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Number theory Difficulty 7.6 National Olympiad, round 2 Prove it Turkey

Find all prime numbers pp for which the number
3p+4p+5p+9p98 3^p + 4^p + 5^p + 9^p - 98
has at most 6 positive divisors.

Solution

Answer: p=2,3,5p = 2, 3, 5.
Let f(p)=3p+4p+5p+9p98f(p) = 3^p + 4^p + 5^p + 9^p - 98. The primes 2,3,52, 3, 5 satisfy the following conditions: f(2)=311f(2) = 3 \cdot 11, f(3)=7112f(3) = 7 \cdot 11^2, f(5)=79049f(5) = 7 \cdot 9049.

Let p>5p > 5. Since 77 divides 3p+4p3^p + 4^p, 5p+9p5^p + 9^p and 9898 we get 7f(p)7 \mid f(p). Now since
p1(mod10)    f(p)3+4+5+9+10(mod11), p \equiv 1 \pmod{10} \implies f(p) \equiv 3+4+5+9+1 \equiv 0 \pmod{11},
p3(mod10)    f(p)5+9+4+3+10(mod11), p \equiv 3 \pmod{10} \implies f(p) \equiv 5+9+4+3+1 \equiv 0 \pmod{11},
p7(mod10)    f(p)9+5+3+4+10(mod11), p \equiv 7 \pmod{10} \implies f(p) \equiv 9+5+3+4+1 \equiv 0 \pmod{11},
p9(mod10)    f(p)4+3+9+5+10(mod11) p \equiv 9 \pmod{10} \implies f(p) \equiv 4+3+9+5+1 \equiv 0 \pmod{11}
we get that 11f(p)11 \mid f(p). Since f(p)f(p) has at most 6 positive divisors the only prime divisors of f(p)f(p) are 77 and 1111. On the other hand, for p>5p > 5 we have f(p)>7112>7211>711f(p) > 7 \cdot 11^2 > 7^2 \cdot 11 > 7 \cdot 11. Therefore, there is no p>5p > 5 satisfying conditions.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.