Question #161102
  • Show that there are infinitely many primes p≅ 1(mod5).
1
Expert's answer
2021-02-12T18:06:16-0500

Let us show that there are infinitely many primes p1(mod 5)p\equiv 1(mod\ 5). Let us prove by contraposition. Suppose that there are finitely many such primes, and let these primes be pip_i for 1in1\le i\le n. Let m=5p1p2...pn+1m=5p_1p_2...p_n+1. Then m1(mod 5)m\equiv 1(mod\ 5) and m>pim>p_i for all  1in1\le i\le n. Therefore, mm is a composite number, and thus psp_s divides mm for some 1sn1\le s\le n. Consequently, m=kpsm=kp_s, that is ps(5p1...ps1ps+1...pn)+1=kpsp_s(5p_1...p_{s-1}p_{s+1}...p_n)+1=kp_s. It follows that psp_s divides kpsps(5p1...ps1ps+1...pn)=1kp_s-p_s(5p_1...p_{s-1}p_{s+1}...p_n)=1. It is impossible because of ps>1.p_s>1. This contradictions proves that there are infinitely many primes p1(mod 5)p\equiv 1(mod\ 5).



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