains a $K_t$-fa
ctor, that is, vertex-disjoint copies of $K_t$ covering every vertex of $G$.
We consider extremal problems for subgraphs of pseudorandom graphs. For graphs $F$ and $Gamma$ the generalized Turan density $pi_F(Gamma)$ denotes the density of a maximum subgraph of $Gamma$, which contains no copy of~$F$. Extending classical Turan
type results for odd cycles, we show that $pi_{F}(Gamma)=1/2$ provided $F$ is an odd cycle and $Gamma$ is a sufficiently pseudorandom graph. In particular, for $(n,d,lambda)$-graphs $Gamma$, i.e., $n$-vertex, $d$-regular graphs with all non-trivial eigenvalues in the interval $[-lambda,lambda]$, our result holds for odd cycles of length $ell$, provided [ lambda^{ell-2}ll frac{d^{ell-1}}nlog(n)^{-(ell-2)(ell-3)},. ] Up to the polylog-factor this verifies a conjecture of Krivelevich, Lee, and Sudakov. For triangles the condition is best possible and was proven previously by Sudakov, Szabo, and Vu, who addressed the case when $F$ is a complete graph. A construction of Alon and Kahale (based on an earlier construction of Alon for triangle-free $(n,d,lambda)$-graphs) shows that our assumption on $Gamma$ is best possible up to the polylog-factor for every odd $ellgeq 5$.
Given an $n$-vertex pseudorandom graph $G$ and an $n$-vertex graph $H$ with maximum degree at most two, we wish to find a copy of $H$ in $G$, i.e. an embedding $varphicolon V(H)to V(G)$ so that $varphi(u)varphi(v)in E(G)$ for all $uvin E(H)$. Particu
lar instances of this problem include finding a triangle-factor and finding a Hamilton cycle in $G$. Here, we provide a deterministic polynomial time algorithm that finds a given $H$ in any suitably pseudorandom graph $G$. The pseudorandom graphs we consider are $(p,lambda)$-bijumbled graphs of minimum degree which is a constant proportion of the average degree, i.e. $Omega(pn)$. A $(p,lambda)$-bijumbled graph is characterised through the discrepancy property: $left|e(A,B)-p|A||B|right |<lambdasqrt{|A||B|}$ for any two sets of vertices $A$ and $B$. Our condition $lambda=O(p^2n/log n)$ on bijumbledness is within a log factor from being tight and provides a positive answer to a recent question of Nenadov. We combine novel variants of the absorption-reservoir method, a powerful tool from extremal graph theory and random graphs. Our approach is based on that of Nenadov (emph{Bulletin of the London Mathematical Society}, to appear) and on ours (arXiv:1806.01676), together with additional ideas and simplifications.
We study the appearance of powers of Hamilton cycles in pseudorandom graphs, using the following comparatively weak pseudorandomness notion. A graph $G$ is $(varepsilon,p,k,ell)$-pseudorandom if for all disjoint $X$ and $Ysubset V(G)$ with $|X|gevare
psilon p^kn$ and $|Y|gevarepsilon p^ell n$ we have $e(X,Y)=(1pmvarepsilon)p|X||Y|$. We prove that for all $beta>0$ there is an $varepsilon>0$ such that an $(varepsilon,p,1,2)$-pseudorandom graph on $n$ vertices with minimum degree at least $beta pn$ contains the square of a Hamilton cycle. In particular, this implies that $(n,d,lambda)$-graphs with $lambdall d^{5/2 }n^{-3/2}$ contain the square of a Hamilton cycle, and thus a triangle factor if $n$ is a multiple of $3$. This improves on a result of Krivelevich, Sudakov and Szabo [Triangle factors in sparse pseudo-random graphs, Combinatorica 24 (2004), no. 3, 403--426]. We also extend our result to higher powers of Hamilton cycles and establish corresponding counti
We establish a so-called counting lemma that allows embeddings of certain linear uniform hypergraphs into sparse pseudorandom hypergraphs, generalizing a result for graphs [Embedding graphs with bounded degree in sparse pseudorandom graphs, Israel J.
Math. 139 (2004), 93-137]. Applications of our result are presented in the companion paper [Counting results for sparse pseudorandom hypergraphs II].
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