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Section 16.7 Exercises

Exercise 16.23.

Let \(p \colon E \to B\) be a vector bundle of rank \(k\text{.}\) Suppose that \(\Phi \colon p^{-1}(U) \to U \times \mathbb{R}^k\) and \(\Psi \colon p^{-1}(V) \to V \times \mathbb{R}^k\) are two local trivializations with \(U \cap V \neq \emptyset\text{.}\) Show that there is a continuous map \(\tau \colon U \cap V \to \mathrm{GL}(k, \mathbb{R})\text{,}\) where \(\mathrm{GL}(k, \mathbb{R})\) is the space of invertible real \(k \times k\) matrices, such that the composite
\begin{equation*} \Phi \circ \Psi^{-1} \colon (U \cap V) \times \mathbb{R}^k \longrightarrow (U \cap V) \times \mathbb{R}^k \end{equation*}
has the form \((\Phi \circ \Psi^{-1})(p, v) = (p, \tau(p)v)\text{.}\)

Exercise 16.24.

If \(M\) is a filtered colimit of commutative monoids \(M_\alpha\text{,}\) show that \(M^{\mathrm{gp}}\) is the filtered colimit of the \(M_\alpha^{\mathrm{gp}}\text{.}\)

Exercise 16.25.

Show that if \(A\) is a commutative semi-ring (i.e. a commutative ring but without additive inverses), then \(A^{\mathrm{gp}}\) is a commutative ring.

Exercise 16.26.

Show that \(K(S^1) \cong \mathbb{Z}\text{.}\)
Hint: cut \(S^1\) into two arcs, over each of which a bundle is trivial, and use Exercise 16.23 to describe the bundle by a single map \(S^0 \to \mathrm{GL}(k, \mathbb{C})\text{.}\) Then use that \(\mathrm{GL}(k, \mathbb{C})\) is path-connected (Hatcher, Vector Bundles and K-Theory, Example 1.12).