1669

1669 is a prime number.

1667 and 1669 form a twin prime pair.

1669 is the smallest prime that appears in the same position as its own value when the Roman numerals (from 1 to 3999) are placed in lexicographic order. The other primes with this property are 3623 and 3631.

1669 is the smallest number whose 9th power has 29 digits.

1669² = 2785561, and 278 * (5/5) * 6 + 1 = 1669.

1669 has a representation as the sum of two squares: 1669 = 15² + 38².

1669 is the hypotenuse of a primitive Pythagorean triple: 1669² = 1140² + 1219².

Source: Prime Curios!