Some 0/1 polytopes need exponential size extended formulations
  We prove that there are 0/1 polytopes P that do not admit a compact LP formulation. More precisely we show that for every n there is a sets X \subseteq {0,1}^n such that conv(X) must have extension complexity at least 2^{n/2 * (1-o(1))}. In other words, every polyhedron Q that can be linearly projected on conv(X) must have exponentially many facets.   In fact, the same result also applies if conv(X) is restricted to be a matroid polytope.   Conditioning on NP not contained in P_{/poly}, our result rules out the existence of any compact formulation for the TSP polytope, even if the formulation may contain arbitrary real numbers. 
Thomas Rothvo{\ss}
math.CO cs.CC cs.DM