A prime $p$ is in class $1$ if the only prime divisors of $p+1$ are
$2$ or $3$. In general, a prime $p$ is in class $r$ if every prime factor
of $p+1$ is in some class $\leq r-1$, with equality for at least one prime factor.
If $p_r$ is the least prime in class $r$, then how does $p_r^{1/r}$ behave?
Selfridge conjectured that this is bounded.
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