Maths Olympiad Prep

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Number theory Difficulty 5.1 AIME, harder Prove it Singapore

Suppose that a1,,a15a_1, \dots, a_{15} are prime numbers forming an arithmetic progression with common difference d>0d > 0. If a1>15a_1 > 15, prove that d>30,000d > 30,000.

Solution

Lemma: Suppose pp is prime and a1,,apa_1, \dots, a_p are primes forming an A.P. with common difference dd. If a1>pa_1 > p, we claim that pdp \mid d.

Proof: Since pp is prime and every aia_i is a prime >p> p, pp does not divide aia_i for any ii. By the pigeonhole principle, there exist 1i<jp1 \le i < j \le p so that aiaj(modp)a_i \equiv a_j \pmod{p}. Now ajai=(ji)da_j - a_i = (j-i)d, and pp does not divide jij-i. So pp must divide dd.

Apply the Lemma to the sequences a1,,aka_1, \dots, a_k for k=2,3,5,7,11k = 2, 3, 5, 7, 11 and 1313. Then all such kk's are factors of dd. So d>23571113>30,000d > 2 \cdot 3 \cdot 5 \cdot 7 \cdot 11 \cdot 13 > 30,000.

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