Let be a prime number. Show that cannot be the fourth power of a prime number.
Solution
1. Let be a prime number greater than 5. Assume for the sake of contradiction that is the fourth power of a prime number. Let be a prime number such that .
2. Then we have:
3. We can rewrite using the Sophie Germain identity:
4. Since is a prime number and , both factors and are greater than 1.
5. For to be a prime number, one of the factors must be equal to 1. However, we will show that neither nor can be equal to 1.
6. Consider the factor :
But is not a prime number, so .
7. Now consider the factor :
But is not a prime number, so .
8. Since neither nor can be equal to 1, both factors are greater than 1. This implies that is a product of two integers greater than 1, which contradicts the assumption that is a prime number.
Therefore, cannot be the fourth power of a prime number.