91

Number
$91$ (ninety-one) is:


 * $7 \times 13$


 * The $13$th triangular number after $1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78$:
 * $91 = \displaystyle \sum_{k \mathop = 1}^{13} k = \dfrac {13 \times \left({13 + 1}\right)} 2$


 * The $7$th hexagonal number after $1, 6, 15, 28, 45, 66$:
 * $91 = 1 + 5 + 9 + 13 + 17 + 21 + 25 = 7 \left({2 \times 7 - 1}\right)$


 * The $6$th centered hexagonal number after $1, 7, 19, 37, 61$:
 * $91 = 1 + 6 + 12 + 18 + 24 + 30 = 6^3 - 5^3$


 * The $6$th square pyramidal number after $1, 5, 14, 30, 55$:
 * $91 = 1 + 4 + 9 + 14 + 25 + 36$