# Elimination Ordering¶

Many algorithms for NP-hard problems are exponential in treewidth. However, finding a lower bound on treewidth is in itself NP-complete. [GD] describes a branch-and-bound algorithm for computing the treewidth of an undirected graph by searching in the space of perfect elimination ordering of vertices of the graph.

A clique of a graph is a fully-connected subset of vertices; that is, every pair of vertices in the clique share an edge. A simplicial vertex is one whose neighborhood induces a clique. A perfect elimination ordering is an ordering of vertices $$1..n$$ such that any vertex $$i$$ is simplicial for the subset of vertices $$i..n$$.

 chimera_elimination_order(m[, n, t, coordinates]) Provides a variable elimination order for a Chimera graph. elimination_order_width(G, order) Calculates the width of the tree decomposition induced by a variable elimination order. Determines whether a node n in G is almost simplicial. is_simplicial(G, n) Determines whether a node n in G is simplicial. Computes an upper bound on the treewidth of graph G based on the max-cardinality heuristic for the elimination ordering. Computes a lower bound for the treewidth of graph G. Computes an upper bound on the treewidth of graph G based on the min-fill heuristic for the elimination ordering. Computes an upper bound on the treewidth of graph G based on the min-width heuristic for the elimination ordering. pegasus_elimination_order(n[, coordinates]) Provides a variable elimination order for the Pegasus graph. treewidth_branch_and_bound(G[, ...]) Computes the treewidth of graph G and a corresponding perfect elimination ordering.