403

403 = 13 x 31. It is the smallest semiprime whose factors form a pair of emirps (13 and 31).

403 is 110010011 in base 2.

403 has a unique representation as a sum of three squares: 403 = 3² + 13² + 15².

403 is a heptagonal number.

403 = 2³ + 3³ + 3³ + 5³ + 6³.

Source: Prime Curios!