39

Number
$39$ (thirty-nine) is:


 * $3 \times 13$


 * The smallest positive integer having a multiplicative persistence of $3$.


 * 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 $28$th integer $n$ such that $2^n$ contains no zero in its decimal representation:
 * $2^{39} = 549 \, 755 \, 813 \, 888$


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


 * Suggested by in his $1986$ book  as being the smallest uninteresting number, which fact makes it intrinsically interesting.

Also see

 * Interesting Number Paradox