On metric properties of maps between Hamming spaces and related graph homomorphisms

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• 作者：Yury Polyanskiy ^{yp@mit.edu" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Error-correcting codes ; Graph homomorphism ; Schrijver's θ-function ; Projective geometry over F2F2
• 刊名：Journal of Combinatorial Theory, Series A
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：145
• 期：Complete
• 页码：227-251
• 全文大小：532 K

文摘

A mapping of k-bit strings into n  -bit strings is called an (α,β)-map if k-bit strings which are more than αk apart are mapped to n-bit strings that are more than βn   apart in Hamming distance. This is a relaxation of the classical problem of constructing error-correcting codes, which corresponds to α=0. Existence of an (α,β)-map is equivalent to existence of a graph homomorphism , where H(n,d) is a Hamming graph with vertex set {0,1}ⁿ and edges connecting vertices differing in d or fewer entries.

This paper proves impossibility results on achievable parameters (α,β) in the regime of n,k→∞ with a fixed ratio . This is done by developing a general criterion for existence of graph-homomorphism based on the semi-definite relaxation of the independence number of a graph (known as the Schrijver's θ-function). The criterion is then evaluated using some known and some new results from coding theory concerning the θ  -function of Hamming graphs. As an example, it is shown that if β>1/2 and – integer, the -fold repetition map achieving f2ffb" title="Click to view the MathML source">α=β is asymptotically optimal.

Finally, constraints on configurations of points and hyperplanes in projective spaces over F₂ are derived.