In a book on complex analysis, the authors prove:
Given finitely many (non-trivial) arithmetic progressions of natural numbers a1,a1+d1,a1+2d1,⋯
a2,a2+d2,a2+2d2,⋯,
ak,ak+dk,ak+2dk,⋯,
their totality (union) is never N.
Given: any k (non-trivial) arithmetic progressions.
Let S denote the set of those numbers which are not in any of above k arithmetic progressions.
If S would have been finite, then we can form new finitely many arithmetic progressions which covers S, and combining them with given progressions, we will cover N by finitely many arithmetic progressions, a contradiction.
Thus S, the complement of all the given k progressions, must be infinite.
Question Does S contain infinitely many primes?
A non-trivial arithmetic progression means, the common difference is not 1, i.e. it is not of the type
a,a+1,a+2,⋯
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