A number which is not prime, but such that
is divisible by
is called a base-2 Fermat pseudoprime, or simply
a Poulet or Sarrus number.
Up to there are
1801533
Poulet numbers.
The first Poulet numbers are 341, 561, 645, 1105, 1387, 1729, 1905, 2047, 2465, 2701, 2821, 3277, 4033, 4369, 4371, 4681, 5461, 6601 more terms