Inapproximability of Treewidth, One-Shot Pebbling, and Related Layout   Problems
  We study the approximability of a number of graph problems: treewidth and pathwidth of graphs, one-shot black (and black-white) pebbling costs of directed acyclic graphs, and a variety of different graph layout problems such as minimum cut linear arrangement and interval graph completion. We show that, assuming the recently introduced Small Set Expansion Conjecture, all of these problems are hard to approximate within any constant factor. 
Per Austrin, Toniann Pitassi, Yu Wu
cs.CC cs.DS