Parity problem
question of how many primes less than a given integer have an even (or odd) number of prime factors
en-GB/pæɹɪtiː ˈpɹɒbləm/
en-US/ˈpæɹɪti ˈpɹɑbləm/
question of how many primes less than a given integer have an even (or odd) number of prime factors