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Robin Chapman
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As Daniel has pointed out, there is an elementary proof that for each $n$ there are infinitely many primes $p$ with $p\equiv1$ (mod $n$). There is an also an elementary proof that for each $n$ there are infinitely many primes $p$ with $p\equiv-1$ (mod $n$). This can be found in Nagell's Introduction to Number Theory section 50 in the second edition.

Robin Chapman
  • 20.8k
  • 2
  • 66
  • 81