# Projections from Dynamic Programming Based Extended Formulations

Some combinatorial optimization problems can be efficiently solved with dynamic programming. Such a dynamic programming approach provides an extended formulation of the given problem. From the extended formulation we want to derive facets of the associated polyhedron in original space.

### Illustration by a shortest path problem

We illustrate our approach by a shortest path problem. Let *D=(N,A)* be a directed graph on node set *N = {0,...,n}* with costs *c(a), a ∈ A* on the arcs. Of course, the shortest path problem on *D* can be solved in polynomial time, for instance with the Bellman-Ford algorithm, if *D* has no negative cycles. The Bellman-Ford algorithm finds the optimal solution by generating the shortest paths between all pairs of nodes *i,j* with at most *k* arcs for *k=0,1,...,n*. Hence, also the *k*-shortest path problem on *D* can be solved in polynomial time.

We are interested in facets of the associated path polytopes *P(0,n)* and *P ^{k}(0,n)*, that is, the convex hull of the incidence vectors of

*(0,n)*-paths and

*(0,n)*-paths of cardinality at most

*k*, respectively. If

*D*is an acyclic digraph, then

*P(0,n)*has a simple characterization. In this case the flow conservation constraints and the nonnegativity constraints are sufficient to determine

*P(0,n)*. However, a complete linear description of

*P*defined on an acyclic graph

^{k}(0,n)*D*is unknown.

Applying the DP-approach to the dominant of *P ^{k}(0,n)* one can derive at least a characterization of the facet defining 0-1-inequalities of

*P*.

^{k}(0,n)#### The extended formulation

The dynamic programming approach provides an extended formulation of the *k*-shortest path problem in higher dimensional space. It transforms the cardinality restricted path problem on *D* into a non-cardinality restricted path problem on another acyclic digraph *D' =(N',A')* with start node *(0,0)* and target node *(n,k)*. An illustration of this approach is shown in Figure 1.

#### The projection

Figure 2 encodes a *((0,0),(n,k))*-cut of *D'* with partitions *S* and *T*. If a node is labeled with 1, it belongs to *S*, otherwise to *T*. The associated cut-inequality *y(S:T) ≤ 1* is valid for the dominant of the path polytope *P((0,0),(n,k))*. This inequality can be identified with a valid 0-1-inequality *bx ≤ 1* for the dominant of *P ^{k}(0,n)*. We identified all those

*((0,0),(n,k))*-cut inequalities

*y(S:T) ≤ 1*whose associated inequalities

*bx ≤ 1*in original space are facet defining for the dominant of

*P*. Figure 2 shows one such

^{k}(0,n)*((0,0),(n,k))*-cut.

Figure 1) DP-based extended formulation | Figure 2) Illustration of a ((0,0),(n,k))-cut |

### Poster

- Projections from Dynamic Programming Based Extended Formulations (05/2008) [(pdf)]