A generalization of extension complexity that captures $P$
  In this paper we propose a generalization of the extension complexity of a polyhedron $Q$. On the one hand it is general enough so that all problems in $P$ can be formulated as linear programs with polynomial size extension complexity. On the other hand it still allows non-polynomial lower bounds to be proved for $NP$-hard problems independently of whether or not $P=NP$. The generalization, called $H$-free extension complexity, allows for a set of valid inequalities $H$ to be excluded in computing the extension complexity of $Q$. We give results on the $H$-free extension complexity of hard matching problems (when $H$ are the odd set inequalities) and the traveling salesman problem (when $H$ are the subtour elimination constraints). 
David Avis, Hans Raj Tiwary
cs.CC math.CO