Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2019/ii/paper-1/21f/c/solution

No. The monodromy action of a covering space for a two-sheeted covering takes values in . Its nonidentity transposition commutes with every subgroup of , so the deck transformation group as a monodromy centralizer always contains that transposition. Equivalently, every double cover has a deck involution of a double covering that exchanges the two points in each fibre. Thus a two-sheeted cover cannot have trivial deck group.

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