1521

1521 = 3 x 3 x 13 x 13.

1521 is a perfect square: 1521 = 39².

1521 is the smallest number that can be written as the sum of four distinct cubes in three ways: 1521 = 11³ + 5³ + 4³ + 1³ = 10³ + 8³ + 2³ + 1³ = 9³ + 8³ + 6³ + 4³.

1521 has a representation as a sum of two squares: 1521 = 15² + 36².

1521 is a divisor of 22³ - 1.

1521 is a centered octagonal number.

Source: What's Special About This Number?