The Semispin Groups in String Theory
In discussions of the T-duality between the two heterotic string theories, the duality is actually implemented through the "common" SO(16) x SO(16) subgroup of "SO(32)" and E_8 x E_8. In fact, however, a global investigation shows that no such "common" subgroup exists. This paper is a survey of the relevant global properties of Spin(32)/Z_2 and its subgroups.