Anne Condon Joan Feigenbaum Carsten Lund Peter W. Shor Probabilistically checkable debate systems and approximation algorithms for PSPACE-hard functions. 305-314 1993 conf/stoc/STOC25 STOC http://doi.acm.org/10.1145/167088.167190 db/conf/stoc/stoc1993.html#CondonFLS93