Solution

ID: past-exam-of-the-numerical-linear-algebra-and-continuous-optimisation-course-of-the-university-of-oxford/2010/a/3/c/solution

An eigenvalue of is a scalar for which has a nonzero eigenvector. Allowing initially , symmetry of the real matrix gives , so is real. Since ,
This proves reality without presuming it when choosing the eigenvector. Once is real, the real or imaginary part of supplies a nonzero real eigenvector.

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