Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2010/ii/paper-3/11g/i/solution

Since is odd, and
is a product of two odd integers greater than one. Thus is odd and composite. Also . Fermat's theorem modulo gives , and , so . This number is even; hence . Therefore
proving that is a base-two Fermat pseudoprime.

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