GSO projection
The Gliozzi–Scherk–Olive (GSO) projection is an ingredient used in constructing a consistent model in superstring theory. The projection is a selection of a subset of possible vertex operators in the worldsheet conformal field theory (CFT)—usually those with specific worldsheet fermion number and periodicity conditions. Such a projection is necessary to obtain a consistent worldsheet CFT. For the projection to be consistent, the set A of operators retained by the projection must satisfy:Closure — The operator product expansion (OPE) of any two operators in A contains only operators which are in A.
Mutual locality — There are no branch cuts in the OPE of any two operators in the set A.
Modular invariance — The partition function on the two-torus of the theory containing only the operators in A respects modular invariance.
Source: Wikipedia
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