They use the Lucas–Lehmer primality test [0], along with trial factoring candidate numbers up to a given bit depth to sift out some composites from the prime ca

They use the Lucas–Lehmer primality test [0], along with trial factoring candidate numbers up to a given bit depth to sift out some composites from the prime candidates quicker than a full LL test. [0] https://en.wikipedia.org/wiki/Lucas%E2%80%93Lehmer_primality_test https://en.wikipedia.org/wiki/Lucas%E2%80%93Lehmer_primality...