Solution

ID: past-exam-of-the-mathematics-course-of-the-year-2-of-the-natural-sciences-course-of-the-university-of-cambridge/2019/paper-1/8/b/solution

After normalization, and . Both are analytic at zero, so . At one,
are analytic, whereas themselves have poles. Hence .
Insert . The coefficient of gives ; for the coefficient of gives
Thus for . The initial data give , , so
The radius is one because the three cube roots of unity are poles of this solution. A direct structural check observes that the differential equation is exactly . Thus , and the initial data select . This is the product-derivative reduction of a second-order differential equation.

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