Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2003/ib/paper-3/19g/a/solution

For any idempotent linear map , each vector decomposes as
If , write ; then . Thus the kernel of a linear map and image of a linear map give a direct sum.
For the self-adjoint map in the question, and satisfy . Hence the direct sum is an orthogonal decomposition:

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