Janis Kaneps Rusins Freivalds Running Time to Recognize Nonregular Languages by 2-Way Probabilistic Automata. 174-185 1991 conf/icalp/1991 ICALP db/conf/icalp/icalp91.html#KanepsF91 http://dx.doi.org/10.1007/3-540-54233-7_133