Number theoryDifficulty 6.0National olympiadProve itSaudi Arabia
Find all primes q1,q2,q3,q4,q5 such that q14+q24+q34+q44+q54 is the product of two consecutive even integers.
Solution
Assume that q14+q24+q34+q44+q54=2k(2k+2) for some positive integer k. That is q14+q24+q34+q44+q54=4k(k+1) If p is a prime, p=2, then p4≡1(mod8). Assume that q1≤q2≤q3≤q4≤q5. A simple parity argument shows that q1=2. If q2=2, then we have q14+q24+q34+q44+q54≡4(mod8) Since 8∣4k(k+1)−16, we get a contradiction. Therefore q2=2. In similar way it follows q3=q4=q5=2. The only solution is q1=q2=q3=q4=q5=2, and k=4.
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Source: MathNet,
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