561

Number
$561$ (five hundred and sixty-one) is:


 * $3 \times 11 \times 17$


 * The $16$th hexagonal number after $1, 6, 15, 28, 45, 66, 91, 120, 153, 190, 231, 276, 325, 378, 435, 496$:
 * $561 = \displaystyle \sum_{k \mathop = 1}^{17} \left({4 k - 3}\right) = 17 \left({2 \times 17 - 1}\right)$


 * The $2$nd Poulet number after $341$:
 * $2^{561} \equiv 2 \pmod {561}$: $561 = 3 \times 11 \times 17$