671

Number
$671$ (six hundred and seventy-one) is:


 * $11 \times 61$


 * The magic constant of the order $11$ magic square:
 * $671 = \dfrac {11 \left({11^2 + 1}\right)} 2$


 * The $4$th Fermat pseudoprime to base $3$ after $91$, $121$, $286$:
 * $3^{671} \equiv 3 \pmod {671}$