Past exam of the mathematics course of the University of Cambridge 2019 ii Paper 1 21F b Solution Created 2026-09-24 Updated 2026-09-29
Take , the wedge of two circles, and label its oriented loops and . For a fixed , letbe permutations of the fibre . Construct a labelled graph with these vertices, an oriented -edge from to , and an oriented -edge from to . Map every vertex to the wedge point and each labelled edge homeomorphically onto its corresponding loop. Every vertex has exactly one incoming and one outgoing edge of each label, so this is an -sheeted permutation covering of a wedge of circlesIt is connected because the -cycle acts transitively on the fibre.
A deck transformation induces a permutation of the vertices that commutes with both monodromy permutations and . Conversely, such a permutation determines a deck transformation, by the deck transformation group as a monodromy centralizer. The permutations and generate : the conjugates include the adjacent transpositions, which generate the symmetric group. Hence centralizes and lies in its center of a group. For , the center of is trivial, so . A deck transformation fixing one point is the identity by part (a), and therefore