1,747,515

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Number

$1 \, 747 \, 515$ (one million, seven hundred and forty-seven thousand, five hundred and fifteen) is:

$3 \times 5 \times 7 \times 11 \times 17 \times 89$

The $1$st of the $26$th pair of triangular numbers whose sum and difference are also both triangular:

$1 \, 747 \, 515 = T_{1869}$, $2 \, 185 \, 095 = T_{2090}$, $2 \, 185 \, 095 + 1 \, 747 \, 515 = T_{2804}$, $2 \, 185 \, 095 - 1 \, 747 \, 515 = T_{935}$

The $1869$th triangular number:

$1 \, 747 \, 515 = \ds \sum_{k \mathop = 1}^{1869} k = \dfrac {1869 \times \paren {1869 + 1} } 2$

Also see

• Previous  ... Next: Triangular Number
• Previous  ... Next: Triangular Number Pairs with Triangular Sum and Difference

Sources

• 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $1,747,515$
• 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $1,747,515$