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北美退伍军人节(Veterans Day),12 日无更新
arXiv 0806.1919cs.ITmath.IT
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非线性索引编码优于线性最优编码

Non-linear index coding outperforming the linear optimum

Eyal Lubetzky, Uri Stav

摘要:

Birk和Kol引入了以下信源编码问题:发送方持有字$x\in\{0,1\}^n$,并希望向$n$接收方$R_1,...,R_n$广播码字。接收机$R_i$对$x_i$感兴趣,并且具有包括$n$比特的某个子集的在先\emph{side information}。这对应于$n$顶点上的有向图$G$,其中$i j$是边,当且仅当$R_i$知道位$x_j$。用于$G$的\emph{index code}是使每个$R_i$在给定其边信息的情况下总是能够重构$x_i$的编码方案。Bar-Yossef、Birk、Jayram和Kol(FOCS 2006)研究了索引代码的最小字长。他们引入了一个图形参数$\minrk_2(G)$,它完全表征了$G$的最佳\emph{linear}索引代码的长度。BBJK的作者表明,在各种情况下,线性码达到最佳字长,并证明线性索引编码实际上是\emph{always}最佳的。在这项工作中,我们反驳了BBJK的主要猜想在以下强烈的意义:对于任何$ε> 0$和足够大的$n$,有一个$n$顶点图$G$,使每个线性索引代码的$G$需要码字的长度至少为$n^{1-ε}$,但一个非线性索引代码的$G$有一个$n^ε$字的长度。这是实现了一个明确的建设,它扩展了阿隆的变体著名的拉姆齐建设的弗兰克尔和威尔逊。此外,我们研究了最佳索引代码在各种,限制较少,自然的模型,并证明了几个相关的性质的图形参数$\minrk(G)$。

英文摘要:

The following source coding problem was introduced by Birk and Kol: a sender holds a word $x\in\{0,1\}^n$, and wishes to broadcast a codeword to $n$ receivers, $R_1,...,R_n$. The receiver $R_i$ is interested in $x_i$, and has prior \emph{side information} comprising some subset of the $n$ bits. This corresponds to a directed graph $G$ on $n$ vertices, where $i j$ is an edge iff $R_i$ knows the bit $x_j$. An \emph{index code} for $G$ is an encoding scheme which enables each $R_i$ to always reconstruct $x_i$, given his side information. The minimal word length of an index code was studied by Bar-Yossef, Birk, Jayram and Kol (FOCS 2006). They introduced a graph parameter, $\minrk_2(G)$, which completely characterizes the length of an optimal \emph{linear} index code for $G$. The authors of BBJK showed that in various cases linear codes attain the optimal word length, and conjectured that linear index coding is in fact \emph{always} optimal. In this work, we disprove the main conjecture of BBJK in the following strong sense: for any $ε> 0$ and sufficiently large $n$, there is an $n$-vertex graph $G$ so that every linear index code for $G$ requires codewords of length at least $n^{1-ε}$, and yet a non-linear index code for $G$ has a word length of $n^ε$. This is achieved by an explicit construction, which extends Alon's variant of the celebrated Ramsey construction of Frankl and Wilson. In addition, we study optimal index codes in various, less restricted, natural models, and prove several related properties of the graph parameter $\minrk(G)$.

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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