668

668 = 2² x 167.

668 is the number of integer partitions of 36 into distinct parts.

668 is a number that cannot be written as a sum of three squares.

668 = 3⁶ - 61.

334,668 = 668² - 334² = (334 + 668) x 334, where 668 = 2 x 334.

668 is the number of legal pawn moves in chess.

Source: What's Special About This Number?