421

421 is a prime number.

419 and 421 are twin primes.

421 is a centered square number.

421 is the smallest number formed by the powers of two in logical order from right to left.

421 is 111 in base 20.

421 is the sum of five consecutive prime numbers: 421 = 73 + 79 + 83 + 89 + 97.

421 has a representation as a sum of two squares: 421 = 14² + 15².

421 is the hypotenuse of a primitive Pythagorean triple: 421² = 29² + 420².

All integers ever tested in the Collatz problem eventually end in the sequence 4-2-1.

Source: Prime Curios!