# (Aug 13 3-5pm) Pre-meeting Overview: Wainwright / Jordan, ch 8, p214-233

Section 8.4.3 till 8.5

The main points that will be covered in these subsections are

(1) Extending the LP view of mode finding that we saw for max-product on tree graphs to reweighted-max-product.

(2) Examples of first order LP relaxations.

(3) Higher order LP approximations and how they can be more accurate.

Section 8.4.3 extends the arguments from earlier subsections in this chapter to show that reweighted max-product (and tree-reweighted Bethe variational problem) can be thought of as an LP problem. Conditions 8.23 have to be met for the max-product solution and LP problem solution to be dual of one another.

Section 8.4.4 discusses examples where an LP problem is formulated from a graph problem. Example 8.3 shows how the Error-control coding problem may be thought of as an LP problem. Example 8.4 shows the same for the (a) Ising model, (b) (Maximum Weight) Independent Set problem, (c) (Minimum Weight) Vertex Cover problem, (d) Max-cut problem. These are all examples of intractable problems, but a first oder LP approximate solution is possible. Example 8.5 is another classic graph problem, Maximum Weight Matching problem, solved using an LP.

Section 8.5 discusses higher order LP relaxations by using the marginal polytopes defined on hypergraphs (rather than the polytopes defined by pairwise interactions in a tree). The main claim and proof in this section is that higher order relaxation LPs are more accurate the lower order relaxation LPs for the same graphical model. The relaxation LP solution, where is the tree-width of the hypergraph for the graphical model, is the same as the exact solution on the original polytope .

Finally, example 8.6 shows how the second order relaxation adds additional constraints that makes the solution “tighter” than in a first order relaxation. This is done by solving the same problem solved earlier in Example 8.3 by the two methods, and comparing the constraints and considering one solution .

We will be reading papers next week, so please come with suggestions for papers you think are interesting.