Banach algebras, Samelson products, and the Wang Differential
  Supppose given a principal $G$ bundle $\zeta : P \to S^k$ (with $k \geq 2$) and a Banach algebra $B$ upon which $G$ acts continuously. Let \[ \zeta\otimes B : \qquad P \times_G B \longrightarrow S^k \] denote the associated bundle and let \[ A_{\zeta\otimes B} = \Gamma (S^k, P \times_G B) \] denote the associated Banach algebra of sections. Then $\pi_*\GL A_{\zeta \otimes B} $ is determined by a mostly degenerate spectral sequence and by a Wang differential \[ d_k : \pi_*(\GL B) \longrightarrow \pi_{*+k-1} (\GL B) .\] We show that if $B$ is a $C^*$-algebra then the differential is given explicitly in terms of an \esp\, with the clutching map of the principal bundle. Analogous results hold after localization and in the setting of topological $K$-theory.   We illustrate our technique with a close analysis of the invariants associated to the $C^*$-algebra of sections of the bundle \[ \zeta\otimes M_2 : \qquad S^7 \times_{S^3} M_2 \to S^4 \] constructed from the Hopf bundle $\zeta: \,S^7 \to S^4$ and by the conjugation action of $S^3$ on $M_2 = M_2(\CC)$. We compare and contrast the information obtained from the homotopy groups $\pi_*(A_{\zeta\otimes M_2})$, the rational homotopy groups $\pi_*(A_{\zeta\otimes M_2})\otimes\QQ $ and the topological $K$-theory groups $K_*(A_{\zeta\otimes M_2})$. 
Claude L. Schochet
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