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 $5$th second pentagonal number after $2, 7, 15, 26$:
 * $40 = \dfrac {5 \left({3 \times 5 + 1}\right)} 2$


 * The $10$th generalized pentagonal number after $1, 2, 5, 7, 12, 15, 22, 26, 35$:
 * $40 = \dfrac {5 \left({3 \times 5 + 1}\right)} 2$


 * The $4$th pentagonal pyramidal number after $1, 6, 12$:
 * $40 = 1 + 5 + 12 + 22 = \dfrac {4^2 \left({4 + 1}\right)} 2$


 * 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 $6, 12, 18, 20, 24, 28, 30, 36$:
 * $40 = 2 + 8 + 10 + 20$


 * The only number whose name in the English language has all its letters in alphabetical order:
 * forty