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Let $\ n\ $ be an arbitrary natural number ($\ 1\ 2\ \ldots).\ $ Then

  • $\ n\ $ is coarse $\ \Leftarrow:\Rightarrow\ $ there exists a prime divisor $p$ of $\ n\ $ such that $\ p^3>n.$;
  • $\ n\ $ is a p-cube $\ \Leftarrow:\Rightarrow\ $ the positive cubical root of $\ n\ $ is a prime number;
  • $\ n\ $ is fine $\ \Leftarrow:\Rightarrow\ p^3<n\ $ for every prime divisor $\ p\ $ of $\ n$.

Example:   Natural $\ 64\ $ and $$ 4095=64^2-1\ = 3^2\cdot 5\cdot 7\cdot13 $$ are both fine. However,

$$ 4097=64^2+1=17\cdot241 $$

is coarse.

QUESTION   Does there exist a fine natural number $\ n\ $ such that both $\ n^2-1\ $ and $\ n^2+1\ $ are fine too? (My guess: perhaps NOT).

Also, I don't expect that there is any p-cube $\ n\ $ such that both $\ n^2-1\ $ and $\ n^2+1\ $ are fine.

On the other hand, I believe that there are infinitely many coarse $\ n\ $ such that both $\ n^2-1\ $ and $\ n^2+1\ $ are fine (as rare as they may be).

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    $\begingroup$ I expect the answer to be positive: being fine has positive probability (related to Dickman distribution), and it is unlikely that being of the $n^2+1$ changes this dramatically or that these events are highly dependent $\endgroup$ Commented Jun 28, 2021 at 7:41
  • $\begingroup$ When $\ n\ $ is fine then it either has several different prime divisors or some divisors are raised to a power in $\ n.\ $ Thus, in the first case, these prime divisors are avoided by both $\ n^2\pm1,\ $ and in the other case, the powers $\pm1\ $ tend to lean toward large prime divisors -- my vague intuitions. $\endgroup$
    – Wlod AA
    Commented Jun 28, 2021 at 7:49
  • $\begingroup$ @PeterTaylor, Thank you for catching my error. $\endgroup$
    – Wlod AA
    Commented Jun 28, 2021 at 7:57
  • $\begingroup$ @PeterTaylor, I fixed my strange error, thank you. $\endgroup$
    – Wlod AA
    Commented Jun 28, 2021 at 8:11

2 Answers 2

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$n = 2673$ has largest prime factor $11$ whose cube is $1331$.
$n^2 - 1 = 7144928$ has largest prime factor $191$ whose cube is $6967871$.
$n^2 + 1 = 7144930$ has largest prime factor $61$ whose cube is $226981$.

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    $\begingroup$ $n=10625$ is the only other example up to 12000. $\endgroup$ Commented Jun 28, 2021 at 8:15
  • $\begingroup$ I risked (and lost :-)). One could still try for infinitely many examples. @FedorPetrov, see above, feels that there should be an infinite sequence. – $\endgroup$
    – Wlod AA
    Commented Jun 28, 2021 at 8:39
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    $\begingroup$ 2673, 10625, 12168, 14651, 24167, 28800, 32085, 34162, 48279, 50692, 59892, 60928, 61347, 61952, 64960, 66125, 66339, 70400, 71995, 74704, 80730, 83028, 89424, 93024, 96348, .... Not yet in OEIS. @WlodAA would you like to contribute it? $\endgroup$ Commented Jun 28, 2021 at 13:35
  • $\begingroup$ @RobertIsrael. that's very kind of you. I have all kinds of problems with my Internet and computer system. I'd be very grateful if you'd like to insert this sequence into OEIS. $\endgroup$
    – Wlod AA
    Commented Jun 29, 2021 at 5:05
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    $\begingroup$ OEIS sequence A345896. $\endgroup$ Commented Jul 1, 2021 at 17:24
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For any $k\in\mathbb{N}$, the number $n=2^3\cdot3^{72k}$ works:

Obviously, $n$ itself is fine.

For $n^2-1 = 2^6\cdot3^{144k}-1 = (2^2\cdot 3^{48 k} + 2\cdot 3^{24 k} + 1)(2^2\cdot 3^{48 k}-2\cdot 3^{24 k} + 1)(2\cdot3^{24 k} + 1) (2\cdot3^{24 k} - 1)$, the first factor is divisible by $7$, the others are small, hence $n^2-1$ is fine.

For $n^2+1 = 2^6\cdot3^{144k}+1 = (2^2\cdot3^{48k}+2^2\cdot3^{36k}+2\cdot3^{24k}+2\cdot3^{12k}+1)(2^2\cdot3^{48k}-2^2\cdot3^{36k}+2\cdot3^{24k}-2\cdot3^{12k}+1)(2\cdot3^{24k}+2\cdot3^{12k}+1)(2\cdot3^{24k}-2\cdot3^{12k}+1)$, the first factor is divisible by $13$, the others are small, hence $n^2+1$ is fine.

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  • $\begingroup$ Excellent! A nice elementary approach. This went in a perfect direction; it suggests general related results. Perhaps you could use this method in the other thread too. One "simply" select proper exponents and proper bases that go well with the exponents and extra divisors. $\endgroup$
    – Wlod AA
    Commented Jun 29, 2021 at 8:42

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