We show that closed string states in bosonic string field theory are encodedin the cyclic cohomology of cubic open string field theory (OSFT) which, inturn, classifies the deformations of OSFT. This cohomology is then shown to beindependent of the open string background. Exact elements correspond to closedstring gauge transformations, generic boundary deformations of Witten's3-vertex and infinitesimal shifts of the open string background. Finally it isargued that the closed string cohomology and the cyclic cohomology of OSFT areisomorphic to each other.
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