65

65 = 5 x 13.

65 is the smallest number that becomes square if its reverse is either added to or subtracted from it: 65 + 56 = 121 = 11²; 65 - 56 = 9 = 3².

65 is the smallest hypotenuse of two different primitive Pythagorean triangles (33, 56, 65 and 16, 63, 65).

65 is the second number to be the sum of two squares in two ways: 65 = 8² + 1² = 7² + 4². It is the first such number to be the sum of two cubes: 65 = 4³ + 1³.


65 is the magic constant of the 5 by 5 magic square.

Source: Number Gossip