Exact Fractional Inference via Re-Parametrization Interpolation between Tree-Re-Weighted- and Belief Propagation- Algorithms
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Inference efforts – required to compute partition function, Z, of an Ising model over a graph of N “spins" – are most likely exponential in N. Efficient variational methods, such as Belief Propagation (BP) and Tree Re-Weighted (TRW) algorithms, compute Z approximately minimizing respective (BP- or TRW-) free energy. We generalize the variational scheme building a λ-fractional-homotopy, Z^(λ), where λ=0 and λ=1 correspond to TRW- and BP-approximations, respectively, and Z^(λ) decreases with λ monotonically. Moreover, this fractional scheme guarantees that in the attractive (ferromagnetic) case Z^(TRW)≥ Z^(λ)≥ Z^(BP), and there exists a unique (“exact") λ_* such that, Z=Z^(λ_*). Generalizing the re-parametrization approach of <cit.> and the loop series approach of <cit.>, we show how to express Z as a product, ∀λ: Z=Z^(λ) Z^(λ), where the multiplicative correction, Z^(λ), is an expectation over a node-independent probability distribution built from node-wise fractional marginals. Our theoretical analysis is complemented by extensive experiments with models from Ising ensembles over planar and random graphs of medium- and large- sizes. The empirical study yields a number of interesting observations, such as (a) ability to estimate Z^(λ) with O(N^4) fractional samples; (b) suppression of λ_* fluctuations with increase in N for instances from a particular random Ising ensemble.
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