521

521 is a prime number.

521 has a representation as a sum of two squares: 521 = 11² + 20².

521 is the 13th Lucas number.

521 and 523 for a twin prime pair.

521 is the hypotenuse of a primitive Pythagorean triple: 521² = 279² + 440².

521 has the representation 521 = 2⁹ + 9.

521 is the exponent of the Mersenne prime 2⁵²¹ - 1.

521 is the smallest prime whose reversal is a cube: 125 = 5³.

Galaxy cluster Abell 521 is located about 2.9 billion light-years from Earth.

Source: Prime Curios!