253

This number is a composite.

2⁵³ - 1 is the largest prime that the prime number tester on this site will test. [Dunn]

2^5-3 is prime. [Dunn]

The smallest value of A(3,k) which is not a prime or prime power. A(m,n) is Ackerman's function. [Hartley]

The only prime-digit brilliant number, (253 = 11 x 23), whose concatenations of prime factors in both orders, i.e., 1123 and 2311, are primes. [Loungrides]