671

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


 * $11 \times 61$


 * The $3$rd integer after $3$, $228$ whose square is the sum of $2$ coprime cubes:
 * $671^2 = 65^3 + 56^3$


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


 * The magic constant of a magic square of order $11$, after $1$, $(5)$, $15$, $34$, $65$, $111$, $175$, $260$, $369$, $505$:
 * $671 = \displaystyle \dfrac 1 {11} \sum_{k \mathop = 1}^{11^2} k = \dfrac {11 \paren {11^2 + 1} } 2$