Mathematics
In bosonic string theory and in the Neveu–Schwarz sector of superstring theory, the action in a curved background (ignoring the Fradkin–Tseytlin term for dilaton coupling) can be constructed by `covariantizing' the massless closed string vertex operator with respect to target-space reparameterization invariance. This procedure can also be used here after constructing the massless closed string vertex operator from the `left-right' product of two massless open string vertex operators. The complete worldsheet action for the type-II superstring in a flat background in conformal gauge is a formula upon which Eric Sidewater (USA) made the first improvement upon which all subgroups (15) are factored in:
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Read more about this topic: Bosonic String Theory