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Ailian Chen

Publications and source records attributed to Ailian Chen.

6 recordsLinked to original sources

Pósa-type resilience for Hamilton Berge cycles in random hypergraphs

We prove a degree-sequence resilience theorem for Hamilton Berge cycles in random uniform hypergraphs. For every fixed integer $k\ge3$ and every $γ>0$, there exists $C=C(k,γ)>0$ such that if $C\log n/n^{k-1}\le p\le1$, then asymptotically almost surely every spanning subhypergraph of $H^{(k)}(n,p)$ with increasing vertex-degree sequence $g_1\le\cdots\le g_n$ satisfying \[ g_i\ge\left(\left(\frac{i}{n}\right)^{k-1}+γ\right) \binom{n-1}{k-1}p \qquad(1\le i<n/2) \] contains a Hamilton Berge cycle. The term $(i/n)^{k-1}$ agrees with the normalized asymptotic degree bound in Salia's deterministic Pósa-type theorem. Our result gives a hypergraph analogue of the random-graph theorem of Condon et al. and strengthens the sharp Dirac-type resilience theorem of Chen and Zhang at the same logarithmic probability order. The deletion-resilience formulation has optimal probability order up to constant factors. The coefficient $1$ of $(i/n)^{k-1}$ cannot be replaced by a smaller constant. The proof controls adaptive rotations and representative-edge incidences uniformly. It also gives an endpoint-growth estimate under nonuniform deletion bounds.

math.CO↗

Local Fan-type obstructions and Ore-type conditions for rainbow Hamiltonicity in graph systems

Let $G=(G_1,...,G_n)$ be a graph system on a common $n$-vertex set $V$. For $v\in V$, set \[ δ(v)=\min_{i\in[n]}d_{G_i}(v), \qquad E_{\cap}=\bigcap_{i=1}^n E(G_i), \] and let $d^c(u,v)$ denote the length of a shortest rainbow $u$--$v$ path. We study Hamiltonicity conditions imposed only on pairs at small rainbow distance. Suppose that every $G_i$ is $2$-connected and that \[ δ(u),δ(v)<\frac n2, \qquad d^c(u,v)\le2 \quad\Longrightarrow\quad uv\in E_{\cap}. \] Our main result shows that any system satisfying this local Fan-type condition but containing no rainbow Hamilton cycle must satisfy \[ E_{\cap}=\varnothing, \qquad \sum_{v\in V}δ(v)\le\frac{n^2-1}{2}. \] Consequently, either $E_{\cap}\ne\varnothing$ or $\sum_{v\in V}δ(v)\ge\lceil n^2/2\rceil$ guarantees a rainbow Hamilton cycle. We further prove that the local Ore-type condition \[ δ(u)+δ(v)<n, \qquad d^c(u,v)\le2 \quad\Longrightarrow\quad uv\in E_{\cap} \] already forces a rainbow Hamilton cycle. Along the way, the Fan-type hypothesis yields a rainbow Hamilton path and a rainbow $(n-1)$-cycle. For diagonal systems, our conditions reduce to the classical Fan and local Ore conditions. The proofs combine rainbow rotation--extension with a double-counting argument in an auxiliary digraph. Keywords: graph system; rainbow Hamilton cycle; rainbow distance; Fan-type condition; Ore-type condition; rotation--extension.

math.CO↗

A Fan-Type Theorem for Hamiltonian Berge Cycles in 3-Uniform Hypergraphs

Let $H$ be a simple $3$-uniform hypergraph on $n\ge7$ vertices, and put $τ_n=\binom{\lfloor(n-1)/2\rfloor}{2}$. We prove that $H$ has a Hamiltonian Berge cycle if its $2$-shadow is $2$-connected, every pair at Berge distance two has an endpoint of hypergraph degree at least $τ_n+1$, and every pair of codegree one has both endpoints above $τ_n$. The distance-two threshold is sharp, and the order range cannot be extended to $n=4,5,6$. The proof separates two structural arguments. A low-degree endpoint of an extremal longest Berge path forces Hamiltonicity, independently of the order and the degree threshold. For high-degree endpoints, injective charging and its equality cases force pairs of codegree one that permit further rotations. The cases $n=7,8$ are settled by exhaustive verification, with a completeness proof and a second implementation using direct vertex--hyperedge cycle search. Keywords: Hamiltonian Berge cycle; Fan-type condition; 3-uniform hypergraph; longest-path rotation; exhaustive verification.

