2556

Number
$2556$ (two thousand, five hundred and fifty-six) is:
 * $2^2 \times 3^2 \times 71$


 * The $71$st triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $1891$, $1953$, $2016$, $2080$, $2145$, $2211$, $2278$, $2346$, $2415$, $2485$:
 * $2556 = \displaystyle \sum_{k \mathop = 1}^{71} k = \dfrac {71 \times \left({71 + 1}\right)} 2$


 * The $36$th hexagonal number after $1$, $6$, $15$, $28$, $45$, $66$, $91$, $\ldots$, $1431$, $1540$, $1653$, $1770$, $1891$, $2016$, $2145$, $2278$, $2415$:
 * $2556 = \displaystyle \sum_{k \mathop = 1}^{36} \left({4 k - 3}\right) = 36 \left({2 \times 35 - 1}\right)$


 * The largest term of the amicable triplet $\left({1980, 2016, 2556}\right)$:
 * $\sigma \left({1980}\right) = \sigma \left({2016}\right) = \sigma \left({2556}\right) = 6552 = 1980 + 2016 + 2556$