39

Number
$39$ (thirty-nine) is:


 * $3 \times 13$


 * The $15$th semiprime after $4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38$:
 * $39 = 3 \times 13$


 * The $21$st after $1, 2, 4, 5, 6, 8, 9, 12, 13, 15, 16, 17, 20, 24, 25, 27, 28, 32, 35, 26$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes.


 * The number of convex polygons that can be assembled from the complete set of $12$ hexiamonds.