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Strong data-processing inequalities for channels and Bayesian networks

  • Polyanskiy, Yury
  • Wu, Yihong
Publication Date
May 19, 2016
Submission Date
Aug 25, 2015
arXiv ID: 1508.06025
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The data-processing inequality, that is, $I(U;Y) \le I(U;X)$ for a Markov chain $U \to X \to Y$, has been the method of choice for proving impossibility (converse) results in information theory and many other disciplines. Various channel-dependent improvements (called strong data-processing inequalities, or SDPIs) of this inequality have been proposed both classically and more recently. In this note we first survey known results relating various notions of contraction for a single channel. Then we consider the basic extension: given SDPI for each constituent channel in a Bayesian network, how does one produce an end-to-end SDPI? Our approach is based on the (extract of the) Evans-Schulman method, which is demonstrated for three different kinds of SDPIs, namely, the usual Ahslwede-G\'acs type contraction coefficients (mutual information), Dobrushin's contraction coefficients (total variation), and finally the $F_I$-curve (the best possible SDPI for a given channel). As an example, we demonstrate how to obtain SDPI for an $n$-letter memoryless channel with feedback given an SDPI for $n=1$. Finally, we discuss a simple observation on the equivalence of SDPI and comparison to erasure channel (in the sense of "less noisy" order). This leads to a simple proof of a curious inequality of Samorodnitsky (2015).

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