1399

This number is a prime.

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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]