651

Number
$651$ (six hundred and fifty-one) is:


 * $3 \times 7 \times 31$


 * The $21$st pentagonal number after $1$, $5$, $12$, $22$, $35$, $51$, $70$, $92$, $117$, $145$, $176$, $210$, $247$, $287$, $330$, $330$, $376$, $425$, $477$, $532$, $590$:
 * $651 = \displaystyle \sum_{k \mathop = 1}^{21} \left({3 k - 2}\right) = \dfrac {21 \left({3 \times 21 - 1}\right)} 2$