39

39 = 3 x 13.

39 is the sum of the numbers 4 through 9: 4 + 5 + 6 + 7 + 8 + 9 = 39.

39 is the sum of the sixth, seventh, and eighth odd numbers: 11 + 13 + 15 = 39.

39 is the smallest number whose sum of digits is larger than the sum of digits of its square (39² = 1521): 3 + 9 = 12; 1 + 5 + 2 + 1 = 9.

39 is the smallest number with six representations as a sum of three distinct primes: 39 = 3 + 5 + 31 = 3 + 7 + 29 = 3 + 13 + 23 = 3 + 17 + 19 = 5 + 11 + 23 = 7 + 13 + 19.

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

39 is 33 in base 12.

The Thirty-Nine Steps is a spy story by John Buchan.

Source: Number Gossip