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Yian Xu

Publications and source records attributed to Yian Xu.

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Opt-Verifier: Unleashing the Power of LLMs for Optimization Modeling via Dual-Side Verification

Building mathematical optimization models is critical in operations research (OR), while it requires substantial human expertise. Recent advancements have utilized large language models (LLMs) to automate this modeling process. However, existing works often struggle to verify the correctness of the generated optimization models, without checking the rationality of the constraints and variables or the validity of solutions to the generated models. This hampers the subsequent verification and correction steps, and thus it severely hurts the modeling accuracy. To address this challenge, we propose a novel LLM-based framework with Dual-side Verification (Opt-Verifier) from both structure and solution perspectives, thereby improving the modeling accuracy. The structure-side verification ensures that the modeling structure of the generated optimization models aligns with the original problem description, accurately capturing the problem's constraints and requirements. Meanwhile, the solution-side verification interprets and evaluates the solutions' validity, confirming that the optimization models are logically and mathematically sound. Experiments on popular benchmarks demonstrate that our approach achieves over 20\% improvement in accuracy.

cs.AI

The optimal binding function for (cap, even hole)-free graphs

A {\em hole} is an induced cycle of length at least 4, an {\em even hole} is a hole of even length, and a {\em cap} is a graph obtained from a hole by adding an additional vertex which is adjacent exactly to two adjacent vertices of the hole. A graph $G$ obtained from a graph $H$ by blowing up all the vertices into cliques is said to be a clique blowup of $H$. Let $p, q$ be two positive integers with $p>2q$, let $F$ be a triangle-free graph, and let $G'$ be a clique blowup of $F$ with $ω(G')\leq\max\{\frac{2q(p-q-2)}{p-2q}, 2q\}$. In this paper, we prove that for any clique blowup $G$ of $F$, $χ(G)\leq\lceil\frac{p}{2q}ω(G)\rceil$ if and only if $χ(G')\leq\lceil\frac{p}{2q}ω(G')\rceil$. As its consequences, we show that every (cap, even hole)-free graph $G$ satisfies $χ(G)\leq\lceil\frac{5}{4}ω(G)\rceil$, which affirmatively answers a question of Cameron {\em et al.} \cite{CdHV2018}, we also show that every (cap, even hole, 5-hole)-free graph $G$ satisfies $χ(G)\leq\lceil\frac{7}{6}ω(G)\rceil$, and the bound is reachable.

math.CO

The chromatic number of ($P_{5}, K_{5}-e$)-free graphs

Let $G$ be a graph. We use $χ(G)$ and $ω(G)$ to denote the chromatic number and clique number of $G$ respectively. A $P_5$ is a path on 5 vertices. A family of graphs $\mathcal{G}$ is said to be {\it$χ$-bounded} if there exists some function $f$ such that $χ(G)\leq f(ω(G))$ for every $G\in\mathcal{G}$. In this paper, we show that the family of $(P_5, K_5-e)$-free graphs is $χ$-bounded by a linear function: $χ(G)\leq \max\{13,ω(G)+1\}$.

math.CO

The chromatic number of (P_5, HVN )-free graphs

Let $G$ be a graph. We use $χ(G)$ and $ω(G)$ to denote the chromatic number and clique number of $G$ respectively. A $P_5$ is a path on 5 vertices, and an $HVN$ is a $K_4$ together with one more vertex which is adjacent to exactly two vertices of $K_4$. Combining with some known result, in this paper we show that if $G$ is $(P_5, \textit{HVN})$-free, then $χ(G)\leq \max\{\min\{16, ω(G)+3\}, ω(G)+1\}$. This upper bound is almost sharp.

math.CO

A tight linear bound to the chromatic number of $(P_5, K_1+(K_1\cup K_3))$-free graphs

Let $F_1$ and $F_2$ be two disjoint graphs. The union $F_1\cup F_2$ is a graph with vertex set $V(F_1)\cup V(F_2)$ and edge set $E(F_1)\cup E(F_2)$, and the join $F_1+F_2$ is a graph with vertex set $V(F_1)\cup V(F_2)$ and edge set $E(F_1)\cup E(F_2)\cup \{xy\;|\; x\in V(F_1)\mbox{ and } y\in V(F_2)\}$. In this paper, we present a characterization to $(P_5, K_1\cup K_3)$-free graphs, prove that $χ(G)\le 2ω(G)-1$ if $G$ is $(P_5, K_1\cup K_3)$-free. Based on this result, we further prove that $χ(G)\le $max$\{2ω(G),15\}$ if $G$ is a $(P_5,K_1+( K_1\cup K_3))$-free graph, and construct an infinite family of $(P_5, K_1+( K_1\cup K_3))$-free graphs such that every graph $G$ in the family satisfies $χ(G)=2ω(G)$.

math.CO

The chromatic number of heptagraphs

A hole is an induced cycle of length at least 4. A graph is called a pentagraph if it has no cycles of length 3 or 4 and has no holes of odd length at least 7, and is called a heptagraph if it has no cycles of length less than 7 and has no holes of odd length at least 9. Let $ł\ge 2$ be an integer. The current authors proved that a graph is 4- colorable if it has no cycles of length less than $2ł+1$ and has no holes of odd length at least $2ł+3$. Confirming a conjecture of Plummer and Zha, Chudnovsky and Seymour proved that every pentagraph is 3-colorable. Following their idea, we show that every heptagraph is 3-colorable.

