651

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


 * $3 \times 7 \times 31$


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


 * The $41$st generalized pentagonal number after $1$, $2$, $5$, $7$, $12$, $15$, $\ldots$, $392$, $425$, $442$, $477$, $495$, $532$, $551$, $590$, $610$:
 * $651 = \displaystyle \sum_{k \mathop = 1}^{21} \left({3 k - 2}\right) = \dfrac {21 \left({3 \times 21 - 1}\right)} 2$


 * The $2$nd positive integer after $383$ such that its fourth power is the sum of $4$ other fourth powers of positive integers with no common factors:
 * $651^4 = 240^4 + 340^4 + 430^4 + 599^4$


 * The product with its reversal equals the product of another $3$-digit number with its reversal:
 * $651 \times 156 = 372 \times 273$


 * The magic constant of a magic cube of order $6$, after $1$, $(9)$, $42$, $130$, $315$:
 * $651 = \displaystyle \dfrac 1 {6^2} \sum_{k \mathop = 1}^{6^3} k = \dfrac {6 \paren {6^3 + 1} } 2$