1.II.5B

Algebra and Geometry | Part IA, 2003

(a) In the standard basis of R2\mathbb{R}^{2}, write down the matrix for a rotation through an angle θ\theta about the origin.

(b) Let AA be a real 3×33 \times 3 matrix such that detA=1\operatorname{det} A=1 and AAT=IA A^{\mathrm{T}}=I, where ATA^{\mathrm{T}} is the transpose of AA.

(i) Suppose that AA has an eigenvector v\mathbf{v} with eigenvalue 1 . Show that AA is a rotation through an angle θ\theta about the line through the origin in the direction of v\mathbf{v}, where cosθ=12(\cos \theta=\frac{1}{2}( trace A1)A-1).

(ii) Show that AA must have an eigenvector v\mathbf{v} with eigenvalue 1 .

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