24n+1 cannot be a prime if 3|n and n>0
My Try:
2^{12k}+1\equiv (-1)^{3k}+1 \equiv0\pmod{17}
So it divisible by 17 for odd k. But how to complete the proof?
Answer
Hint: 16^{3k} +1 = (16^k+1)(16^{2k}-16^k+1).
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