Let us show that there are infinitely many primes p≡1(mod 5). Let us prove by contraposition. Suppose that there are finitely many such primes, and let these primes be pi for 1≤i≤n. Let m=5p1p2...pn+1. Then m≡1(mod 5) and m>pi for all 1≤i≤n. Therefore, m is a composite number, and thus ps divides m for some 1≤s≤n. Consequently, m=kps, that is ps(5p1...ps−1ps+1...pn)+1=kps. It follows that ps divides kps−ps(5p1...ps−1ps+1...pn)=1. It is impossible because of ps>1. This contradictions proves that there are infinitely many primes p≡1(mod 5).
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