Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-oxford/2023/prelims/mathematics-iv/7/c/ii/solution

The gradient and constant Hessian matrix are
At , and . Hence one multivariable Newton iteration gives
At this point the gradient vanishes, so every subsequent iterate is . The one-step result follows because the gradient of a quadratic polynomial is affine and its linearization is exact. Since the leading principal minors of are and , it is a positive-definite matrix. Alternatively,
directly proves that is the unique global minimum, with value .

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