### Speaker

Yuji Okawa

### Description

In open bosonic string field theory with the cubic interaction in terms of the star product a gauge-invariant operator can be defined for each on-shell closed string state. In the theory on N coincident D-branes we claim that the evaluation of correlation functions of the gauge-invariant operators in the 1/N expansion can be interpreted as a closed string perturbation theory in a low-energy limit. Therefore, open string field theory can provide a nonperturbative definition of closed string theory if it is a consistent quantum theory for finite N, which motivates us to explore the generalization to open superstring field theory.