18,446,744,073,709,551,617

Number
$18 \, 446 \, 744 \, 073 \, 709 \, 551 \, 617$ is:


 * $274 \, 177 \times 67 \, 280 \, 421 \, 310 \, 721$


 * The $6$th Fermat number after $3$, $5$, $17$, $257$, $65 \, 537$, $4 \, 294 \, 967 \, 297$:
 * $18 \, 446 \, 744 \, 073 \, 709 \, 551 \, 617 = 2^{\paren {2^6} } + 1 = 2^{64} + 1$
 * and the $2$nd not to be prime.

Also see

 * Prime Decomposition of 6th Fermat Number