We point out that the choice of phases in Gliozzi-Scherk-Olive projections can be accounted for by a choice of fermionic symmetry-protected topological phases on the world sheet of the string. This point of view not only easily explains why there are essentially two type II theories, but also predicts that there are unoriented type 0 theories labeled by $n$ mod 8 and that there is an essentially unique choice of the type I world sheet theory. We also discuss the relationship between this point of view and the $K$-theoretic classification of D-branes.

中文翻译：

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通过拓扑阶段对字符串理论进行分类

我们指出，Gliozzi-Scherk-Olive投影中相的选择可以通过选择弦世界上受铁离子对称性保护的拓扑相来解决。这种观点不仅可以轻松地解释为什么本质上存在两种II型理论，而且可以预测存在以标记的无方向性0型理论。$ñ$mod 8，而我对类型I世界工作表理论有一个本质上独特的选择。我们还讨论了这种观点与$ķ$脑的理论分类。