The no-ghost theorem for string theory in curved backgrounds with a flat timelike direction Original Research Article

It is well-known that the standard no-ghost theorem can be extended straightforwardly to the general c=26 CFT on Rd−1,1×K , where 2≤d≤26 and K is a compact unitary CFT of appropriate central charge. We prove the no-ghost theorem for d=1 , i.e., when only the timelike direction is flat. This is done using the technique of Frenkel, Garland and Zuckerman.