30

Number
$30$ (thirty) is:


 * $2 \times 3 \times 5$


 * The $4$th square pyramidal number after $1, 5, 14$:
 * $30 = 1 + 4 + 9 + 14$


 * The $5$th abundant number after $12, 18, 20, 24$:
 * $1 + 2 + 3 + 5 + 6 + 10 + 15 = 42 > 30$


 * The $3$rd primorial, after $1, 2, 6$ (counting $1$ as the zeroth):
 * $30 = 5\# = 2 \times 3 \times 5$
 * Hence the smallest positive integer with $3$ distinct prime factors.


 * The $7$th semiperfect number after $6, 12, 18, 20, 24, 28$:
 * $30 = 2 + 3 + 10 + 15$


 * The smallest Giuga number:
 * $\dfrac 1 2 + \dfrac 1 3 + \dfrac 1 5 - \dfrac 1 {30} = 1$

Also see

 * Pythagorean Triangles whose Area equal their Perimeter