How Many Variables Does One Need to Prove PSPACE-hardness of Modal Logics.

Alexander V. Chagrov Mikhail N. Rybakov How Many Variables Does One Need to Prove PSPACE-hardness of Modal Logics. 71-82 2002 conf/aiml/2002 Advances in Modal Logic db/conf/aiml/aiml2002.html#ChagrovR02