364

Number
$364$ (three hundred and sixty-four) is:


 * $2^2 \times 7 \times 13$


 * The $12$th tetrahedral number, after $1$, $4$, $10$, $20$, $35$, $56$, $84$, $120$, $165$, $220$, $286$:
 * $364 = \ds \sum_{k \mathop = 1}^{12} \frac {k \paren {k + 1} } 2 = \dfrac {12 \paren {12 + 1} \paren {12 + 2} } 6$
 * Hence the total number of gifts in.


 * The $21$st number whose divisor sum is square: