Maths Olympiad Prep

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Number theory Difficulty 6.7 National olympiad Prove it

Let p>5p>5 be a prime number. Show that p4p-4 cannot be the fourth power of a prime number.

Solution

1. Let p p be a prime number greater than 5. Assume for the sake of contradiction that p4 p - 4 is the fourth power of a prime number. Let q q be a prime number such that p4=q4 p - 4 = q^4 .

2. Then we have:
p=q4+4 p = q^4 + 4

3. We can rewrite q4+4 q^4 + 4 using the Sophie Germain identity:
q4+4=(q2+2)2(2q)2=(q22q+2)(q2+2q+2) q^4 + 4 = (q^2 + 2)^2 - (2q)^2 = (q^2 - 2q + 2)(q^2 + 2q + 2)

4. Since q q is a prime number and q>1 q > 1 , both factors q22q+2 q^2 - 2q + 2 and q2+2q+2 q^2 + 2q + 2 are greater than 1.

5. For p p to be a prime number, one of the factors must be equal to 1. However, we will show that neither q22q+2 q^2 - 2q + 2 nor q2+2q+2 q^2 + 2q + 2 can be equal to 1.

6. Consider the factor q22q+2 q^2 - 2q + 2 :
q22q+2=1    q22q+1=0    (q1)2=0    q=1 q^2 - 2q + 2 = 1 \implies q^2 - 2q + 1 = 0 \implies (q-1)^2 = 0 \implies q = 1
But q=1 q = 1 is not a prime number, so q22q+21 q^2 - 2q + 2 \neq 1 .

7. Now consider the factor q2+2q+2 q^2 + 2q + 2 :
q2+2q+2=1    q2+2q+1=0    (q+1)2=0    q=1 q^2 + 2q + 2 = 1 \implies q^2 + 2q + 1 = 0 \implies (q+1)^2 = 0 \implies q = -1
But q=1 q = -1 is not a prime number, so q2+2q+21 q^2 + 2q + 2 \neq 1 .

8. Since neither q22q+2 q^2 - 2q + 2 nor q2+2q+2 q^2 + 2q + 2 can be equal to 1, both factors are greater than 1. This implies that p p is a product of two integers greater than 1, which contradicts the assumption that p p is a prime number.

Therefore, p4 p - 4 cannot be the fourth power of a prime number.

\blacksquare

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.