Tightness conjecture for the MAP conditional-entropy bound

Let hnh_n denote the normalized conditional entropy in the code ensemble, and let τ\tau and ρ\rho be the degree sequences appearing in Theorem 1. For each admissible random variable VV, let ϕV\phi_V be the corresponding trial-entropy functional. Under the hypotheses of Theorem 1, tightness conjecture.

limnhn=supV,ϕV,\lim_{n\to\infty}h_n=\sup_V\\,\phi_V,

where the supremum is taken over the space of admissible random variables, with the degree sequences used as arguments of ϕV\phi_V fixed to be those of Theorem 1. The conjecture asserts that the upper bound from Theorem 1 is asymptotically tight; establishing equality requires optimizing the trial entropy over all admissible random variables.

Sources & referencesView supporting material

Primary source

Andrea Montanari, “Tight bounds for LDPC and LDGM codes under MAP decoding”, arXiv:cs/0407060 (2005).

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