67,280,421,310,721

Number
$67,280,421,310,721$ is:


 * The $n$th prime, where $n$ remains to be determined


 * One of the two prime factors of the $6$th Fermat number $2^{\paren {2^6} } + 1$, the other being $274 \, 177$:
 * $67 \, 280 \, 421 \, 310 \, 721 = 262 \, 814 \, 145 \, 745 \times 2^8 + 1 = 5 \times 47 \times 373 \times 2998279 \times 2^8 + 1$


 * The largest prime to have been identified as such without the use of a computer. This was done by subsequent to his factorisation of $F_6$ in $1880$.