A new multilayered PCP and the hardness of hypergraph vertex cover

Given a k-uniform hypergraph, the Ek-Vertex-Cover problem is to find the smallest subset of vertices that intersects every hyperedge. We present a new multilayered probabilistically checkable proof (PCP) construction that extends the Raz verifier. This enables us to prove that Ek-Vertex-Cover is NP-hard to approximate within a factor of (k-1-epsilon) for arbitrary constants epsilon>0 and k >= 3. The result is nearly tight as this problem can be easily approximated within factor k. Our construction makes use of the biased long-code and is analyzed using combinatorial properties of s-wise t-intersecting families of subsets. We also give a different proof that shows an inapproximability factor of [k/2]-epsilon. In addition to being simpler, this proof also works for superconstant values of k up to (logN)(1/c), where c>1 is a fixed constant and N is the number of hyperedges.