Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2024/iii/paper-101/2/iv/solution

Yes. Let be maximal and set
Then is a field generated as a -algebra by countably many elements. Since the polynomial ring in countably many variables has a countable monomial basis, has at most countable dimension as a -vector space.
Suppose were transcendental over . The family
would be linearly independent over . Indeed, after multiplying a finite relation by , evaluation at forces the th coefficient to vanish. This would be an uncountable linearly independent subset of the countable-dimensional vector space , a contradiction.
Thus is algebraic. Since is an algebraically closed field, . If is the image of , the quotient map is evaluation at and

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