131071

This number is a prime.

131071 is the largest known prime of form 2^(2^k+1)-1, where k = 4. [Luhn]

The concatenation of first four happy numbers in reverse order is prime. [Gupta]

Pietro Cataldi verified that 131071 is a Mersenne prime in the year 1588. Did the defeat of the Spanish Armada have something to do with it?

The largest nth Mersenne prime with n digits. [Beedassy]

131071 is the 35th Fibonacci 17-step number, the first odd prime of this type. Note that 2^17-1 = 131071, making it the 6th Mersenne prime. [Loungrides]