2401

This number is a composite.

2401 = (2+4+0+1)^pi(2+4+0+1). It is the smallest multidigit number with this property and there exists only one larger such number. [Firoozbakht]

Smallest fourth power that becomes prime when its most significant (or left-most) digit is deleted. The next is 279841. [Post]

The smallest multidigit square, such that the sum of its aliquot divisors is also a square. [Gupta]

The smallest square n such that sigma(n) and sigma(n) + 2 are both primes. [Gupta]