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Approximating the Bethe Partition Function
Proceedings of the 30th Conference on Uncertainty in Artificial Intelligence, PMLR R12:264-273, 2014.
Abstract
When belief propagation (BP) converges, it does so to a stationary point of the Bethe free en- ergy F, and is often strikingly accurate. How- ever, it may converge only to a local optimum or may not converge at all. An algorithm was recently introduced by Weller and Jebara for at- tractive binary pairwise MRFs which is guaran- teed to return an $\epsilon$-approximation to the global minimum of F in polynomial time provided the maximum degree $\Delta$= O(log n), where n is the number of variables. Here we extend their ap- proach and derive a new method based on an- alyzing first derivatives of F, which leads to much better performance and, for attractive mod- els, yields a fully polynomial-time approxima- tion scheme (FPTAS) without any degree restric- tion. Further, our methods apply to general (non- attractive) models, though with no polynomial time guarantee in this case, demonstrating that approximating log of the Bethe partition func- tion, log ZB = -min F, for a general model to additive $\epsilon$-accuracy may be reduced to a discrete MAP inference problem. This allows the merits of the global Bethe optimum to be tested.