Madhav V. Marathe Harry B. Hunt III Richard Edwin Stearns Venkatesh Radhakrishnan Approximation Algorithms for PSPACE-Hard Hierarchically and Periodically Specified Problems http://arxiv.org/abs/cs.CC/9809064 1998 CoRR cs.CC/9809064 db/journals/corr/corr9809.html#cs-CC-9809064 informal publication