Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2025/ii/paper-4/18j/c/solution

For , the answer is yes. A root of is or , so
For , the answer is no. Every is finite abelian Galois. If belonged to , then it would belong to some . The subextensions of an abelian Galois extension result would make
Galois. This is false: has two nonreal roots absent from the real field . Hence is a proper subfield of .
For , the answer is yes. Given , take
which is coprime to . Every root of in characteristic has exact multiplicative order . The degree
is the least positive such that . The value works, and no can work because
Therefore
Every element algebraic over lies in some finite field , so
These are respectively the maximal cyclotomic extension of the real numbers, the maximal cyclotomic extension of the rational numbers, and the maximal cyclotomic extension of a finite field.

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