Dichotomy Theorems for Alternation-Bounded Quantified Boolean Formulas
  In 1978, Schaefer proved his famous dichotomy theorem for generalized satisfiability problems. He defined an infinite number of propositional satisfiability problems, showed that all these problems are either in P or NP-complete, and gave a simple criterion to determine which of the two cases holds. This result is surprising in light of Ladner's theorem, which implies that there are an infinite number of complexity classes between P and NP-complete (under the assumption that P is not equal to NP).   Schaefer also stated a dichotomy theorem for quantified generalized Boolean formulas, but this theorem was only recently proven by Creignou, Khanna, and Sudan, and independently by Dalmau: Determining truth of quantified Boolean formulas is either PSPACE-complete or in P.   This paper looks at alternation-bounded quantified generalized Boolean formulas. In their unrestricted forms, these problems are the canonical problems complete for the levels of the polynomial hierarchy. In this paper, we prove dichotomy theorems for alternation-bounded quantified generalized Boolean formulas, by showing that these problems are either $\Sigma_i^p$-complete or in P, and we give a simple criterion to determine which of the two cases holds. This is the first result that obtains dichotomy for an infinite number of classes at once. 
Edith Hemaspaandra