math.CO

On coloring of graphs of girth 2l + 1 without longer odd holes

A hole is an induced cycle of length at least 4. Let $ł\ge 2$ be a positive integer, let ${\cal G}_l$ denote the family of graphs which have girth $2ł+1$ and have no holes of odd length at least $2ł+3$, and let $G\in {\cal G}_ł$. For a vertex $u\in V(G)$ and a nonempty set $S\subseteq V(G)$, let $d(u, S)=\min\{d(u, v):v\in S\}$, and let $L_i(S)=\{u\in V(G) \mbox{ and } d(u, S)=i\}$ for any integer $i\ge 0$. We show that if $G[S]$ is connected and $G[L_i(S)]$ is bipartite for each $i\in\{1, \ldots, \lfloor{ł\over 2}\rfloor\}$, then $G[L_i(S)]$ is bipartite for each $i>0$, and consequently $χ(G)\le 4$, where $G[S]$ denotes the subgraph induced by $S$. Let $θ^-$ be the graph obtained from the Petersen graph by deleting three vertices which induce a path, let $θ^+$ be the graph obtained from the Petersen graph by deleting two adjacent vertices, and let $θ$ be the graph obtained from $θ^+$ by removing an edge incident with two vertices of degree 3. For a graph $G\in{\cal G}_2$, we show that if $G$ is 3-connected and has no unstable 3-cutset then $G$ must induce either $θ$ or $θ^-$ but does not induce $θ^+$. As corollaries, $χ(G)\le 3$ for every graph $G$ of ${\cal G}_2$ that induces neither $θ$ nor $θ^-$, and minimal non-3-colorable graphs of ${\cal G}_2$ induce no $θ^+$.

math.CO

On the chromatic number of some $P_5$-free graphs

Let $G$ be a graph. We say that $G$ is perfectly divisible if for each induced subgraph $H$ of $G$, $V(H)$ can be partitioned into $A$ and $B$ such that $H[A]$ is perfect and $ω(H[B])<ω(H)$. We use $P_t$ and $C_t$ to denote a path and a cycle on $t$ vertices, respectively. For two disjoint graphs $F_1$ and $F_2$, we use $F_1\cup F_2$ to denote the graph with vertex set $V(F_1)\cup V(F_2)$ and edge set $E(F_1)\cup E(F_2)$, and use $F_1+F_2$ to denote the graph with vertex set $V(F_1)\cup V(F_2)$ and edge set $E(F_1)\cup E(F_2)\cup \{xy\;|\; x\in V(F_1)\mbox{ and } y\in V(F_2)\}$. In this paper, we prove that (i) $(P_5, C_5, K_{2, 3})$-free graphs are perfectly divisible, (ii) $χ(G)\le 2ω^2(G)-ω(G)-3$ if $G$ is $(P_5, K_{2,3})$-free with $ω(G)\ge 2$, (iii) $χ(G)\le {3\over 2}(ω^2(G)-ω(G))$ if $G$ is $(P_5, K_1+2K_2)$-free, and (iv) $χ(G)\le 3ω(G)+11$ if $G$ is $(P_5, K_1+(K_1\cup K_3))$-free.

math.CO

Universality in Kinetic Models of Circadian Rhythms in Arabidopsis thaliana

Biological evolution has endowed the plant Arabidopsis thaliana with genetically regulated circadian rhythms. A number of authors have published kinetic models for these oscillating chemical reactions based on a network of interacting genes. To investigate the hypothesis that the Arabidopsis circadian dynamical system is poised near a Hopf bifurcation like some other biological oscillators, we varied the kinetic parameters in the models and searched for bifurcations. Finding that each model does exhibit a supercritical Hopf bifurcation, we performed a weakly nonlinear analysis near the bifurcation points to derive the Stuart-Landau amplitude equation. To illustrate a common dynamical structure, we scaled the numerical solutions to the models with the asymptotic solutions to the Stuart-Landau equation to collapse the circadian oscillations onto two universal curves -- one for amplitude, and one for frequency. However, some models are close to bifurcation while others are far, some models are post-bifurcation while others are pre-bifurcation, and kinetic parameters that lead to a bifurcation in some models do not lead to a bifurcation in others. Future kinetic modeling can make use of our analysis to ensure models are consistent with each other and with the dynamics of the Arabidopsis circadian rhythm.

physics.bio-ph

The NNN-Property of Cyclic Groups

A Cayley graph is said to be an NNN-graph if it is both normal and non-normal for isomorphic regular groups, and a group has the NNN-property if there exists an NNN-graph for it. In this paper we investigate the NNN-property of cyclic groups, and show that cyclic groups do not have the NNN-property.

math.CO

Constructing 2-Arc-Transitive Covers of Hypercubes

We introduce the notion of a symmetric basis of a vector space equipped with a quadratic form, and provide a sufficient and necessary condition for the existence to such a basis. Symmetric bases are then used to study Cayley graphs of certain extraspecial 2-groups of order 2^{2r+1} (r\geq 1), which are further shown to be normal Cayley graphs and 2-arc-transitive covers of 2r-dimensional hypercubes.

math.CO