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arXiv 2307.06155math.COcs.ITmath.IT
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论图的 Shannon 数之比

On the Ratio of Shannon Numbers of Graphs

Sharareh Alipour, Amin Gohari, Mehrshad Taziki

AI总结:

本文定义并研究图的相对分数独立数,证明其与两种构造方法等价,并给出图的 Shannon 容量等不变量之比的首个非平凡界及应用。

摘要:

设 $\Gamma$ 是一个将任意两个图 $G$ 和 $H$ 映射到非负实数的函数,满足 $$\alpha(G^{\boxtimes n})\leq \alpha(H^{\boxtimes n})\Gamma(G,H)^n,$$ 其中 $n$ 为任意自然数,$G^{\boxtimes n}$ 是 $G$ 与其自身的 $n$ 次强积。我们建立了寻找这种函数 $\Gamma$ 的两种不同方法之间的等价性。通过任一方法得到的共同解被称为“图 $G$ 相对于另一图 $H$ 的相对分数独立数”。我们用 $\alpha^*(G|H)$ 表示这一函数,并讨论其若干性质。特别地,我们证明了 $$\alpha^*(G|H)\geq \frac{X(G)}{X(H)} \geq \frac{1}{\alpha^*(H|G)},$$ 其中 $X(G)$ 可以是图 $G$ 的独立数、Shannon 容量、分数独立数、Lovász 数,或 Lovász 数的 Schrijver 或 Szegedy 变体。该不等式是前文所述两个任意图的不变量之比的首个显式非平凡上界,也可用于获得这些不变量的上界或下界。作为显式应用,我们给出了两个 Cayley 图的 Shannon 容量之比的新上界,并计算了某些 Johnson 图的 Shannon 容量的新下界(从而得到其 Haemers 数的精确值)。此外,我们表明 $\alpha^*(G|H)$ 可用于给出著名的 No-Homomorphism Lemma 的一个更强版本。

英文摘要:

Let $Γ$ be a function that maps two arbitrary graphs $G$ and $H$ to a non-negative real number such that $$α(G^{\boxtimes n})\leq α(H^{\boxtimes n})Γ(G,H)^n$$ where $n$ is any natural number and $G^{\boxtimes n}$ is the strong product of $G$ with itself $n$ times. We establish the equivalence of two different approaches for finding such a function $Γ$. The common solution obtained through either approach is termed ``the relative fractional independence number of a graph $G$ with respect to another graph $H$". We show this function by $α^*(G|H)$ and discuss some of its properties. In particular, we show that $α^*(G|H)\geq \frac{X(G)}{X(H)} \geq \frac{1}{α^*(H|G)},$ where $X(G)$ can be the independence number, the Shannon capacity, the fractional independence number, the Lovász number, or the Schrijver's or Szegedy's variants of the Lovász number of a graph $G$. This inequality is the first explicit non-trivial upper bound on the ratio of the invariants of two arbitrary graphs, as mentioned earlier, which can also be used to obtain upper or lower bounds for these invariants. As explicit applications, we present new upper bounds for the ratio of the Shannon capacity of two Cayley graphs and compute new lower bounds on the Shannon capacity of certain Johnson graphs (yielding the exact value of their Haemers number). Moreover, we show that $α^*(G|H)$ can be used to present a stronger version of the well-known No-Homomorphism Lemma.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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