821

821 is a prime number.

821 has a representation as a sum of two squares: 821 = 14² + 25².

821 and 823 form a twin prime pair.

821 is the hypotenuse of a primitive Pythagorean triple: 821² = 429² + 700².

821 is the maximum number of regions into which 40 lines divide the plane.

821 is the smallest prime whose reversal is a seventh power: 128 = 2⁷.

821 = 6! + 5! - 4! + 3! - 2! + 1!

Aeroflot Flight 821 crashed on approach to Perm airport on Sept. 14, 2008.

Source: Prime Curios!