657

657 = 3 x 3 x 73.

657 is 1010010001 in base 2 (binary). It is 1221 in base 8.

657 has a representation as a sum of two squares: 657 = 9² + 24².

657 is the largest known integer that cannot be written as the sum of a semiprime and a square.

657 is the number of ways to tile a 4 x 22 rectangle with 4 x 1 rectangles.

657 = 219 + 438 uses each of the digits from 1 to 9 once.

Source: Prime Curios!