520 = 23 x 5 x 13.
520 has two representations as a sum of two squares: 520 = 62 + 222 = 142 + 182.
520 is the number of ways to place two non-attacking kings on a six-by-six chessboard.
520 is 4040 in base 5 and 1010 in base 8.
520 is the sum of two successive primes: 520 = 257 + 263.
NGC 520 appears to be a pair of colliding galaxies.
