Bourbaki

Exercise 9, § 3

Integration, VII, p. 93

Let (e1,e2,e3)(e_1, e_2, e_3) be the canonical basis of R3\mathbf{R}^3. Every matrix σ∈SO(3,R)\sigma \in SO(3, \mathbf{R}) such that σ(e3)≠e3\sigma(e_3) \neq e_3 and σ(e3)≠−e3\sigma(e_3) \neq -e_3 may be written in a unique way as

σ(φ,ψ,θ)=(cos⁡φcos⁡ψ−sin⁡φsin⁡ψcos⁡θ−cos⁡φsin⁡ψ−sin⁡φcos⁡ψcos⁡θsin⁡φsin⁡θsin⁡φcos⁡ψ+cos⁡φsin⁡ψcos⁡θ−sin⁡φsin⁡ψ+cos⁡φcos⁡ψcos⁡θ−cos⁡φsin⁡θsin⁡ψsin⁡θcos⁡ψsin⁡θcos⁡θ), \sigma(\varphi, \psi, \theta) = \begin{pmatrix} \cos \varphi \cos \psi - \sin \varphi \sin \psi \cos \theta & -\cos \varphi \sin \psi - \sin \varphi \cos \psi \cos \theta & \sin \varphi \sin \theta \\ \sin \varphi \cos \psi + \cos \varphi \sin \psi \cos \theta & -\sin \varphi \sin \psi + \cos \varphi \cos \psi \cos \theta & -\cos \varphi \sin \theta \\ \sin \psi \sin \theta & \cos \psi \sin \theta & \cos \theta \end{pmatrix},

where 0<θ<π0 < \theta < \pi, 0≤φ<2π0 \leq \varphi < 2\pi, 0≤ψ<2π0 \leq \psi < 2\pi (Euler's angles). Show that sin⁡θ dθ dφ dψ\sin \theta \, d\theta \, d\varphi \, d\psi is a Haar measure on SO(3,R)SO(3, \mathbf{R}). (Identify the homogeneous space SO(3,R)/SO(2,R)SO(3, \mathbf{R})/SO(2, \mathbf{R}) with S2S_2, use Exer. 8, and Th. 3 of §2.)

Solution

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Transcribed from the printed volume. Not translated.