math.CO↗

Local Ore Conditions for Rainbow Pancyclicity and Vertex-Pancyclicity in Graph Systems

We investigate rainbow pancyclicity and vertex-pancyclicity of graph systems satisfying a local Ore-type degree condition. All graphs in the system share the same vertex set. We prove that if each graph is 2-connected and satisfies our local Ore condition, then the graph system is either rainbow pancyclic and vertex-pancyclic for cycles of length from 4 to n, or all graphs are identical balanced complete bipartite graphs on an even vertex set. A stronger degree threshold ensures full rainbow vertex-pancyclicity. All thresholds we obtain are tight. Our proofs rely on cycle extension techniques and neighborhood separation arguments.

math.CO↗

Rainbow Berge Hamiltonicity in edge-colored random $k$-uniform hypergraphs

Let $H \sim H^{k}_c(n,p)$ be an edge-colored random $k$-uniform hypergraph on the vertex set $[n]$, where each edge $e \in \binom{[n]}{k}$ is included independently with probability $p$ and is uniformly and independently assigned a color from the color set $[c]$. For $k = 2$, Ferber and Krivelevich (2016) established that if $c = (1+o(1))n$ and $p = (\log n + \log \log n + ω(n))/n$, then with high probability the edge-colored random graph $H \sim H^2_c(n,p)$ contains a rainbow Hamilton Berge cycle. Subsequently, Bal, Berkowitz, Devlin, and Schacht (2021) determined the threshold for the appearance of a (non-rainbow) Hamilton Berge cycle in random $k$-uniform hypergraphs. In this paper, we generalize the results to all integers $k \ge 3$. We prove that if $c = (1+o(1))n$ and $p = (k-1)! \frac{\log n + \log\log n + ω(n)}{n^{k-1}}$, then with high probability $H \sim H^{k}_c(n,p)$ contains a rainbow Hamilton Berge cycle. Furthermore, both conditions on $c$ and $p$ are asymptotically tight. \noindent\emph{Key words:} Rainbow subgraph, Hamiltonicity, Berge cycle, Random hypergraph.

math.CO↗

A Bondy-type theorem for rainbow pancyclicity in graph systems

We establish a Hamiltonian-to-pancyclic analogue of Bondy's theorem for graph systems under an aggregate degree condition. Let $\G=(G_1,\ldots,G_n)$ be a graph system on a common $n$-vertex set $V$, and write $δ(v)=\min_{i\in[n]}d_{G_i}(v)$. If $\G$ contains a rainbow Hamilton cycle and \[ \sum_{v\in V}δ(v)\ge \left\lceil\frac{n^2}{2}\right\rceil-1, \] then $\G$ is rainbow pancyclic, unless $n$ is even and every member is the same balanced complete bipartite graph. For even $n$ the threshold is exact at the integer level. Unlike the usual transversal Dirac- or Ore-type hypotheses, our condition is not layerwise: the member attaining $δ(v)$ may depend on $v$, and some vertices may have $δ(v)<n/2$. Relative to a fixed rainbow Hamilton cycle, we count shortcuts whose colors are released by the Hamilton arcs they replace. A missing cycle length forces complementary shortcut supports to cross-intersect. A counting gap settles even shortening, while equality or near equality in odd shortening yields a distance-two exchange whose orbits force the balanced bipartite obstruction. At the lower integer threshold an exact defect identity shows that only one or two units of slack are available. \noindent\textbf{Keywords:} graph system; rainbow cycle; pancyclicity; Hamilton cycle; extremal graph theory.

math.CO↗