40

Number
$40$ (forty) is:


 * $2^3 \times 5$


 * The $4$th octagonal number, after $1, 8, 21$:
 * $40 = 1 + 7 + 13 + 19 = 4 \left({3 \times 4 - 2}\right)$


 * The $7$th abundant number after $12, 18, 20, 24, 30, 36$:
 * $1 + 2 + 4 + 5 + 8 + 10 + 20 = 50 > 40$


 * The $9$th semiperfect number after $6, 12, 18, 20, 24, 28, 30, 36$:
 * $40 = 2 + 8 + 10 + 20$