1399
This number is a prime.
The consecutive primes 1399 and 1409 have the property that
1399*1409 +/- (1399+1409) are both prime. Moreover,
7^1399-6^1399 is prime, and so is (12*10^1399-21)/99 =
1212...2121, a smoothly undulating palindromic prime with
1399 digits. [Hasler]
In 1882, Lucas noted that Le Lasseur found the first known prime factor (1399)
of the Mersenne number 2233-1.
The sum of all 3 3-digit primes ending with 33 is prime. [Bopardikar]
1399 is the smallest prime such that 1399+10, 1399+100,
1399+1000, 1399+10000 and 1399+100000 are primes. [Rivera]
The 65th emirp (1399) has a Collatz trajectory length of
65. [Gaydos]
The smallest prime number whose prime index (222) and sum
of digits (22) together use only one digit. The next two
such primes are 867746501 (44444444 and 44) and 1097774749
(55555555 and 55). [Gaydos]