Infinitude of Sophie Germain primes
Determine whether there are infinitely many primes p such that 2p + 1 is also prime (i.e., whether there are infinitely many Sophie Germain primes).
References
Note that to this day it is still not known if there are infinitely many such primes.
— Understanding Fermat's Last Theorem's Proofs
(2508.10362 - Qiu et al., 14 Aug 2025) in Section 2.1 (Classical Contributions)