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Sep 3 at 19:37 comment added Max Alekseyev @joro: Ok, why are you asking then? Btw, the same argument applies to $2^{2k}-1$, where $k\equiv 1,5\pmod 6$, not necessarily prime.
Sep 3 at 16:22 comment added joro It is proved in the question: $4^p-1=(2^p+1)(2^p-1)$ and $2^p+1$ is divisible by $3$ with exponent one.
Sep 3 at 16:16 comment added Max Alekseyev @joro: Most likely, but I don't see how to prove that.
Sep 3 at 11:40 comment added joro So if $p$ is prime, $4^p-1$ is never powerful, right?
Aug 22 at 20:46 history edited Max Alekseyev CC BY-SA 4.0
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Aug 22 at 20:41 history answered Max Alekseyev CC BY-SA 4.0