2^64 - 39 ...

[Paul Stoeber]

... is the greatest composite OpenBSD prime.

$ uname -srm
OpenBSD 5.7 amd64
$ n=$(echo "2^64 - 39" | bc)
$ /usr/games/factor $n
18446744073709551577: 18446744073709551577
$ /usr/local/bin/gfactor $n
18446744073709551577: 139646831 132095686967