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Number theory Difficulty 6.0 National olympiad Prove it Saudi Arabia

Find all primes q1,q2,q3,q4,q5q_{1}, q_{2}, q_{3}, q_{4}, q_{5} such that q14+q24+q34+q44+q54q_{1}^{4} + q_{2}^{4} + q_{3}^{4} + q_{4}^{4} + q_{5}^{4} is the product of two consecutive even integers.

Solution

Assume that q14+q24+q34+q44+q54=2k(2k+2)q_{1}^{4} + q_{2}^{4} + q_{3}^{4} + q_{4}^{4} + q_{5}^{4} = 2k(2k+2) for some positive integer kk. That is
q14+q24+q34+q44+q54=4k(k+1) q_{1}^{4} + q_{2}^{4} + q_{3}^{4} + q_{4}^{4} + q_{5}^{4} = 4k(k+1)
If pp is a prime, p2p \neq 2, then p41(mod8)p^{4} \equiv 1 \pmod{8}.
Assume that q1q2q3q4q5q_{1} \leq q_{2} \leq q_{3} \leq q_{4} \leq q_{5}. A simple parity argument shows that q1=2q_{1} = 2. If q22q_{2} \neq 2, then we have
q14+q24+q34+q44+q544(mod8) q_{1}^{4} + q_{2}^{4} + q_{3}^{4} + q_{4}^{4} + q_{5}^{4} \equiv 4 \pmod{8}
Since 84k(k+1)168 \mid 4k(k+1) - 16, we get a contradiction. Therefore q2=2q_{2} = 2. In similar way it follows q3=q4=q5=2q_{3} = q_{4} = q_{5} = 2.
The only solution is q1=q2=q3=q4=q5=2q_{1} = q_{2} = q_{3} = q_{4} = q_{5} = 2, and k=4k = 4.

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