Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2005/calculus-linear-algebra-discrete-mathematics-logic-and-proof-and-models-of-computation/2/c/solution

Let be the single eigenvalue. By diagonalizability, there is a basis of eigenvectors, and every corresponding eigenvalue is . For any , linearity gives
Thus , a scalar multiple of the identity map.
A counterexample to the conclusion without diagonalizability is
Its characteristic polynomial is , but . The eigenspace for is therefore , of dimension one. There is no basis of eigenvectors in dimension two, so is not diagonalizable.

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